Full text of "Manipulation of the microscope"

See other formats

LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.

Class

MANIPULATION

OF THE

MICROSCOPE

EDWARD BAUSCH

ILLUSTRATED

FOURTH EDITION
TWENTIETH THOUSAND

PUBLISHED BY

PUBLICATION DEPARTMENT

BAUSCH ,t LOME OPTICAL CO

ROCHESTER N. Y.

OF THE

UNIVERSITY

BAUSCH & LOME OPTICAL CO.
1901

PRESS OF THE

BAUSCH * LOMB OPTICAL CO
ROCHESTER, N. Y.

PREFACE TO FIRST EDITION.

It may seem to some persons an act of presumption
for a mailer of microscopes and microscopical acces-
sories to enter the field of authorship and attempt to
supplement the valuable labors which in recent years
have made the use of the microscope an indispensible
aid in the advancement of science.

To such, if any, I submit that, being a producer
of microscopes and their accessories, I have had
opportunity to become acquainted with the lack of
general knowledge of the fundamental principles of
the instrument and the best methods of technique, even
among owners of microscopes. Indeed, with so many
complications, with almost unlimited powers and uses
of the instrument, the beginner cannot fail to feel the
need of a guide and adviser.

In order to accomplish the greatest good, I have
started out in this little Manual with the supposition
that the purchaser, or owner, is a beginner, and
absolutely ignorant of the microscope and everything
which pertains to it, and therefore have attempted to
convey, step by step, in as simple language as I could
command, information which will, I trust, lead to ease
of manipulation and give both pleasure and profit to
those for whom it was specially written.

With these, its purposes and hopes, I beg for my
self-imposed labor a friendly reception.

EDWARD BAUSCH.

June i, 1885.

206598

OF THE

UNIVERSITY

PREFACE TO SECOND EDITION.

The demand for this book having considerably
exceeded the expectations of its author, and the com-
ments on its utility having been so favorable, lead to
the view that it fills a gap in microscopical literature.

In preparing for a new edition an opportunity has
been given for enlarging on some of the subjects and
rewriting others, so as to make them conform to the
changes which the last five years have brought about
in the construction of apparatus.

While it may be true that many of the subjects
might be treated much more extensively, the writer has
purposely refrained from doing so, because he has con-
sidered it beyond the province of his intention, and
because books giving more extensive information are
available.

An intending purchaser of a microscope finds it
more or less difficult to make a suitable selection and,
while it is always best to consult an experienced
microscopist, the writer has endeavored to convey
information which, he hopes, will aid in this direction.

THE AUTHOR.
May, 1891.

PREFACE TO THIRD EDITION.

The past demand for this little volume makes
extended remarks superfluous, the new edition appear-
ing as evidence that it is considered of some value.

This edition has been almost entirely rewritten to
bring it in accord with the advance which has been
made in the construction of microscopes -and acces-
sories and while it is not expected to be a complete
guide, it is, nevertheless, hoped that it will lighten
the labor of the beginner.

Since its first issue there have appeared two books
covering the same purpose: " The Microscope and
Microscopical Methods " by Prof. S. H. Gage of
Cornell University and " Microscopical Praxis " or
" Simple Methods of Ascertaining the Properties of
Various Microscopical Accessories" by Dr. A. C. Stokes,
both of which are heartily commended to the micro-
scopist. Neither should be wanting in a microscopical
library. The writer is also pleased to acknowledge the
suggestions of an enlarged scope for this book, which
he has obtained by a perusal of them, as well as from
the admirable work of Dr. W. H. Dallinger in the
latest issue of Carpenter, " The Microscope and its
Revelations ", which may be commended to those who
wish to study more deeply the principles of the micro-
scope and learn its history and development.

The writer trusts that omissions will be pardoned,
as the only time which it has "been possible to devote
to this work has been the spare moments of a busy life,
and hopes sufficient information may be obtained to
give full compensation for such defects.

THE AUTHOR.
March i, 1897.

PREFACE TO FOURTH EDITION.

In the revision for this edition an effort has been
made to reach greater conciseness in the explanations
and in paragraphing the instructions in the use of
various kinds of apparatus. Many of the illustrations
have been replaced by new ones, and others have been
added.

This book has been adopted in many schools as a
text book and it is hoped that its usefulness in this
direction may be extended.

THE AUTHOR.
October, 1901.

VII

OPTICAL PROPERTIES OF LENSES.

Purpose of the Microscope. The microscope
is an instrument which magnifies small objects, so that
we are better able to examine their structure than is
possible with unassisted vision.

Simple and Compound Microscopes. Micro-
scopes are divided into two classes — simple and
compound, — the difference between the two being as
follows:

The simple microscope is usually of low magnify-
ing power and consists of one lens or a single system
of lenses through which the object is viewed directly
and the image is seen erect, or in its real position.

The compound microscope gives a higher magnify-
ing power and the image formed by one system of
lenses is observed through a second system of lenses,
and the final magnified image appears reversed, so
that what is right in the object is left in the image.

Lenses. As microscopes depend upon the action
of lenses it seems fitting that they, as well as their
action on light passing through them, should receive
attention.

Every person has unquestionably observed that
• when a spoon is placed in a tumbler of water it is
apparently bent at the surface of the water, or when
looking at an object lying in the bottom of a dish, it is
apparently at a different point than when viewed from
the side. This is caused by the deflection or bending
of the rays of light as they pass from one transparent
medium into another of greater or less density and is
called refraction. The amount of refraction increases as
the difference in the density of the two media becomes
greater. We also know that in viewing objects through
a glass prism, they apparently lie in a direction differ-
ent from their real one. The amount of this deviation
depends for the one factor upon the density of the

glass composing the
prism and for the
other upon its shape.

Fig. i represents
a cross section of
a prism and shows

how the ray on

- , r

entering at the first

surface undergoes refraction and how on emerging at
the second surface a second refraction takes place.
It will be noticed that the light is bent downward
or toward the base of the prism and if the prism
be imagined reversed with its base upward, the action

of the prism on the light will be the same, but the
light will be bent upward, always toward the base.

Now, a lens in either of its two principal forms is in
effect a combination of two prisms. In Fig. 2, where

the two bases are
placed together, is
shown how the light
is refracted toward
the bases and thereby
converged. In Fig. 3,
where the bases are
in reverse position, the

fig. 2.

action of the prisms

on the light is the same, the rays being refracted
toward the bases thus causing them to diverge or
separate.

In Fig. 4 is
shown how, by in-
creasing the number
of prism faces, the
form of a lens is
gradually approach-
ed.

If the combina-
tions of prisms be
imagined with curved surfaces instead of flat ones,
their action on light passing through them will be

Fig. 3.

precisely the same and we have the two great classes
of lenses, converging or convex and diverging or
concave, as shown in Fig. 5.

Fig. 4.

Fig. 5.

These two classes are again subdivided, Fig. 6
showing the three forms of the convex and concave

Fig. 6.

types which are, beginning at the left:
/ — double convex, II — plane convex, III — convex
meniscus, IV — double concave, V — plane concave,
VI — concave meniscus.

Fig. 7.

In the convex lenses, parallel or nearly parallel
rays are converged to one point as shown in Fig. 7.
This point c is the
principal focus, or
focal point, and the
distance from the
point b, called the
principal point, to
ris the focal length.
With the same lens,
if a flame be placed
at c, all the rays
which strike the lens will emerge in a parallel direction
on the opposite side. The straight line d b c which
passes through the middle of the lens is called the
principal axis and the distance a c, which for the sake
of simplicity has been taken from the focal point as
center, the radius.

The radius of curvature in combination with the
refracting power of the glass determines the converg-
ing quality of a lens and consequently its focal length,
and as the radius is lengthened the focus becomes
longer. In the ordinary lenses the glass used is, with
very slight variation, of the same refracting power, so
that the difference in focus is dependent upon the curva-
ture of the surfaces. If in a double convex lens the
radius of each surface is one inch, the lens has a focus
of one inch, and if in a plane convex lens the convex

surface has this same radius, the refracting power is
one-half and the focus is twice as long, or two inches.

In a concave lens, the action of the lens is opposite;
instead of converging the light toward the axis, it
diverges the rays from the axis as shown in Fig. 8.

Fig. 8.

The imaginary extension of the diverging rays
should meet at e and the distance t b indicates the
virtual focus, in contradistinction to the real focus in
the convex lens.

To Determine the Focal Length of a
Convex Lens. The focal length of the lens may
be quite accurately determined by the following
methods :

/ — Hold the lens toward the sun with one hand.
With the other hold a piece of paper under it.
Move the paper slowly toward the lens. A large bright
spot will appear, which, as the paper is brought nearer,
will decrease in size but increase in intensity until
it becomes quite small, and as the paper is brought
still closer to the lens the image will be found to
enlarge again. Return again until the spot decreases
to smallest she. The distance between lens and paper
is the focal length. By holding sufficiently long,
the paper, especially if it be dark, will be found to
burn, due to the concentration of heat rays and hence
this point is called the burning point or focal point.

II — In a room opposite a window or at a con-
siderable distance from a lamp, hold the end of a
ruler against a white wall and place the edge of
the lens against it, so that the axis of the lens will
be parallel with the ruler. Move the lens sfowly
toward or away from the wall until a greatly
reduced but bright image of either object appears
sharply defined. Read off the distance between the
wall and edge of lens. This is the focal length.

In a lens of considerable thickness measure from
the centre of the lens.

Magnifying Power. Magnifying power of a
convex lens depends upon its converging power.

In Fig. 9, a b represents the object, c d the lens,
<? the pupil of the eye. iy following the coirse of

rays from the object it will be noted how they are
refracted by the lens and intercepted by the pupil of
the eye. If now the lines between c and d be pro-
longed, they will be found to meet beyond a b and
there form a virtual image. It will be well to under-
stand at this point the difference between a real and
a virtual image. The real image is one which can

Fig. 9.

be accurately seen and projected upon a surface, as
with the magic lantern, or in the photographic camera.
The virtual image cannot be so projected, although
readily seen by looking through the lens.

In a lens of less convexity or longer focus, there is
less convergence of rays ; the size of the virtual image
is consequently reduced and thus the magnification is
less.

Spherical Aberration. In considering the refrac-
tion of light by a lens, we have up to this point
purposely avoided mentioning another quality which is
incident to it. In magnifying an object with a single
lens it will be noticed that the virtual image as seen
through its central portion is quite clear, while that
near the margin or edge is quite indistinct. This is
due to spherical aberration and the extent of this
aberration increases with the power of the lens.

It is due to the difference in refraction of those
rays passing near the margin and those passing through
the central portion, so that the rays, instead of combin-
ing at the focal point, come together at different inter-
vals along the central line or principal axis.

By reference to Fig. 10 it will be seen that the
outer or marginal rays are refracted at e and f so

that they will combine at o, and the inner or central
rays are refracted at g and k so that they will meet

at /. In the same manner will the rays which enter
between c h and / d come together at intermediate
points between o and /, and those of the central portion
between h and / will fall beyond /. Spherical aberra-
tion increases with the decrease in the focus of a lens
and in lenses of the same focus but different form, is
greatest in the double convex and least in the so-called
crossed lens, in which the two convex surfaces are of
different radii and in the proportion of i to 6, on
condition, however, that the surface of shorter radius is
directed toward the object.

This latter form of magnifying lens is seldom used,
as it shows the greatest amount of aberration if used in
the reversed position. The most common form is the
double convex with equal curvatures and when con-
siderable magnifying power is desired, the defects of
spherical aberration are partially overcome by the
interposition of an opaque plate, with round opening,
to shut out the marginal rays. This plate is called
a diaphragm and when used the lens is said to be
stopped down.

Chromatic Aberration. In magnifying an object
with a single lens it will be noticed that it has not only
the defect of spherical aberration, but that the object
appears fringed with colors, predominantly violet and
red, or if objects are viewed through a prism, we have
not only an apparent change of position, but a decided

appearance of so-called rainbow colors. This appear-
ance is called chromatic aberration and is a result of
refraction. It is caused by dispersion or the dispersive
quality, the separation of light into its primary colors,
violet, indigo, blue, green, yellow, orange and red, in the
Order given.

indiifo
•iolet

Fig. 11.

The dispersion of light by a prism is shown in
Fig. 1 1 , and it will be seen that the ray of white light
a b on entering the prism at b is separated into the
different colors in the order given, and that on
emergence at the second surface the violet ray at v has
undergone the greatest amount of refraction and the
red ray at r the least. The band of colors formed
is called the spectrum. The results of dispersion are
nicely illustrated in the diamond, this having a very
high degree of refracting power and is polished or cut
to make a many-sided prism in which each face or

facet creates refraction with its consequent dispersion
and play of colors.

With the light as refracted by a lens or prism, on
its emergence the violet ray, being the more refrang-
ible, will be principally affected and be brought to a
focus within the principal focus, and the red ray will
be brought to a focus beyond that of the violet.

To avoid spherical and chromatic aberrations to
the greatest possible extent has been for many years
the study of opticians.

Fig. 12.

How this was successfully accomplished is explained
by reference to Fig. 1 2 where it will be seen that the
ray of white light is separated into the various colored
rays, from red to violet, in passing through the first
prism which is made of crown glass, and that the rays
would continue to diverge in the direction r r' for the
red, and v v' for the violet. If now a second prism of

suitable flint glass with about one-half the refracting
angle and in reversed position is placed close to the
first or crown glass prism the red ray will be refracted
in the direction rr r" , and the violet in the direction
7/ z/', so that on emerging at r" and v" respectively,
the red and blue rays will continue in a parallel course
and thus be recomposed into a beam of white light,
which is changed in its direction from a b d to r" e.

Thus it is possible not only to avoid dispersion but
to obtain a converging effect. If now the prisms will
be imagined as a combination of lenses
we have a so-called corrected or achro-
matic combination or lens.

If the chromatic and spherical aberra-
tions are both corrected it is called
aplanatic. The convex lens is made of
crown glass and the concave <& flint glass.

The corrected lens shown in Fig. 13
is an achromatic lens of the simplest
form and while not absolutely corrected is generally
used when the demands on it are not too great.
Better correction is obtained when two or more of
these lenses are used in combination and they are
thus used in some of the lenses of the compound
microscope. The variety of forms, due to the variety
of glass from which combinations may be made, is
almost infinite.

Fig. 13.

SIMPLE MICROSCOPES.

Simple microscopes are usually termed magnifiers
and whether consisting of one or several lenses in
close contact, always remain simple. They are made
to be held in the hand or are fixed on a stand or

mounting, which is pro-
vided with adjustments
for focusing, thus giving
steadiness and leaving the
hands free for dissecting
Fl8' 14' or moving the object

during observation. Magnifiers are made in a large
variety of forms, the difference appearing in their
optical as well as mechanical construction. The most
common are those with one or several double convex
lenses mounted in hard
rubber or vulcanite,
nickel, aluminum, cel-
luloid, etc., and ar-
ranged to be folded for

Fig. 15.

pocket use as shown in

Fig. 14 and Fig. 15. Those containing several lenses

are preferable, since they offer a variety of magnify-

ing powers and in combination give the greatest
magnification admissible with single lenses.

Doublet Magnifier. This lens, Fig. 16, recently
introduced, is composed of two 'separated plane convex

lenses and, while not so
compact for pocket use,
eliminates some of the
optical defects of the
ordinary magnifiers.

Fig. 16.

Coddington Lens. A lens of greater efficiency
than either the ordinary magnifier or the doublet
magnifier is the Coddington lens, Fig. 17'.

While this is also a single double convex lens
it, will be noticed that it has considerable thickness,
being really the central portion of a sphere and
provided with a circular incision at the middle, which
is blackened and thus
acts as a diaphragm,
shutting out the marginal
rays and correcting the
spherical aberration, at
the same time, however,
limiting the size of field.

In selecting a Coddington, one should avoid those
which are of ten 'offered as the real and which consist

of two double convex lenses, separated by a black
diaphragm, giving the external appearance of a Cod-
dington while in reality being only an inferior Doublet.

Aplanatic Triplet. The best type of magnifier
is the Aplanatic Triplet, Fig. 18. It is composed of
three lenses cemented together in such a manner that
they are virtually one. For many years this form was,

Fit. 18. Fig. 19.

on account of cost, used only to a limited extent, but
an increased demand has brought about a greatly
reduced price. Its advantage lies in the fact that both
the spherical and chromatic aberrations are corrected
to a high degree, thus enabling the observation of
minute detail and giving a large field which is flat to
the extreme margin. It can be highly recommended
to those wishing a good magnifier.

Hastings' Aplanatic Triplet. This form of
lens, Fig. 19, has been computed by Prof. C. S. Hastings
of Yale University and is a modification of the Aplanatic
Triplet, giving not only the highest spherical and chro-

matic corrections, but a considerably flatter and larger
angular field and longer working distance ; that is, the
distance between lens and object is greater and is an
important feature in higher powers, as it admits of
better illumination of the object and greater ease in
working.

In all the better types of magnifiers the vulcanite
mountings are discarded for those made of metal,
especially German silver, although many are used of
pure silver and some even of gold.

Reading Glass. When magnifying lenses reach
a diameter of 2 inches or more they are usually termed
Reading Glasses and are then provided with a "handle.
They are used to examine large objects or areas, where
a low magnification is sufficient, to enlarge small
printed matter, or to determine detail in engravings
and photographs.

Bruecke Lens. This lens, named after its
inventor, is designed to give a longer working distance
between lens and object than can be obtained with
simple lenses of the same magnifying power and to
give an erect image at the same time, such qualities
being manifestly desirable for dissecting and similar
work.

A combination of achromatic Jenses forms the
image which is viewed through an achromatic con-

cave eye lens by means of which magnifications from
5 to 100 diameters are obtained. Change in magnify-
ing power is conveniently made either by changing
the object lenses or by varying the position of the eye
lens. The field of this lens is necessarily small but it
is the only form yet devised by which a long working
distance combined with high magnifying power can be
obtained.

Holders, Stands and Dissecting Micro-
scopes. There is a great variety of mechanical contri-
vances for holding magnifiers, to give steadiness as
well as to leave the hands free for moving and working

on the object and adjusting
for focus. When provision
is made to hold and adjust
the lens the apparatus may
be called a lens holder or
stand, but when a platform
called a stage, upon which
the object can be placed,
Fig- 20. and a mirror for illuminating

the object properly, are added, it is called a dissecting
microscope.

There are two forms of magnifiers which contain
within themselves some properties of stands. One, the
Tripod Magnifier, Fig. 20, rests upon three legs and
has a screw for adjustment of focus. Its optical parts

are two convex lenses, usually having a power of
ten diameters, which are separated by a diaphragm,
giving a large, fairly flat field. It is especially used in
primary botanical and zoological work.

Another is the Linen Tester, Fig. 21. It is made
in various sizes, but the ordinary form has a lens of
i inch focus. It is arranged
to fold into small compass for
convenience in carrying, and
when opened for use is placed
over the object so that this
comes into the square opening
in the base and is then exactly
in the focus of the lens. Its
F. 21 principal use, as its name in-

dicates, is in the textile indus-
tries for counting the number of threads which appear
in the standard openings of \ or \ inch.

The simplest form of holder is a base to which is
fixed a series of ball and socket joints, which offer
means of adjusting the magnifier in every direction.
A spring clip is provided for holding magnifiers of
different sizes. The lens is focused by raising and
lowering the lens clip by hand. In some of the more
complex holders there is a rack and pinion for focusing
the lans which adds to convenience in woiking.

For finer dissecting work a stand with fiim base,
stage, lans holder, and mirror, and with adjustment for

the magnifier is required. Of this type Fig. 22 repre-
sents one of the most simple and inexpensive forms;

Fig. 22. Barnes Dissecting Microscope.

Fig. 23 a compact form in which all the parts may
be placed in the box which forms the base so that the

Fig. 23.

whole may be carried in the pocket, while Fig. 24
shows a form recommended by the highest authorities
for advanced work.

How to use Magnifiers and Dissecting
Microscopes. It is generally admitted that the
intelligent use of a magnifier is a great aid in micro-
scopical Studies and while its use is a simple matter,
some words of advice may be of aid in obtaining better
results, or lead to doing work with more comfort. In
all work, whether with simple or compound micro-
scopes, it is a good plan to start out with the principle
not to use a greater magnifying power than is necessary
to accomplish the' results in view.

It should be made a
habit at the outset to
keep both eyes open.

Keep the eye com-
fortably near to the
upper surface of the
lens, as the angular
view or field is in-
creased, there is the
least spherical aberra-
tion, and the focal dis-
tance is the greatest.
This can be easily
Fig 24. tested by gradually in-

creasing the distance

between the eye and lens, when it will be found that
the lens must be brought nearer to the object. In
single lenses the spherical and chromatic aberrations

become more pronounced and the field smaller as the
distance between eye and lens is increased.

When magnifiers are used on opaque objects —
those which are not transparent and which are illumin-
ated by reflected light not transmitted through them
— a position should be chosen opposite a window
or flame that the greatest amount of light will reach
the object. If a hat is worn place it back on the head
so that the rim will not cut off the light.

Holding the object in one hand, take the magnifier
between the thumb and forefinger of the other and place
the fingers of the hand holding the lens in such a manner
that they shall rest upon the other hand, thus insuring
steadiness of the lens and object and adding consider-
ably to the comfort of working.

While it seldom occurs that magnifiers are made
with other than double convex surfaces, single achro-
matic lenses with plane convex surfaces are sometimes
used. These should be used with their convex sur-
faces toward each other.

In magnifiers containing several lenses, when they
are used together, the one of highest power should be
nearest the object.

When reversed the angular field is greater, but the
spherical and chromatic aberrations are correspondingly
greater.

In simple dissecting microscopes like the Barnes, in
which the mirror is in a fixed position, the microscope
should be set squarely before
the source of light. Diffused
day light is always preferable
to any artificial illumination.
Whatever the source of
light may be it is seldom too
strong when viewing opaque
objects; in fact the want of
sufficient light is too often ex-
perienced, especially indoors.
With transparent objects,
however, whether viewed by
looking toward the light
or by means of a reflecting
mirror the contrary is too
often the case, so that an
object is viewed in a glare
of light and is liable to in-
juriously affect the eye.
When the light is too strong reduce it or change the
position of the body or the instrument.

While in some classes of work it is perhaps unnec-
essary to have the very best magnifiers, such as the
Aplanatic or Hastings' Aplanatic Triplets, the latter
can always be recommended, when the means will
permit, on account of the higher results and greater

Correct manner of holding
magnifier and object.

degree of satisfaction and comfort derived from them.

Caution. Unless a microscope, whether simple
or compound, is known to come from the hands of a
reliable maker, any claim as to magnifying power
should be accepted with reserve. In former years,
when the country was overrun with cheap foreign pro-
ductions, the most fanciful claims were made in this
direction. Avoid strolling or street venders who, as a
rule, not only make the most ridiculous claims as to
magnifying power, but charge much higher prices than
the same articles can be bought for from reliable
opticians and generally offer worthless articles or, at
best, of doubtful value.

Some precaution should also be used in reference
to quality in purchasing a magnifier. As competition
causes a downward tendency in prices, it unfortunately
often involves a deterioration in quality. The ordinary
forms are mounted in vulcanite; black horn is 'often
palmed off as such, but is a poor substitute as it warps
and cracks. The surfaces of lenses, instead of being
perfectly polished, are often scratched and unfinished,
showing small pitholes or undulating surfaces. This
is common among cheap Coddington lenses and
naturally destroys the distinctness of the image.

Magnifying Power. It is evident that a lens
magnifies an object equally in all directions ; this is
said to be in areas, and is the square of the linear, or
diameter, so that if an object is magnified four times

in the linear, it is magnified sixteen times in area. The
commonly accepted term to express magnifying power
of simple as well as compound microscopes is in
diameters* (linear).

To Determine Magnifying Power. For simple
lenses the magnifying power may be determined by
dividing ten by the focal length in inches. Thus a
single lens of i inch focus magnifies about ten dia-
meters; one of 2 inch focus, about five diameters;
one of \ inch focus, twenty diameters, and so on. In
a lens of high magnifying power, the focus is ordinarily
made about twice the diameter, so that if a lens is
\ inch diameter its focus is about i inch.

While the determination of focus in single lenses
gives approximate magnifying power, it will not do so
in some of the forms which have been described. The
following method, if carefully followed, will give very
accurate results and is withal simple and interesting:
Place a sheet of white paper on the table. With the
source of light at the right or left hand, arrange a pile
of books to such height that when the magnifier to be
tested is placed upon the pile with the lens projecting
toward the observer the distance between the upper
surface of the lens and the paper \\ill be exactly ten
inches. The magnifier can be held in place by
another book placed upon it. Place a pocket ruler
between the leaves of the upper book so that when the
edge is close to the magnifier the divisions on the ruler

will be exactly in focus. Place the ruler so that it will
not cover much of the lens. It is immaterial what the
divisions on the ruler are, whether inches or milli-
meters, so long as they are reasonably fine. View the
divisions with the right eye and open the left eye, when
it will be found that the divisions are apparently pro-
jected upon the paper. Take a pencil and outline one
of the spaces upon the paper. By dividing this
enlarged space by the actual number of divisions on
the ruler, the exact magnifying power will be determ-
ined. Thus, if it is found that the one space on the
paper contains five spaces on the ruler, the magnifying
power is five, and the focus of the lens 2 inches ; or if
ten spaces, it is ten, with a focus of i inch. One or
several lenses in conjunction may be examined in this
way. Some difficulty may and probably will be
experienced in seeing the divisions on the ruler and
on the paper at the same time, but this will be over-
come with a little practice. Indeed, it is well to point
out at this stage that both eyes should be kept open in
viewing objects through simple as well as compound
microscopes, as continued work can be done with
infinitely more comfort and, while at first some diffi-
culty may be experienced, it will be found that after
a little earnest effort, both eyes unconsciously remain
open and the prominence with which objects appear
to the unoccupied eye diminishes as the mind becomes
intent upon the object it is viewing.

THE COMPOUND MICROSCOPE.

As has been stated a magnified image is observed
in the Compound Microscope. Any two lenses of
suitable focus, placed sufficiently far apart, will attain
this object, and this was for years the method of
construction.

In any microscope, whether simple or compound,
the difficulty of holding it or the object steady during
observation increases with the increase in magnifying
power, and in the compound form with only a
moderately high power it is utterly impossible to
retain sufficient steadiness to make any reliable obser-
vation. Mechanical contrivances therefore are a neces-
sity and were applied in the very earliest constructions
of the microscope. Even when such a luxury as an
achromatic lens was unknown they were all made to
embody the following essential parts:

A platform or stage for holding the object.

A means of adjustment for properly focusing the
lenses on the object.

Provisions for suitably illuminating the object.

From what may be called a crude attainment of
these three purposes, the construction gradually became
more complex. Many additions have been made which
have proven useful and have remained, while others
have been discarded. As the first microscope was
constructed in 1590, it has required nearly three
centuries to bring the instrument up to its present
general form, and it is interesting to note that many
improvements which have been introduced within the
last forty or fifty years have been used and lost sight
of within this time.

While certain parts are necessary to make up a
modern instrument, no one design of construction is
followed. The forms are innumerable, each maker
following his own inclination in variety, design, number
of parts, and material. For the latter, brass predomin-
ates, although bronze and iron are used to a consider-
able extent. The first two metals are usually highly
finished and, as they easily tarnish, are protected by
lacquer, which not only is serviceable in this direction
when of proper composition and rightly applied, but
offers a means of ornamentation. Iron is covered
with a heavy coating of japan and being dark is on
this account often recommended as being agreeable
for the eyes.

The entire apparatus, including the optical parts,
is called a microscope, whereas, without them, it is
termed a stand.

The microscope is called by some a " machine ",
but we earnestly protest against this harsh term being
applied to an instrument of such precision.

As it is necessary for the student to become
conversant with the names of the various parts and
to understand their use, we give an illustration, Fig. 25,
with the parts lettered for better identification, and
append a list giving their names. We recommend
that they be impressed upon the memory, as they
are the basis of microscopical language.

A. Base, the foundation of the instrument. It
usually rests upon three points (or should do so) and
is of such weight that it keeps the instrument firm
when it is in an upright or inclined position. The
two principal forms are the horseshoe and tripod.

B. Pillar, the vertical column which is fastened
to the base and carries in its upper end the joint or
axis which is provided for inclining the instrument.
It generally consists of one piece, either round or
square, but, in larger instruments, is often made in two
columns.

C. Arm, supports all the upper working parts of
the instrument and carries the adjustments for focus.

D. Body, the tube portion to which the optical
parts are attached.

Fig. 25.

E. Nose- Piece, an extra piece which is attached
to the lower part of the tube to hold the objective.

Society Screw, a standard screw which is cut
into the* nose-piece and is called so from the fact that
it was first recommended by the Royal Microscopical
Society of London. It is also called the universal
screw and is in general use in this country.

F. Objective, contains the object lenses, is
screwed into the nose-piece and is called so because
it is nearest the object. It is the most important of the
two optical parts (of the microscope proper) and upon
its perfection the distinctness of the image and there-
fore the value of the instrument almost entirely
depends.

G. Eyepiece or Ocular, the remaining optical
part, and called so because it is nearest the eye. It
magnifies the image given by the objective. This and
the objective will be fully treated later on.

H. Draw-Tube, the inner tube of the body which
moves in the outer sheath and which receives the eye-
piece. It permits adjustment for different tube lengths
and variations in magnifying power.

I. Collar, a ring which is attached to the draw-
tube and is usually provided with a milled or knurled
edge.

J. Coarse Adjustment, a mechanism for moving
the body quickly back and forth for adjusting the
focus approximately. It consists of a slide attached
to the body, and a straight rack, the former being
fitted to a recess, the latter engaging the teeth of a
pinion which is stationary in the arm. In instruments
of very simple form the coarse adjustment is obtained
by fitting the body in an outer stationary sheath.

K. Milled-Heads, the large buttons attached
to the shanks of the pinion, which is revolved by
means of them. They are usually large to give
sensitiveness to the movement, and should be placed
wide apart so that the fingers may be entirely free
from the body.

L. Micrometer Screw, a fine screw provided
with milled head. It acts upon the body either
directly or by levers. It has a slow and delicate
movement and provides the fine adjustment. This as
well as the coarse adjustment should be extremely
sensitive and should not have the least side or lateral
motion. The fact that either of them has it, is
evidence of poor workmanship.

M. Stage, consists of a strong metal plate, placed
at a right angle to the body, and provided with an
opening for the passage of light iiom below. The
object is placed upon it for examination.

Centering Screws are provided in some stands
for moving the stage in different directions to bring
the center of its revolving motion in the center of
the field. In a limited sense they offer means of
mechanical movement for the object.

N. Clips, two springs attached to the upper
surface of the stage and used to hold the object in
position.

O. Mirror, used for reflecting and condensing
light upon the object. As a rule there are two mirrors,
one plane and the other concave. The former gives a
comparatively weak light, while the latter concentrates
it and gives more intensity.

P. Mirror Bar, carries the mirror and is pivoted
in order to illuminate the object from different
directions.

Q. Substage, a tube or attachment below the
stage, of standard size, to receive various accessories
which may be required. It is sometimes immovably
fixed to the stage but in the best instruments is pro-
vided with an adjustment to vary its distance from the
object.

S. Diaphragm, a provision for increasing or
decreasing the amount of light which illuminates the
object.

Optical Axis, an imaginary line which passes
from the center of the eyepiece through the centers of
the body, objective, stage and substage to the mirror.
Whatever lies in it is said to be centered.

Object, that which is examined.

Slide or Slip, a plate of glass upon which the
object is placed or mounted ; the prevailing standard
being 3 inches long by i inch wide.

Cover Glass, a thin piece of glass cut circular
or square, which is placed upon the object, either for
flattening or preserving it, or both. The thickness
varies from 1-50 to 1-250 inch and this variation has
a very important bearing on the optical effect of the
microscope.

Classification of Microscopes. Until recently
microscopes were divided into two classes, the Jackson
and the Ross models. While the latter was for many
years very popular, particularly with the English
makers, it has been almost entirely superseded by the
Jackson form, and with good reason. In the former
the means of adjustment were applied, as near as
consistent with the construction, to the body tube;
whereas in the Ross form they were placed at the back
or more distant point in the instrument, thus increasing
by means of the connecting arm the faults which might
exist in the adjustment.

A form of stand which is now very popular is
called the Continental pattern, from the fact that it
was originally made on the continent of Europe, and is
a combination of the Jackson and the Ross models.
The coarse adjustment, when consisting of a rack and
pinion, is placed close to the tube while the fine adjust-
ment is placed in the arm.

Tube Length. The Continental form, Fig. 25,
which is quite generally used in all countries, has a
short tube from 160.0 to 170.0 mm. (6.3 to 6.7 inches)
whereas in England the long tube from 216.0 to 250.0
mm. (8.5 to 10 inches) is still retained to a consider-
able extent. The short tube contracts the height of
the instrument, this being the vital point in the con-
struction of the Continental stand.

Until recently this subject was given little attention,
each maker following a standard which he had adopted
for himself. The injurious influence of this diversity
was not appreciated by the public, as it was not
acquainted with the products of different makers, until
Prof. S. H. Gage made it the subject of a paper before
the American Microscopical Society and, as a result of
his inquiries, prepared the table of standards, Fig. 26,
followed by the different makers.

Acting on his suggestion a committee was appointed
to consider this subject, as well as that of eyepiece,
objective and thickness of cover glass, to which we will

Parts included
in Tube-length.
See Diagram.

r Grunow,

Tube -length
in
Millimeters.

203

t.:

X^-

-J£

E. Leitz, -

170

-£

Nachet et Fils,

146 or

200

fl-fl?< Powell and Lealand, -

254

C. Reichert, -
Spencer Lens Co.,
,W. Wales,

160 to
235 or
254

180
160

^- —

•"

r Bausch & Lomb Opt. Co

,216 or

160

Bezu, Hausser et Cie.,

220

Klonne und Muller, -

160-180 or 254

b-d-

W. & H. Seibert, -

190

Swift & Son,

165 to

228%

C. Zeiss,

160 or

250

f Gtmdlach Optical Co.,

254

1 R. Winkel, - -

220

$L-dc-d Ross &Co., -

254

^ —

—
x

^-^ R. & J. Beck, -
r-/ J. Green, - -
— f Hartnack, - -.

254
254
160-180

, —
^»~_

— .

__- -^

Verick, -
Watson & Sons,

160-200
160-250

Fig. 26-

recur farther on, and reported in favor of the adoption
of two standards for tube length, viz., short standard
1 60.0 mm. (6.3 inches), long standard 216 mm. (8.5
inches); that the tube length shall be considered those
parts between the upper end of the tube where the
ocular is inserted and the lower end of the tube where
the objective is inserted. There are no optical advant-
ages in the one or the other. The short length is
almost a necessity, however, in the Continental pattern
of microscopes as compactness is the special desidera-
tum; but, while this subject will be given more extended
attention and optically considered farther on, it might
be stated here that when an objective, except perhaps
in the very low powers, is constructed to be used with
a certain length of tube, it should be used with this
length only. This statement cannot be made too
prominent and will bear repetition.

Stage. This should be of such strength that, under
considerable magnification, the object may be moved
in different directions without displacement of focus.
This depends upon the matei ial of which it is made,
its thickness, and the strength of attachment between
it and the arm. Absolute ligidity is practically impos-
sible when considerable force is exerted, as can easily
be determined in the best instruments, and it is a
mistake to condemn an instrument for this cause as is
sometimes done. If the object will remain in fociis
under a high power with a fair amount of pressure

above that which is required in moving the object
about, the stability may be considered ample. In older
instruments the fault often occurred of making the stage
unnecessarily thick, which for present day require-
ments or with modern substage appliances would inter-
fere with the accomplishment of the best results. At
the present time the stages of instruments from reput-
able makers are of ample strength without undue
thickness although in cheap foreign products this is too
often not the case. It is of the greatest importance,
however, that the surface of the stage should be square
with the tube in all directions. Any deviation pro-
duces inferior optical results. In the better class of
instruments a vulcanite plate attached to the upper
part of the stage has proven very successful. The
peculiar gritty feeling due to small particles of dust
between the stage and slide is not so noticeable as on
a metal surface, and it is not much affected by acids or
alkalies and will therefore retain its neat appearance
almost indefinitely.

Revolving Stage. While in the largest number
of instruments the stage is fixed and generally square,
there are others in which it is revolving, that is, may
be revolved around the optical axis. Such a stage is
an absolute necessity in the examination of crystals
and rock sections, for which pjurpose graduations in
degrees or fractions of degrees, by means of which
the angles of the objects may be measured, are pro-

vided. It is supplied in most of the better class of
instruments and is a convenience in almost all kinds
of microscopical work. As a slight deviation of the
center of revolving motion from exact coincidence with
the optical axis will cause the object to swing out of
the field, centering screws are provided by which this
error can be quickly corrected. These screws are
also convenient, within narrow limits, in providing a
mechanical method of moving the object in different
directions over the field of view. It sometimes occurs,
as the stage is revolved, that an object at the edge
of the field, which is in focus, gradually becomes
indistinct, showing poorest at the half revolution and
as the stage is brought around to the first point again
comes into focus. This may be due to poor fitting of
the parts, or to the fact that the stage is not square in all
directions with the body; in either case a serious defect.

Glass Stage. This with the slide carrier, Fig. 27,
is a device for moving the object more steadily and

smoothly than can be
directly on
It is made

done

the

stage. It is made de-
tachable from the micro-
scope and consists of a
glass plate, in a metal
frame, upon which the
slide carrier, resting on four points, moves. At each
end a spring clip passes around the glass plate and

27.

presses against its lower surface, thus offering the
minimum of friction, with sufficient resistance to make
an easy movement.

Mechanical Stage. This is a very important
form in which the movements are mechanical, in two
directions at right angles to each other, motion being
transmitted through the milled heads by a rack and
pinion or screw. It is a most useful accessory and
with it work can be done systematically and rapidly
with the assurance that every portion of the field has
been covered and with a degree of comfort which must
be experienced to be appreciated. For instance, in a
bacteriological or urinary specimen, where one is
searching the field for certain objects, it is the only
reliable means of covering each portion of it. This
stage is especially valuable for blood counting and
plankton work. While not many years ago it was
spurned by many scientists as a toy, it is now generally
accepted as an invaluable part of a microscope. In
order to be so, however, it must be of the most perfect
workmanship, which is difficult to attain on account of
the necessarily small parts of which it is composed and
the hard usage which it must bear. The movements
must be smooth and easy and on reversing the milled
heads, must not show any lost motion or dead point.
A great deal of attention and ingenuity has been
applied in the development of the modern mechanical

stage. In its early construction it was heavily built
and of a thickness which would make it unsuitable for
use with modern substage accessories.

There are two different types, fixed and attachable.
In the fixed, which is generally also revolving, the

Fig. 28.

mechanical parts are attached to and form an integral
part of the stage. The Attachable Stage, Fig. 28, is
complete in itself, being attachable to the microscope,
and may be removed at will. Both are provided with
graduations, usually divisions in millimeters, in both
directions by which one may read off the amount of
space which is covered and locate the position of an
object for future reference.

In using the mechanical stage, it should first be
determined how many spaces, or how much of one space
is contained within the limits of the field ; then begin
at one edge of the specimen and with the lateral
movement (right to left or vice versa) make it

pass across the field. Move the slide forward with
the vertical movement the amount of space which has
been previously determined and by a return action of
the lateral movement, bring it across again and thus
through the entire specimen, or until the object is

found.

Nose -Piece. This being the lower end of the
body, to which the objectives are attached, it is import-
ant in so far as it must be accurately made. As has
been stated, it has the society screw. Previous to 1857
each maker followed a standard of his own and this to
a great extent is still the case on the Continent. The
Royal Microscopical Society, of London, appreciating
the inconvenience of this diversity, recommended a
standard thread of thirty-six to the inch with an external
diameter of 0.8 inch, which was finally adopted in
England and this country. The Society supplied to
the makers a so-called standard hob or tap with which
to gauge the thread. Unhappily, however, these taps
have not been made to a standard, as there is a
variation in those which are sent out by the Society,
so that, while the public is under the impression that

there are fixed dimensions, there is on this account a
diversity in the products of different makers ; hence it
often happens that the objectives of one maker will not
fit the stands of others. The writer in 1884 read a
paper on this subject before the American Micro-
scopical Society and as a result a committee was
appointed to bring about a better state of affairs. It
failed, however, in obtaining the co-operation of the
Royal Microscopical Society, the main reason being
the expense involved, so that we must continue to
suffer until some concerted action is taken by the
manufacturers themselves, which we trust will not be
far distant.

Revolving Nose-Pieces. Changing one objective
for another to obtain a different power is time consum-
ing and inconvenient; besides,
there is danger of dropping
the objective and thus a
liability of injuring it or dis-
turbing or destroying the
Fig- 29< object. To avoid this the

double, the triple, and quadruple nose-pieces are offered,
to the first of which two, to the next three, and to
the quadruple four objectives may be attached in
such a manner that, when fixed to the nose-piece of
the microscope, each objective may in turn be brought
into use by rotating the nose-piece. Each objective

comes to the center and will be in focus, if not exactly,
at any rate very closely. This statement should per-
haps be modified in so far that as the nose-piece is
rotated each objective should be approximately in focus
when in position, since objectives and nose-pieces are
still supplied by makers with which this is not the case.
Of all the convenient accessories these are the most
useful and in most common use, the writer knowing
from experience that nearly all of the instruments sold
for personal use are supplied with the double nose-
piece when two objectives are used, and the triple
when three are taken.

Bodies or Tubes. These are of two kinds:
monocular, having one body which may contain one
or two draw-tubes, observations being made with one
eye, and binocular for observation with both eyes, the
two tubes being fixed together at the nose-piece and
gradually separating until they reach the pupillary
distance. The first is a monocular microscope, the
second a binocular microscope.

While the methods for transmitting the rays from
the objective to the binocular tubes vary, the construc-
tion in most common use is that introduced by
Mr. Wenham.

By reference to Fig. 30 it will be seen that the rays
from one-half the objective are transmitted uninter-
ruptedly to the vertical tube, while the prism intercepts

the rays from the other
half and reflects them
into the oblique tube.
The result is an image in
each eyepiece, thus giv-
ing stereoscopic vision.
This gives a perception
of depth, a sense of
being able to look into
an object, and conveys
to the mind the im-
pression of roundness
or separate appreciation
of the different planes
of the object which it
is impossible to obtain
with monocular vision.
Its use, however, is
limited to the lower
power objectives.

Coarse Adjust-
ment. In providing
this adjustment, two
methods are followed.
The most simple form is
the sliding tube in which
the body tube, which
carries the nose-piece at
the lower end and draw-

Fig. 30.

tube at the upper end, is moved up and down in an outer
sheath, which is fastened to the arm. The milled ring
is grasped by thumb and fore and middle fingers and
pushed down and drawn up with a spiral motion. It
is not to be commended except for economical reasons,
as it lacks firmness, wears out quickly from the con-
siderable friction, endangers the object and the
objective from the liability to sudden or jerking
motions and does not well permit the application of
the double nose-piece. While a clamping ring which
fastens the tube in a fixed position is provided in some
instruments, especially to permit the use of double
nose-piece, this again has its disadvantages and is
cumbersome. Therefore it is strongly recommended
not to purchase an instrument of this kind if it can be
avoided.

The rack and pinion adjustment is by far preferable
in every respect and has stood the test of many years,
although efforts have been made to introduce other
methods, all of which, however, have become obsolete.
To be satisfactory and lasting, it must be exceedingly
well made and it is safe to advise that any instrument
with this adjustment, which does not work well at the
outset, may be regarded as a poor one. In late years
the pinion with spirally cut teeth and the rack with
diagonal ones has come into common use and is better
than the older form with straight cut teeth. In order
to make the pinion operative, bearings are provided for

it in the arm and its teeth engage in the rack, which is
fastened to a slide and has its bearing in the recessed
vertical length of the arm, as shown in Fig. 31.

Fig. 31.

This adjustment must meet the following conditions
and if it does not, the instrument may be safely con-
demned as faulty :

// must work with the utmost smoothness and with
not the least perceptible jar or grating.

It must be free from lost motion when working with
the highest powers.

The slide must be so perfectly fitted, that it shall
show no play when the tube is moderately forced from
one side to the other.

It is opportune in this connection to state that all
fittings involved in the rack and pinion are necessarily

delicate and great care should be exercised in using
the adjustment and keeping the parts free from dust.
Lubrication should never be necessary between the
teeth of the rack and pinion. When applied to the
spindles of the pinion it should be only a very small
quantity of the best oil, and when lubricating the
sliding parts, wipe these with cambric to which a drop
of oil has first been applied. A surplus amount of
oil acts as a dust catcher.

Fine Adjustment. While this is constructed in
numerous ways, in all it depends upon a screw for the
propelling power. It is sometimes called the slow
motion, as one revolution of the screw seldom gives more
than 1-50 inch motion. This screw is also called the
micrometer screw. The brass button by which it is
rotated is called its head and, when it is provided with
equal divisions upon its upper surface, it is the grad-
uated head. In this case a stationary index is fastened
to the arm.

The fine adjustment, although it should be delicate
and sensitive, must, nevertheless, be rigidly constructed.
Its bearings must be large and still free so as to be
responsive to the movement of the screw, and these
conditions must be maintained with an endless amount
of use.

While the fine adjustment, even more than the
stage, will show displacement with moderate magnify-

ing power by a slight pressure against the tube, it
should return to its position when released.

If a new instrument does not meet the conditions
here set down for testing a fine or coarse adjustment,
it may be put down as of faulty construction, no matter
by whom made or how well made it may appear in
other respects.

A fine adjustment should fullfill the following con-
ditions :

The screw must work freely and smoothly, and
without any side motion or play.

The adjustment should act promptly without the
least particle of hesitation or lost motion.

There should not be the slightest displacement of the
object in the field when the screw-head is worked back
and forth.

Draw-Tube. While this part of the instrument
may be an advantage when judiciously used, it may
have an injurious influence when abused. It will give
both short and long tube standards and should be pro-
vided with a mark to indicate each length, or should
have divisions by which the standard can be read off.
It should not be overlooked, that when a double nose-
piece is used its thickness is added to the optical tube
length and the draw-tube should be shortened an equal
amount. In the cheaper instruments the draw-tube

simply slides in the outer tube, but in the better instru-
ments a special spring sleeve is provided in which the
draw-tube operates. As both of these have the defect
incident to the sliding tube adjustment, a cloth lining
is preferable as the movement is smooth, while firm,
and will remain so for an unlimited time.

Correct position of hands to operate draw-tube.

The draw-tube may be used to vary the magnifying
power, but unless used judiciously may be the cause of
more harm than good. While this feature will be
touched upon again in another chapter, showing the
optical effect, it will suffice at the present to state that
it should be used only with objectives of low power or
with high powers only under well defined conditions.

The draw-tube usually has at its lower end a dia-
phragm to prevent reflection from the inner surfaces of
the tubes, and this also sometimes has a society screw
for attaching very low power objectives or accessories.

Care should be used in moving the draw-tube as a
too sudden movement upward may draw the main tube
with it and thus injure the rack and pinion, or down-
ward, may force the objective onto the object or by the
compression of air in the tube, may force out the eye-
piece. To operate the draw-tube, hold the main tube
with one hand and with the thumb and fore- finger
of the other grasp the milled edge of draw-tube and
move it up or down with spiral movement.

Base. A judicious form and weight of base
adds greatly to the stability of the microscope and it is
a too common fault that in many instruments, even
from reputable makers, this essential feature is sacri-
ficed from wrong motives of economy, portability or
compactness. While it can hardly be expected that
when the arm is inclined to the horizontal position the
microscope shall be stable, as it is never used in
this position except for photography, and must then be
clamped to the table, it is but reasonable to demand
that when the instrument is upright or slightly inclined
under ordinary manipulative operations, it should not
be required that the base be held with one hand while
the other makes the adjustments. We can imagine

nothing more aggravating than a lack of stability.
A considerable weight directly under the pillar is of
little value, and a great expansion of the resting points
with extreme thinness is little better. There should
be a combination of both qualities and if suitable pro-
portions are not maintained, an excess can hardly be
called a fault, whereas too little would certainly be.

Joint for Inclination. This should work
smoothly but firmly, and the arm should remain in
any position in which it is placed. If it has a gritty
sensation, the two parts are liable to " eat " and finally
reach a point where they cannot be moved.

Besides the above qualification a good joint should
work without the slightest back-lash when the arm is
worked quickly back and forth through small arc.

When the arm comes against the stop for upright
position it should not lean forward.

Mirror and Mirror-Bar. The proper illumina-
tion of an object is an important feature and although
there are numerous accessories for properly accomplish-
ing this, the mirrors alone are effective agents when
properly constructed and applied, particularly when no
high magnification is used. The plane mirror is
generally used with very low powers and reflects light
in about its original intensity. The concave mirror,
however, is intended to concentrate the light so that
all the rays which strike its surface are reflected and

come together at some point above, and the rays from
the surface being contained within a comparatively
small space, cause an increased intensity. This point
is called 'the focal point and usually coincides with the
opening of the stage when parallel rays such as from
the sky are used. When the light comes from a source
considerably nearer to the mirror, as from a lamp, and
the rays are diverging, the focal distance becomes con-
siderably Ignger. Some of the intensity is lost in
consequence as well as the degree of convergence.
For this reason some mirror-bars are so arranged that
the distance of the mirror from the stage may be varied
to accommodate the variation in the location of the
source of light. While this is of considerable aid,
there is in some instruments not sufficient room for a
complete accommodation, with the result that, under
certain conditions, the utmost effectiveness of the
microscope is not obtained.

Substage Diaphragm. This is provided for
regulating the amount of light. While it should be
possible to use the mirror at its utmost capacity, it very
often occurs that for certain investigations a profusion
of light is more harmful than otherwise. When there is
too much light objects are said to be drowned in it, and
this often makes it impossible to determine structure.
An intelligent use of the diaphragm is of great service.

The revolving diaphragm is the most simple and
consists of a black disk which rotates on a pivot and

53 ' •

is perforated with a series of openings of different sizes,
each of which may be brought into the optical axis.

Besides this there are other forms which may be
said to be better — for instance the so-called cap dia-
phragms, which require a separate piece for each
aperture and which are held by a special substage
receiver. There are usually three cap diaphragms,
each one having an aperture of different size, and
when attached are located below and near the object.
While the results obtained are much better than with
a revolving diaphragm, a change of diaphragm is
inconvenient as it involves the removal of the slide
from the stage or the receiver from below the stage.
An improved form has of late years been applied to
the better instruments in the iris diaphragm, which
consists of a series of thin overlapping blades placed
around a central opening the size of which may be
varied by means of a lever or milled edge operating
the blades. Besides the possibilities of varying the
size of the aperture there should be an adjustment for
changing the distance from the object. The distance
of the diaphragm from the object is one of considerable
importance. The best position is just below the sur-
face of the stage, but as this is not always possible, it
should be as near as conditions will permit. Very
recently it has been possible so to construct the iris
diaphragm, that it passes up through the opening of the
stage and may thus be brought very close to the object.

OBJECTIVES AND EYEPIECES.

In taking up this subject we would say at the outset
that it is fraught with difficulties, as almost all of the
features are based on scientific facts which can be best
explained by mathematical formulae, but as it is our
purpose to give intelligible explanations to those who
may not be conversant with algebraic expressions,
many of the statements and descriptions will appear
rather dogmatic. We can but advise those who wish
to study the subject further, to consult such books as
contain more explicit information.

For the purpose of simplicity the compound micro-
scope has up to this time been spoken of as being
composed of two lenses, the lower of which magnifies
the object and the upper magnifies the image formed
by the lower lens. While this expresses the principle,
as a matter of fact the microscope is never so con-
structed as the defects of chromatic and spherical aber-
rations would be more pronounced than in the simple
microscope, even to such an extent as to nullify the
benefit which might be derived from the increased
magnifying power alone. In fact, magnifying* power in
itself is of very little value without the attributes

obtained from the chromatic and spherical corrections.
The objective as well as the eyepiece is always com-
posed of a series of lenses, the purpose of which will
be explained as we proceed. The advent of achro-
matic lenses was the first decisive step in advance and
has been the foundation of all latter improvements and
the high standard of the best production of the present
day.

It is a matter of pride to Americans to note that
two of our countrymen, now deceased, were influential
in furthering the progress to a considerable extent, and
their memories should always be honored by the micro-
scopical world. They should be remembered with
feelings of gratitude, particularly as the compensation
for their efforts was extremely limited. The pioneer in
microscopical optics in this country was Charles E.
Spencer who was followed by Robert B. Tolles, and
while both men did a great amount of original advanced
work, it is the latter, particularly, who, by his wonderful
achievements, created a great discussion in European
circles, by obtaining results which for a long time it
was claimed could not be accomplished.

Of inestimable value to the scientific world have
been the labors of that most capable and genial gentle-
man, Prof. E. Abbe, of Jena, to whom, while best
known to the general microscopist for- some of his
more insignificant improvements, such as the Abbe
condenser, camera lucida, apertometer and apochromatic

objectives, we are much more indebted for his profound
disclosure of the principles of microscopical optics,
as well as to the combined efforts of himself and
Dr. Schott for their labors in the art of glass making
and to the large variety of glass which they have
placed at the disposal of opticians, who by this means
have been able to accomplish much higher results
than would otherwise have been the case. All of the
ordinary kinds of glass, faultless as they may appear,
are entirely unsuitable for use in the production of the
higher grade of optical instruments. The production
of optical glass for this use is fraught with many diffi-
culties. It must be absolutely homogeneous, free from
impurities and unchangeable under varying climatic
conditions. The mere fact of the existence of air
bubbles, while undesirable, is not necessarily a defect,
as it has been found impossible to produce certain
kinds of glass without them.

The amount of injury due to air bubbles is in the
loss of light, and as microscope lenses are small the
percentage of this loss may be considerable, therefore
bubbles are carefully avoided.

In this connection it is opportune to state that the
production of this glass, generally termed Jena glass,
has been taken advantage of by unscrupulous parties
in creating the impression that the bare fact of using
this glass gives in itself much better results. Such is
not at all the case. The merit of the production con-

sists mainly in the large variety of glass with different
ratios of refractive index and dispersive power, and its
effectiveness in lens making depends entirely on its
intelligent use by the optician.

As defects in the objective have already been
specially mentioned, it will at this point be well to state
that while an achromatic lens is unnecessary in the
eyepiece, a supplementary lens below and near the
upper lens has been found beneficial in so affecting the
image, by collecting the rays, that it can be viewed at
one glance and without spherical or chromatic aberra-
tion. For this reason the lower lens is called the
field or collective lens, and the upper lens the eye lens.
These lenses are mounted in a tube with fixed
relations, and are then called the eyepiece or ocular.
In the diagram, Fig. 32, the course of rays from the
object through the objective and eyepiece is shown.
g h represents the objective, / / the field lens and op
the eye lens of the eyepiece. As the rays from the
object pass through the objective they are seen to cross
before reaching the field lens, are converged as they
pass through, and further converged by the eye lens op.
At the point c d they form a real image of the object,
which can readily be seen by placing a ground glass or
piece of oiled paper at this point. It is an interesting
experiment and one which we recommend trying.
The eye lens enlarges this image and forms a greatly
magnified virtual image at e f. From this diagram

Fig. 32.

several changes with consequent results can be noted :

If the objective is of shorter focal length, a larger
real image is formed at c d.

If the distance between objective and eyepiece is
increased, a larger real image is formed at c d.

If the eyepiece is of higher power, a larger virtual
image is formed at e f.

In the same manner a reduced magnifying power
may be obtained by reversing these conditions.

As has already been stated, a i inch lens with a
distance of ten inches between it and the image gives
a power of ten diameters, and the eyepiece multiplies
the virtual image by the extent of its power. From
this it can be easily computed that with a i inch
objective used with a tube length of ten inches and
i inch eyepiece, a magnifying power of lox 10 = 100
will be obtained ; or the same combination with a tube
length of five inches will give one-half this power or
fifty.

Objectives are divided into two classes, dry and
immersion. In the dry objectives there is no interven-
ing medium other than air between the lowest lens
surface of the objective and the upper surface of the
cover glass. In the immersion objective a liquid fills
this space. From this fact it is easily seen that liquid
can only be used with objectives which are quite close to
the cover and therefore short focus or high power, and
so, on the other hand, objectives of long focus or low

power can not be immersion. The purpose of immer-
sion is to obtain higher optical results ; is in fact a
necessary condition, and an objective which is con-
structed as a dry one cannot be used as an immersion,
and vice versa. Although there have been objectives
constructed which can be used both as immersion and
dry, they have gone into disuse as they must suffer
when used in one or the other direction or in both.

Water was for many years used as immersion fluid,
but specially prepared cedar oil was found to give
better results and has almost entirely taken its place.
The refractive and dispersive properties of cedar oil
are almost identical with those of crown glass, and as
cover glass has very nearly the same properties as
crown glass, the term homogeneous immersion fluid is
often applied to the oil, but for brevity, objectives
which are constructed to be used with it are called
oil immersion.

Tube Length. Objectives are constructed and
their aberrations corrected for the length of tube with
which they are to be used, and as has been shown in
a previous chapter there are now two generally accepted
standards. They are corrected for either the long or
short tube and specially marked by progressive firms,
and it is hoped that in time this will become a universal
custom. When an objective is not marked the
purchaser should require to know the tube length with
which it is to be used.

Nomenclature, or Rating of Objectives.
While for many years objectives were marked arbi-
trarily by makers and differently by each maker to
designate the power, it is now customary to nnark
objectives so that the figures shall indicate the true
optical value — on the continent of Europe in milli-
meters and in England and this country in inches.
Objectives are rated according to their equivalent
focus. The equivalent focus of a series or combination
of lenses is the same as the focus of a single lens
having the same magnifying power as the series or
combination. This is also true of eyepieces. So, if
two combinations equal in magnifying power a single
lens of i inch focus they are marked i inch or
25.4 mm., or if a collection of four lenses is equal to a
lens of 1-12 inch focus it is marked 1-12 inch, or 2 mm.
Magnifying power increases in proportion to the
•decrease in focal length, so that a 1-12 inch objective,
for instance, will give a real image twelve times larger
than a i inch or, with a tube length of ten inches, a real
magnification of 120 diameters.

Powers. According to their magnifying power,
objectives are called low, medium, or high power and
are classified by Carpenter as follows :

Low powers, 3 inch, 2 inch, 11-2 inch, i inch, 3-4
inch, 2-3 inch.

Medium powers, 1-2 inch, 4-10 inch, 1-4 inch,
1-5 inch.

High powers, 1-6 inch, 1-8 inch, 1-10 inch, 1-12
inch, 1-16 inch, 1-20 inch, 1-25 inch.

t
It might be stated that such powers as 1-20 and

1-25 inch are very rarely constructed at the present
time and that the 1-16 inch may be considered the
maximum, although seldom used. The 1-12 inch is
the highest which is ordinarily used and will give all
the optical advantages, while the higher powers involve
so many mechanical difficulties as to increase the cost
of production very considerably and as a rule rather
detract from than add to the optical qualities.

While the above list of objectives comprises the
variety of powers generally offered by makers, exper-
ience in the different courses of study has shown that
certain objectives, and these generally in sets, are
most suitable. Thus with the short tube the 2-3 is
generally used for a low power, with the 1-6 and less
frequently the 1-8 for a medium, and the 1-12 oil
immersion for a high power. With the long tube the
3-4 and 1-5 are usually selected with the 1-12 for a
high power. When the work requires low and medium
powers only, the 11-2 and 1-2 are preferable. Of the
low powers the 2, i 1-2 or i inch are used. The 3 inch
and lower powers are rarely required, except in photo-
graphy, as the magnifying power is not much greater
than that obtained by a good pocket magnifier.

Special Objectives. While objectives are
divided into two classes, and although there are objec-
tives, distinguished by specific names, of more com-
plicated construction and greater capacity than .those
generally employed, there are other objectives, designed
for special purposes, which are not suitable for ordinary
investigations. An exception may be made in the
so-called variable objective, a low power objective in
which a variation in power is possible by varying the
distance of the lenses by mechanical means.

. Illuminating Objectives. These are absolutely
necessary in metallurgical investigations and for examin-
ing rulings on metallic surfaces. The powers used range
between 11-2 and 1-2 inch. A rectangular prism is fixed
back of the front lens with the diagonal surface over
and covering one-half of its opening. The light passes
into the prism from the side of the microscope and is
deflected by the diagonal surface of the prism through
the front lens, which, acting as a condenser, concen-
trates the light on the surface of the object. The
remaining one-half of the objective serves in the usual
way to form the image. Satisfactory as these objectives
are, they are almost useless on ordinary opaque objects
on account of the reflections due to the prism.

Photographic Microscope Objectives. The
regular objectives, especially low and medium powers,
are not suited for photographic purposes on account of

lack of coincidence of the visual and chemical rays
on the photographic plate ; while an image may be
sharply defined when viewed by the eye, that made on
the plate will be indistinct. For very low powers
standard photographic lenses of short focus may be
used, but the microscope objective requires its chro-
matic corrections changed while retaining the focus,
which very much detracts from its usefulness for
regular microscopical work.

While the regular medium powers give fair results,
they, as well as the low powers, should be adapted to
these changed conditions to give the best results and
particularly with a view to obtaining a greater amount
of illumination.

Projection Objectives. These objectives are
used to throw an enlarged image of an object on a
white screen or wall fifteen feet or more from the
apparatus.

Systems. An objective is said to consist of
systems which may vary in number from one to four
or five. Two systems are generally used in low
power objectives, three and sometimes four in the
medium powers, and four or five in the high powers.
They are the individual portions consisting of one, two
or three lenses. When a system is composed of more
than one lens, the lenses are cemented together by

means of a transparent (colorless) cement in such a
perfect manner that to the unpracticed eye they appear
as one piece of glass. An achromatic objective may
consist of a simple system having two or three lenses,
or it may have two, three or four and even five systems

d
h

Fig. 33.

with as many as eight or ten lenses in all. The
systems are called in their order : anterior or front,
middle, and posterior. When a system consists of two
lenses it is called a doublet, when of three lenses, a
triplet.

Thus in Fig. jj, a is the anterior, m the middle,
and p the posterior system : thus also, h is a single,
d a double, t a triple, and q a quadruple system.

While all the work connected with the microscope
must be extremely accurate, that involved in the produc-
tion of an objective is of the highest degree of accuracy;
in fact there is no production of the human hand which

is more delicate and one which must be so faultless.
It will be noticed from the diagram that objectives of
different power are different in construction and this is
also true of objectives of the same power but different
in efficiency. Objective makers follow different lines
to obtain results, and as a large variety of glass is now
offered from which to select, widely different construc-
tions follow, which in objectives of the same class from
reputable makers are very similar in efficiency.

The optical glass used must be free from defects
and must be absolutely permanent under all climatic
conditions. This glass comes in thick plates from the
maker and is sawed into slabs of suitable thickness
from which are cut pieces of the requisite size for
lenses. These are then cut into lens form and the
various processes of grinding, polishing, and edging
follow. The control of work in these various steps
must be absolute. There must be no pit holes or
scratches, no variation in thickness or diameter, and
the surfaces must be so perfect that all mechanical
means of measurement fail and recourse must be had
to highly sensitive optical tests to determine errors.
The difficulties increase with the increase in the power,
as all defects are magnified to the extent of the magni-
fying power of the microscope, and furthermore the
difficulties of handling and manipulating the lenses
become more serious as the lenses become smaller.
The lenses are then mated and cemented, set and fixed

in suitably recessed brass mountings, carefully main-
taining distances and exact coincidence with the optical
axis. No matter how perfect lenses may be up to, this
point, negligence in mounting will make them utterly
useless. Especially difficult is the setting of the minute
front lenses of the medium and high powers, the latter
being smaller than an ordinary pin head. The amount
of metal which fixes them in position is necessarily
very small, and the mounting thin, and although they
are firmly set they are extremely sensitive to any blow
or unusual pressure. The greatest care must also be
used in cementing lenses together, as the cement
between them is an immeasurably thin film, and if
there is any variation in radii of the two surfaces or in
their spheroidity, the cement will either fail to fill out
the gap or will be under a strain which will destroy their
usefullness. After all these processes the objective
must undergo a critical test under the microscope for
defects and if it fails in any one direction it must
undergo correction. On account of the many pro-
cesses and the several directions in which faults may
exist this is often very difficult. It sometimes happens
that one or another piece of glass possesses faults which
were not previously recognizable, or that there are
slight variations in the refractive power of one or
another kind of glass, either of which may be fatal to-
good, results ; at any rate, whatever faults may exist,
they must be entirely eliminated ; the final result must

be an objective fully up to the standard previously
established by the maker, and in all objectives of the
same class there must be no noticeable variation.

The foregoing is given to emphasize the fact that all
objectives, particularly the higher powers, are unusually
accurate and delicate productions, and that unusual
care should be observed in their use to retain their
efficiency. This too often is not the case. There \is
no reason why under ordinary conditions an objective
should not last a life time with daily use. Ordinary
care and cleanliness will accomplish this. Neverthe-
less, to be more specific, observation of the following
simple rules may be of aid :

Do not allow an object ire to drop.

Do not allow the front of the objective to come in
contact with the cover glass.

Do not separate its systems.

- Remove dust occasionally from the front and rear
surfaces with clean camel's hair brush.

After using an oil immersion objective invariably
remove the fluid with a soft piece of linen.

Never at 'low immersion contact with cover over
ten hours.

Use no alcohol.

The various features which must be considered as
determining the quality of an objective are :

Angular Aperture,

Achromatism,

Resolving Power,

Flatness of Field,

Penetration,

Working Distance,

Magnifying Power.

Although these attributes may be considered separ-
ately, some of them go hand in hand. The presence
or extent of one necessarily involves or precludes
another.

Angular Aperture. The angle which the most
extreme rays transmitted through the objective make
at the point of focus is called the angular aperture, or,

in short, the angle of the
objective, the extent of which
is expressed in degrees, and
of all the qualities in an ideal
objective, this is the most
important. Thus in Fig. 34,
d is considered the point of
focus, and c d e the angular
aperture. However, the above
definition has its limitations.
Fig 34 While in objectives of proper

\;
V

construction it holds true, there are many in which it is
not the case. For instance, an objective may be so
constructed that it may transmit a considerable number
of rays ir> excess of those which combine to form an
image and it is evident that as they do not aid in form-
ing an image, they serve no purpose and therefore
have no value in the consideration of angular aperture.
Objectives of the same power are made of varying
degrees of angular aperture, while others differing in
power may have the same angle.

Diagrammatic representation of light transmitted by objectives of low (a)
and high (b) angular aperture.

White light is radiated by objects equally in all
directions, but only those rays from one-half or up to
1 80 degrees of the object need be considered. It is
evident that the more rays which can be collected to
form an image, the more distinct will it become, thus
making it possible to see more detail. If of two objec-

tives one receives on its front surface and transmits a
larger number of rays than another of equal magnify-
ing power, we have a case where power would indicate
that we should see equally well, but we will find that
there is a difference, due to the amount of angular
aperture, in favor of the wider angle. Or, in the case
of two objectives in which one has one-half the power
of the other, but which transmits the same amount of
rays, it would appear that if the power alone were to
indicate the visibility of the object, the higher one
should show more detail, whereas in reality both show
equally well. The practical value of this fact is most
apparent near the limit of the resolving power of micro-
scope objectives.

Aperture. Without the defining word ' angular,'
aperture indicates a very important feature in an
objective and designates the diameter of the beam or
pencil of light which passes out through the rear lens
of an objective, or in other words, is the effective
diameter of the rear lens.

The word * aperture ' has been used, especially in
England, to designate the angular aperture. This has
created confusion and if an abbreviation be used the
word angle is far more suitable, as Dallinger says in
Carpenter's, " The Microscope and its Revelations,"
" We would, nevertheless, remark that visibility of detail
in, for example, the moon depends on the aperture of

the telescope ; of course what is known as its ' aperture '
is simply estimated by the diameter of the object glass.
The definition of * aperture ' in its legitimate sense of
' opening ' is shown by Abbe to be obtained when we
compare the diameter of the pencil emergent from the
objective with the focal length of that objective."

" Thus we see that, just as in the telescope, the
absolute diameter of the object glass defines the
aperture, so in the microscope, the ratio between the
utilized diameter of the back lens and the focal length
of the objective defines its aperture. This definition is
clearly a definition of aperture in its primary and only
legitimate meaning of an opening, that is, the capacity
of the objective for admitting rays from the object and
transmitting them to the image."

" Hence ' aperture ' means the greater or less
capacity of objectives for gathering in rays from
luminous objects."

Many objectives are made in which the rear lens
is larger in diameter than the beam of rays which,
coining from an object, can be transmitted through it.
This, while not particularly detrimental, has no value
and perhaps leads to a wrong conclusion in reference
to angular aperture, when this is measured, as the
excess of image forming rays, called stray rays, may
indicate a greater angle and resolving power than the
objective really possesses.

Cover Glass. At this point we must introduce
another feature which has not yet been considered but
which has an important bearing in the description of
angular aperture. As has been shown, in describing
the principles of refraction, rays which pass from air
into glass, are bent out of their course or refracted.
The cover glass is, as its name implies, a piece of glass
to cover the object ; but its purpose is really more to
protect and preserve the object, or with soft or limpid
objects to flatten them out, and to obtain by means of
the upper surface of the cover an optically plane sur-
face. Although extremely thin, it has a very pro-
nounced influence on the optical performance of an
objective and this influence increases as the magnifying
power of the objective increases.

For the purpose of illustration, we will imagine a
cover glass of considerable thickness, Fig. 35, in which o

represents the source
of light, or in this
case the object. As
o a and o a' enter at

d\b

i u

b/ &/ the lower surface of

a',

/ ,/ ) the cover glass they

XX / are refracted toward

tne

emerging at the up-

per surface of the

Fig. 35. cover glass are again

refracted away from the axis in the directions b c
and b' <? , which are the original directions of the rays.
The same action will take place with the extreme rays
o e anfl o e' , which will emerge as shown at d f and
d' f . If the front lens of an objective is brought
close to the cover glass we can study its influence and
for this purpose will use the simple but intelligible
illustrations of Prof. Gage, Fig. 36.

I shows a dry objective in which the intervening
medium between the top of the cover glass and the
front of the objective is air.

II is a water immersion objective in which this
space is filled with water.

III is an oil or homogeneous immersion objective
in which the space is filled with oil of cedar.

From what has been said in regard to the laws of
refraction we know that as the medium becomes more
dense the refraction becomes greater and for the same
reason it is clear that as the difference in density
between the two media becomes less the refraction is
proportionately less. As water has a greater density
than air, but less than glass, refraction between
these two media is less than between air and glass.
As the oil of cedar has the same density as glass, the
oil and glass are virtually a homogeneous mass and
no refraction between these two media takes place.

AIR.

\ \\\

II
WATER.

Ill

Ill
OIL.

Fig. 36.

L7 I

zzx

I/I

Referring now to the diagrams and bearing in mind
the course of rays from an object through a cover
glass, we can follow out their action. In the case of a
dry objective, we see that a portion of the rays coming
from the top of the cover glass are so refracted that
they pass outside of the front lens of the objective and
are therefore lost. In the water immersion, however,
the rays are less refracted. With the oil immersion the
only refraction to be considered is that which takes
place at the lower surface of the cover as from that
point the rays pass without change of direction
through the cover, fluid, and front lens, as far as the
convex surface, where they are refracted and carried
through the objective. From a view of the diagram it
might appear that if the lens were enlarged in diameter
more of the extreme rays which are otherwise lost
might be utilized ; but as it is apparent that the radius
of the front lens must then be increased, we know that
this will increase the focal length and consequently
reduce the magnifying power.

Numerical Aperture. The consideration of the
capacity of objectives to take up a greater or less
number of rays diverging from a given object leads
to the question as to whether any optical law may exist
in relation to the angular aperture of objectives, under
the different conditions of their use, either dry, or
with water, oil, or any other immersion fluid, and
Prof. Abbe has found that it is as follows :

" The sine of one-half of the angle of aperture multi-
plied by the refractive index of the medium between the
front of the objective and the cover, is equal to the effec-
tive semi-diameter of the emergent pencil at the point of
its emergence from the posterior lens, divided by the equi-
valent focal length of the objective" This expression
Prof. Abbe calls the Numerical Aperture and its great
value lies in the fact that it serves to designate the
efficiency of an objective as to the quantity of light or
number of rays, essential for the formation of a perfect
image, that can be utilized by it. It has been proven
that the greater the value of the Numerical Aperture,
the more perfect is the delineation of fine structure in
minute objects and the greater is the degree of fineness
of detail that can be seen. Also, if objectives have the
same Numerical Aperture, no matter what their focal
lengths and the intervening media may be, they will
give equally good definition of fine structure.

When designated on objective mountings or used
in tables, the term Numerical Aperture is abbreviated
to N. A. The formula for computing Numerical
Aperture is written, N.A.=/z sine ur where n is the
index of refraction of the medium between front lens
and cover glass, and it the half-angle of aperture.
Since the media are air, water, or oil, it is necessary to
know the refractive index of each, which is i.o for air.
1.33 for water, about 1.52 for oil, and for the regular
crown glass is, closely, the same as for the oil.

This formula can be better illustrated by an example.
Take an objective with an angle of aperture in air of
100 degrees, of which the numerical aperture is to be
determhied. The focal length of the objective is left
out of consideration as the formula depends entirely on
angle and index and it is evident that all objectives
having this same angle, in this case in air, will have
the same N. A.

Considering the angle it, it wijl be readily seen that
this is one-half of 100 degrees, or 50 degrees. Now
referring to a table of natural sines or to a table of
logarithms it will be found that sine //, or the sine of
50 degrees is equal to 0.766, and as the intervening
medium is air, the value of index n is equal to i.o.

Substituting the above values in the formula, we
have N.A.=(/z or i.o) x (sine u or o.766)=o.766.

To make this computation we have always the value
of the index of refraction at hand, but the angular
aperture must be determined, and the method for
accomplishing this will be given in a succeeding
chapter.

As a rule the designation as to power and numerical
aperture engraved on the mounting of an objective
from a responsible firm can be relied upon as being
quite close, the variations seldom being greater than is
incident to accurate human handiwork, and such
variations as do occur have little influence on the

optical capacity. It is manifestly difficult for opticians
who produce large quantities of objectives to measure
and suitably mark each individual product, particularly
when the differences are at best slight.

It is easy to learn the numerical aperture of an
objective after the angular aperture has been deter-
mined, as the various values for different angles have
been computed and are issued in tables. Such tables
can be found in the pages of the Journal of the Royal
Microscopical Society and in the latest edition of
Carpenter's " The Microscope and its Revelations,"

Before the numerical aperture was accepted as the
absolute measure of the optical efficiency of an objec-
tive, it was known that an increase of angular aperture
gave better results, but just why this was so, was not
appreciated. So also was it difficult to understand that
a dry objective of 180 degrees, a water immersion of
96 degrees, and an oil immersion of 82 degrees angular
aperture had the same optical efficiency.

How to Measure Angular Aperture. With
instruments in which the axis of the mirror-bar is in the
plane of the stage and in which the circular part is
graduated, the matter of measuring angular aperture is
quite simple. It was recommended by Mr. Tolles, and
has been carefully worked out by Dr. George E.
Blackham. After the object has been focused upon,
incline the body of the microscope to a horizontal

position, remove the mirror with its bow from the
socket and place therein, or by rubber band attach
to the end of the bar, a toy candle at such height
that the flame will be in the optical axis. The
mirror-bar is now swung to the right and left to
such an extent that one-half the field shall be
illuminated, provided the object shall still be well
defined, or to the point where the definition of the
object shall appear impaired without regard to the
illumination of the field. These limits will mark the
efficient beam of light which passes through the rear
system of the objective. In some objectives the rear
system is larger in diameter than the effective beam of
light transmitted from the object, which will permit
stray rays to reach the eyepiece, but which have no
value in forming the image and therefore are not to be
considered. In instruments which have no graduated
mirror-bar the matter becomes more difficult and less
accurate.

Another method is that of Lister. After the object
has been focused upon, place the body of the micro-
scope in a horizontal position and in front and some
distance from it, a candle or lamp ; if the latter, with
the narrow edge of the wick toward the instrument, but
level with the tube. Move the lamp on each side of
the axis to a point as described in the foregoing
method. Indicate the position of the center of the
lamp at each extreme on the table and beneath the

instrument also a point situated vertically under the
object. Connect these points and with a protractor
measure the angle.

A very accurate method and one which can be
carried out on all instruments is that suggested by
Prof. Abbe and for which the firm of Carl Zeiss supply
an apparatus which is called the apertometer, which
consists of a semi-circular disk of glass having at its
straight edge a beveled surface which reflects the light
through a perforated disk into the objective. Two
strips of brass which act as stops are placed on the arc
and thus indicate the limit of aperture, which, as well
as the corresponding numerical aperture, can be read
off on the scale.

In the foregoing we have gone to some length in
stating the importance of angular and numerical aper-
ture and laid stress upon the influence which these
factors have upon the efficiency of the objective. We
shall now endeavor to explain what these attributes are
which determine the value of an objective.

Resolving Power. Most important of all qualities
in an objective is the resolving power, which is the
power to show intricate structure and minute detail, it
being of course understood that the objective is properly
constructed so that defects shall not detract from this
quality. It is of course clear, that no matter how great
the numerical aperture may be, its effectiveness may

be injured by the presence of chromatic or spherical
aberration or defective mechanical work, and it is a
matter of no uncommon occurrence that in objectives
of the same power and aperture there is a noticeable
difference in resolving power; or in objectives of
different numerical aperture, the one of less aperture
will have a greater resolving power than the other.
If we could accept the statements of makers as true
ones, a portion of this work and a vast amount of
literature would be unnecessary, but the writer has
occasion to know that this is not the case and that
there is a constantly increasing danger and tendency
to allow small defects to pass, in spite of the fact that
the general efficiency of the microscope has increased
in late years.

To resolve a structure is to make it visible and the
resolving power is in direct ratio to the numerical
aperture and can be mathematically calculated. It can
be studied from the aperture tables already mentioned.
It will be seen that an objective with a numerical
aperture of 0.50 will make visible only one-half as
many lines in the same space as one with a numerical
aperture of i.o. This, it will be seen, refers only to
the resolving power of an objective and makes abso-
lutely no reference to its magnifying power. Now, as
we know that the purpose of the compound microscope
is to give magnifying power and that there is a certain
structure which is not visible to the eye, how can we

reconcile this with the fact that numerical aperture only
does it ? Why, if this is true, should we use a higher
power objective with a prescribed aperture in pre-
ference to a lower one of the same aperture ?

The normal eye can see about 200 lines to the inch,
or structures which are 1-200 of an inch apart and it is
therefore evident that any structures under the micro-
scope must be separated at least to this extent in the
virtual image, in order that they may become visible.
While we have shown that magnifying power may be
increased

by increasing the power of the objective,
by increasing the power of the eyepiece,
by increasing the tube length,

we are limited in the last direction mainly by such
length of tube as has been found convenient in the
construction of the stand ; in the case of eyepiece on
the one hand by convenience in use and on the other
by the fact that too great an increase causes an indis-
tinctness, so that, although variation in eyepiece within
narrow limits is admissible, we are compelled to select
an objective which, with a medium power eyepiece,
will give sufficient separation to structure and fine
detail, that it will be visible to the eye without any
undue strain.

Chromatic Aberration. Up to this point we
have spoken of the correction of chromatic aberration by
suitable use of flint glass lenses and for the purpose of
not making the subject too complex, have purposely
refrained from stating that entire freedom from color
is impossible in the ordinary combinations of flint and
crown glass. The correction is for only two colors of
the spectrum, red and violet, leaving as a residue
the other colors which appear as apple green and
purple. These form the so-called secondary spectrum
and on bright objects this can easily be discerned.
For all ordinary purposes the presence of these colors
is not prejudicial to the performance of an objective.
The secondary spectrum becomes more pronounced in
dry objectives of large aperture and when high power
eyepieces are used. Even in properly corrected low and
medium powers of medium aperture and high power
immersion objectives it is noticeable, but certainly
not to any extent to be objectionable, except when
oblique light is used, provided of course that the other
corrections are properly made.

Great care should be used in judging an objective
by its chromatic correction and one should not be led
to false conclusions by the amount of color which an
objective shows. It has been a common experience
with the writer to have objectives complained of which
were properly corrected and which were excellent in
every respect except that they showed the secondary

colors, so that it may safely be said, remembering that
wide aperture involves a greater amount of color, that
an objective showing the proper colors of green and
purple and having proper resolving power may safely
be accepted, or in the choice of objectives between one
showing no color but having no resolving power, the
other having color and resolving power, the latter is
certainly the preferable one.

When colors of green and purple are not sufficiently
pronounced on an object when the mirror is in central
position, they will become more apparent when the
mirror is swung to an oblique position, using any object
suitable to the power of the objective, and preferably
mounted dry.

If the mirror is swung to the left, the object should
be fringed on the right side with yellow-green and on
the left with bluish color, or if the mirror is swung
to the right the conditions are reversed.

When the objective is not properly corrected it is
said to be chromatically under-corrected or over-corrected.

It is well to state here, however, that, with the care
which is exercised by reputable makers, objectives
which deviate from the proper point of correction, or in
which the under- or over-correction is so prominent
as to be noticeable to one inexperienced in this work,
are never allowed to reach the public. These remarks
are not intended to discourage the student from testing

objectives for color correction ; on the contrary we
particularly recommend such tests as will tend to
more critical judgment of the optical properties of
the instrument and work in general. To aid those
who have not had sufficiently long experience to reach
correct conclusions, methods for judging color correc-
tion are given which should be applied to doubtful pro-
ducts ; but the experiments should be repeated quite
often before reaching a conclusion.

To judge under-correction by central light, focus
upon a coarse object and it will be found to have a
distinctly bluish tinge. If not pronounced, by slightly
increasing the distance between objective and object,
the latter will show a blue color and by decreasing
the distance an orange color.

To judge over-correction, after focusing on the
object, this will show an orange color, or by increas-
ing the distance between objective and object the orange
will appear, and by decreasing the distance, the blue.

To judge color correction by oblique light: Under-
correction will show when the mirror is swung to the
left and the object is fringed with orange on the left
and bluish on the right.

Over-correction is apparent when the object is
fringed with blue on the left and orange on the right.

Another method is that recommended by Naegeli and
Schwendener in their work an the microscope. The

right half at the front or back of the objective is
covered with black paper or tinfoil so that only the
other or left half remains optically effective ; take for an
object a line or dot of light which can be easily pro-
duced by blackening one surface of a slide by smoking
in an oil flame and drawing a line across it with a point.

Under-correction shows when the image has on
the right side a violet or blue border and on the left
a red or orange colored border.

Over-correction shows on the other hand when the
left side appears violet or blue and the right red or
orange.

Spherical Aberration and Cover Glass. As

has been stated, there is a residue of chromatic and
spherical aberration in all ordinary achromatic lenses
and objectives. The use of cover glass influences the
spherical correction and while not appreciable to any
extent in low powers, it is very sensible in the medium
and high powers. Referring to Fig. 37 it will be
remembered that the rays from the object do not
reach the surface of the objective front uninterruptedly
but are changed in their course. If we make use of
the same illustration and extend the refracted rays
downward as shown by the dotted lines d e, f c, f c,
d' e until they meet at the axis, these points will be the
apparent location of the object and will appear to meet
in the planes e a and c b instead of at o. To neutralize

this condition the objective will require to be spher-
ically under-corrected, by which is understood that the
marginal rays will eminate from a point near the objec-
tive and the central rays at a greater distance from the

r/d/

1 fi

/ ,

A \

Fig. 37.

objective, or as is shown in the diagram, appear to
come from exactly the same points, which are the
apparent positions of the object or g e,h c,h' c, and gf e.
If a thicker cover is used the objective will require
to be more under-corrected, or if a thinner one, less
under-correction is needed, so that if an objective is
corrected for a definite thickness of cover, its correction
will be disturbed if greater or less thickness be used,

and since resolving power depends primarily upon the
proper correction of the two aberrations, it will be
entirely lost if there is much variation from the normal
thickness.

It must not be forgotten that in microscopical work
we are dealing with minute things and this applies
especially to the cover glass. By studying the table
which follows it will be noted that there is quite a
difference in thickness used by the various makers, and
that the mean thickness is 0.18 mm. or 0.007 inch.

The deviation from standard thickness affects the
distinctness of the image according to its structure and
in proportion to the increase in power. In the low
powers there is no noticeable influence, but with the
1-5, 1-6, and 1-8 it is so marked that with objects of
fine structure. a deviation of 0.05 mm. either thicker
or thinner than the standard is sufficient to totally
obliterate fine structure and have the outlines and
coarse lines only apparent. Slighter variations affect
the image proportionately.

It is surprising to see how little attention is paid to
this subject in the large majority of the standard works
on the microscope. Almost all books give carefully
prepared illustrations and descriptions showing the
effect on the course of light of the interposition of the
cover glass, and after giving conclusive evidence of its
disturbing influence, still, in a general way, say it is of
little moment.

9°

It is a misfortune that cover glasses are not of uni-
form thickness, but the difficulties of producing them
of equal thickness are almost unsurmountable, and it is
also to» be deplored that opticians have not agreed
upon a standard thickness for which they correct their
objectives. The list herewith given, which has been
prepared by Prof. Gage, will show the variations in
thickness used by different opticians as standard.

mm.

T2o°o mm.

TITO mm.

iVo mm.

nro-iV'o mm.

T¥o-T2o°o mm.

ToViTrV mm.

rV'o mm.

To2o-i¥o mm.

ToVrVo mm.

i1.-)0,,- mm.

J. Green,

J. Grunow,

Powell & Lealand,

Spencer Lens Co.,

W. Wales,

Watson & Sons,

Klonne & Muller,

Bausch & Lomb Optical Co.,

E. Leitz,

R. Winkel,

Ross & Co.,

C. Zeiss,

C. Reichert,

Gundlach Optical Co.,

VV. & H. Seibert,

R. & J. Beck,

J. Zentmayer,

Nachet et Fils,

Bezu, Hausser et Cie.,

Swift & Son.

The cover glass may truly be called a necessary
evil ; for, while absolutely required in microscopical
investigations, there is no adjunct to the microscope
that has been and is productive of so much evil, and
has done so much to retard the utilization of benefits
made possible by the advance in the construction of
objectives.

It must be remembered that the majority of objec-
tives will always be dry, and it is an unfortunate
circumstance that with this class of objectives the
influence of variation in thickness of cover glasses is
most apparent; but- since it is so, all possible aid
should be given to enable the student to obtain results
which closely equal those obtained with the conditions
under which the objectives were originally corrected.

With oil immersion objectives a variation in thick-
ness is not very appreciable, provided, however, that
the fluid is of the proper consistency, as there is pract-
ically no refraction between the cover glass, immersion
fluid and front lens of objective. If the fluid, however,
does not have the same refraction as the cover the
sperical aberration may be as pronounced as with dry
objectives, as it becomes more noticeable on account of
higher power. It is therefore important to purchase
the fluid from the maker of the objective or under a
guarantee of the index of refraction.

Objectives with fixed mountings, such as are ordi-
narily used, in which the lenses have fixed relations

correcting the spherical aberration for a definite thick-
ness of cover glass, will not permit of any change to
correct for other thicknesses. With these, correction
can be* made within narrow limits by varying the tube
length.

For a thick cover the tube must be contracted ;
for a thin cover the tube must be extended.

Objectives are, however, also constructed with a
mechanical arrangement by which the distances between
the lenses may be varied and are then called adjustable.

For thick covers the lenses are brought together,
or the adjustment is closed.

For thin covers the lenses are separated, or the
adjustment is opened.

In order therefore to obtain the highest efficiency
of an objective, a cover glass should be used which has
the same thickness as the standard used by the maker
of the objective. Covers of standard thickness may be
purchased, but the cost will be considerably higher
than for the thicknesses regularly listed.

The best plan is to purchase a cover glass guage
by means of which the thickness may be accurately
measured. Under this plan we recommend the follow-
ing procedure :

Select standard thickness and use with high and
medium power dry objectives. Thicker covers use

with low power objectives ; thinner covers use with
oil immersion objectives, as increased working distance
is obtained.

Penetration. Penetrating power is the quality
which enables us to look into an object — to observe
different planes at one time. It depends upon magni-
fying power and angular aperture, and decreases with
the increase of either of these properties. Objectives
are generally not constructed with any reference to it ;
it is, rather, a natural consequence of certain qualities
of the objectives.

Penetration and resolving power are antagonistic, or
at any rate in an inverse ratio, and can only be com-
bined in a definite proportion. In two objectives of
the same power and aperture, one cannot have penetra-
tion as a special feature and the other resolving power ;
they will be similar in these qualities provided they are
similarly corrected. However, if they are not similar
in their angular aperture, the one of smaller aperture
will have more penetration than the other. In objec-
tives of the same angle but different power, the one
of low power will have in itself more penetration.

Low power objectives have a proportionately greater
penetrating power than medium or high powers. In
an object of considerable thickness, different planes
can be observed at one time without focusing on them
and we thus obtain an appreciation of form which is

impossible in the higher powers, as in these, focal
adjustment for different depths is required.

Furthermore the accommodation of the eye is also
a factor as it varies with different persons and thus, to
a certain extent, is a matter of individuality.

Flatness of Field. The field of view or in
brief the field in a microscope is the area which is
observed by looking into the eyepiece. While its area
is constant in an eyepiece, the size of the area being
determined by the diameter of the diaphragm, it is
variable with the change of objectives, the field or
visible portion of the object becoming smaller with the
increase in power. Angular aperture has no influence
on the size of field. The field is said to be flat when
all the parts of the image within its area are sharp at
the same time.

When not fiat, it will be found that as the edge
of the field is approached the image becomes more and
more indistinct, and that the objective must be corre-
spondingly adjusted ; in many cases it remains indis-
tinct or blurred, and this may be considered a most
serious fault. Lack of flatness may be due to the
un evenness of the object, and in order to test the
objective for flatness, an object of assured flatness
should be used, such as a stage micrometer which con-
sists of a series of very fine parallel lines cut into
glass and very accurately spaced according to definite

fractions of millimeters or inches. After focusing upon
the lines, they will appear to become more curved and
less distinct as they near the edge, as shown in
Fig. 38.

Flatness of field mainly depends upon the correction
of the spherical aberration and, as under the best
conditions this cannot be entirely
eliminated, it is impossible to
obtain absolute flatness. It may
also, however, be due to a faulty
eyepiece ; in this case it can
be determined by observing
whether it shows equally in
different objectives. With be-
ginners, especially, it is usually
most complained of, owing probably to the fact that
it is most easily discernable. While it is a desirable
quality and indicates to a considerable degree the
quality of objectives, it is impossible to obtain absolute
flatness of field in objectives of sufficient angular
aperture to meet the requirements of the present day
and especially in higher powers.

Working Distance represents the free space
between the front lens in the objective and the upper
surface of the cover glass which protects the object,
when the objective is in focus and is corrected for that
cover glass.

38.

In objectives of low power, it is of little considera-
tion, but with those of medium and high power, where
it must be expressed in o.oi or o.ooi inch, it becomes
a matter, of importance.

Working distance is spoken of as being long or
short and varies with the power and angular aperture.
Generally the working distance decreases with the
increase in numerical aperture and becomes greater as
the aperture becomes smaller. It was for a long time
considered that these two qualities varied according to
a fixed rule, but this at the present time is not con-
sidered to be the case. While in objectives of the
same aperture it may vary considerably, in others of
different aperture the higher one may have the greater
wrorking distance. The skill of the optician must, to
a considerable degree, determine the amount of it.

It will be seen from the above that working distance
stands in no direct relation to the focal distance of the
objective and it may be added that it is never as
great as the focal distance of a simple lens of the same
magnifying power.

As may be imagined, there is a variety of opinions
as to what constitutes long or short working distance in
a certain objective. No definite rule can be laid down
for this, as it is conditioned on the skill and require-
ments of the manipulator. It has several times occurred
in the experience of the writer that objectives were

complained of as having no working distance (that the
objective could not be focused) when on investigation
it was found that very thick glass had been used as
cover glass.

There is no question about the desirability of a long
working distance in the higher power objectives, as the
comfort of working is greatly increased and the danger
of injury to the objective and also the object which
may sometimes be a rare specimen is further removed ;
but, unfortunately, the difficulty cannot be eliminated
and must be reckoned with. It is therefore important
for the student to know the amount of wrorking distance,
so as to make allowance for it in focusing the objective,
and to know the limit of thickness to use with high
power objectives.

The actual measurement of wwking distance in
microscopes having a graduated micrometer screw is
simple and may be determined as follows :

Lower the eyes to the level of the stage and bring
the front of the objective very carefully just in contact
with the top of the cover glass. Note the division on
micrometer screw and by an upward focusing turn of
the screw bring the object in focus a/id read off the
distance traversed.

With an oil immersion objective follow the same
process by bringing the objective in contact with the

carer glass while dry, then separate slightly and, inclin-
ing the instrument, allow some of the oil to drop in the
space, then focus and read off. Divide the number of
graduations on the screw head by the pitch of the screw
and multiply by the number of divisio?is read off.

Magnifying Power. This is a question of vital
importance in a microscope, not so much as a quality
in itself, as in connection with the resolving power.
The inquiry should not be simply, how many diameters
an instrument will magnify, but what the precision and
extent of its definition is under a certain magnifying

2-3 inch Objective. 1-6 inch Objective. 1-12 inch Oil Immersion

Objective.

power. If a" high magnifying power is all that is
desired, this may be obtained to an almost unlimited
extent by means of simple lenses which may be pro-

cured at a small pecuniary outlay ; but these do not
give a distinct image nor do they make structure visible,
which, be it remembered, it is the purpose of the micro-
scope to do.

The normal eye can distinguish about 200 lines to
the inch and in a microscope such magnifying power
should be used as will apparently separate the structure
which it is sought to see at least to this extent. To
illustrate, take a 1-6 inch objective of 0.85 N. A. and a
2 inch eyepiece. An objective of this kind, properly
corrected, resolves the test-object Pleurosigma angula-
tum, in which the lines average 60,000 to the inch.
With the above eyepiece it is utterly impossible to see
them, while if it is replaced by a 3-4 inch or 1-2 inch
eyepiece, they can easily be distinguished. This is
not owing to any peculiar quality of the eyepiece, but
merely to the fact that by increasing the magnifying
power the dimensions of the object have been increased
and the spaces between the lines have been separated
sufficiently to become visible to the eye.

Beginners as a rule are apt to use too much magni-
fication, or amplification, and often attempt to view a
large surface with an objective which will show but a
small part of it. It must not be forgotten that the
apparent field of view is decreased as higher powers
are used and that a low power will give a better
impression of a large, coarse object and its relative
parts, because it makes a larger surface visible.

100

Objectives of the same power, but having different
angular apertures, will always have the same magni-
fying power and field.

The fellowing table will probably be of assistance
to the beginner. After he has become better acquainted
with his instrument his judgment will dictate to him
what to do.

A power of 25 diameters will show a surface of
about 1-5 inch diameter.

A power of 50 diameters will show a surface of
about i-io inch diameter.

A power of 100 diameters will show a surface of
about 1-20 inch diameter.

A power of joo diameters will show a surface oj
about i-ioo inch diameter.

A power of 1000 diameters will show a surface of
about i -200 inch diameter.

This table is approximately correct with a Huy-
ghenian eyepiece.

As has already been shown, magnifying power may
be obtained by using a higher power objective, or eye-
piece, or lengthening the tube, and it has also been
pointed out that the objective should be relied upon to
obtain this increase.

Objectives with the same angular aperture, but of
different power, will have the same resolving power.

In both objectives and eyepieces the lenses decrease
in size with the increase in power and consequently
gather less light and while this one objection exists in
the objective, an additional one occurs in the eyepiece,
in that the eye must be brought closer to the eye-lens
and must be kept more strictly in the optical axis,
which at a long sitting, becomes fatiguing.

Choice of eyepiece should be determined by require-
ments and individual preference, but the use of high
power eyepieces should be avoided whenever possible.
All responsible makers of microscopes make up such
outfits of stands, objectives, and eyepieces as experience
has taught are most efficient and convenient.

In all work on recognized forms (objects of which
the structure is known) it is safe to follow the rule, not
to use a higher power than is necessary to properly
study them.

Apochromatic Objectives. All achromatic
objectives have a residual chromatic and spherical
aberration, the former being the secondary spectrum :
but Prof. Abbe has in this direction also effected a not-
able improvement, which with a uniform correction of
the spherical aberration corrects for three colors, thus
resulting in a closer concentration of image-forming
rays which with the greater numerical aperture made
possible by these conditions results in a higher resolv-
ing power. It has been found, however, that, in high

powers of wide aperture, even with these objectives
there is an outstanding error which by itself cannot be
corrected. This is balanced in the eyepiece, which is
correspondingly over-corrected, and is called the com-
pensating eyepiece. It has therefore been necessary to
make the low powers under-corrected as well, so that
they may be used with the same eyepieces, for it is
evident that neither these objectives with the Huy-
ghenian eyepiece, nor the compensating eyepiece with
the achromatic objectives will give satisfactory results.

The proper combination of apochromatic objective
and compensating' eyepiece gives a beautiful image
with the maximum of resolving power, unapproached
by any of the achromatic combinations. They further-
more have the advantage that they are exactly suited
to photomicrography, there being exact coincidence
between the photographic and visual image, which in
the achromatic objectives is not the case, as these must
be either corrected for photography, when they are not
satisfactory for ordinary purposes, or an allowance
must be made for the difference between the visual and
chemical images in the camera, which is very difficult.

One objection to the high powers has been the
liability to deterioration of some of the materials used,
which has happily been overcome by Prof. Chas. S.
Hastings in the apochromatic objectives computed
by him.

103

Eyepiece or Ocular. The purpose of the eye-
piece is to refract the diverging rays coming from the
objective so that they will reach the pupil of the eye
and at the same time magnify the image formed by the
objective. While it is the most simple part of the
optical combination of the microscope it is, withal, an
important part of it. In fact it may be said that, owing
probably to its simplicity, it has been neglected, and
very often eyepieces are furnished which are not at all
commensurate with the quality of the objectives.

Designation. While eyepieces are still marked
by some makers arbitrarily without any relation to their
optical value, a rational system is to mark them accord-
ing to the equivalent focus of the two lenses.

Thus an eyepiece marked i inch has an initial
magnification of ten diameters, a 2 inch, five diameters,
and so on.

Field of View. As has been stated this is the
visible area which is limited by the size of the dia-
phragm.

While the relative size of field in eyepieces of
different powers is the same, the actual field of view
as it relates to the image becomes less as the power
of the eyepiece increases. Thus if a i 1-2 inch will
show i- 10 inch of the object within the limits of the
field, a 3-4 inch will show only 1-20 inch. By some

104

persons it has been supposed that an enlargement of
the microscope tube would permit a large field, but this
is not so in ordinary eyepieces as the optical conditions

Fig. 39.

prescribe well defined limits and larger field can only
be obtained with specially constructed eyepieces.

I05

Par Focal Eyepieces. Important as it is to
have objectives par focal, it is fully as much so with
eyepieces as they are often changed with the same

Fig. 40.

objective to obtain a change in magnifying power. It
is a great convenience to use one or another without

106

disturbing the focus more than the fine adjustment
will easily rectify.

Eyepieces are divided into two classes :

Negative eyepieces, Fig. 39, in which the focal point
is within the eyepiece itself and between eye lens e I
and field lens //or at the diaphragm d d.

Positive eyepieces, Fig. 40, in which the focal point
is outside and below the field lens f 1. These eye-
pieces can be used as magnifiers.

Huyghenian Eyepiece. This form is named
after Huyghens, who is said to have first used it. It is
the construction which is in most general use, although
made up in a variety of mountings. It is negative and
consists, as has already been stated, Fig. 39, of an
eye lens, e I, nearest the eye, and a field or collective
lens, f I, which is the large lens nearest the objective.
Between them and placed at the focus of the eye lens is
a perforated, blackened disc, called a diaphragm, d d,
which limits the size of field and shows it within a
sharply defined border. The Huyghenian eyepiece is
made in two forms :

The English type as shown in Fig. 41, which has
a large tube fitting into the microscope tube and a
neck or smaller tube which is usually arranged with
a cap to slide over the eye lens.

The Continental type, Fig. 42, has a straight tube
which drops entirely into the tube of the microscope
and rests upon the mounting of the eye lens.

107

Solid Eyepiece is also a negative eyepiece and is
the invention of the late Robert B. Tolles. It is called
solid, from the fact that instead of being composed of two
lenses, it consists of one piece of glass, Fig. 43, which
is cut to a cylindrical form and on the ends of which
the proper curvatures are ground and polished. The
diaphragm is made by cutting a circular groove into
the glass at the proper distance between the two sur-
faces, which is then filled up with an opaque pigment.

Fig. 41.

Fig. 42.

These eyepieces are made only in high powers, as
optical glass is usually not of sufficient homogeneity
to make low powers, and their cost would be too con-
siderable without a corresponding advantage. They
are usually made only in powers of 1-2 inch and
stronger and for this reason have but a limited use.

1 08

Ramsden Eyepiece is a positive eyepiece, Fig. 40,
and is constructed of two plane convex lenses with the
convex surfaces toward each other. It is especially
useful as a micrometer eyepiece in microscopes and
telescopes, as it is used as a magnifier on the fine
divisions of the micrometer and at the same time gives
the virtual image.

Fig. 44.

Periscopic, Orthoscopic arid Kellner Eye-
pieces are also positive eyepieces which are achro-
matized by having the eye lens a doublet or triplet.
The spherical correction is such as to produce a large
and flat field, Fig. 44.

Projection Eyepiece. This is constructed to
throw an enlarged image on a screen at a distance
from the instrument and for photographing the image.

109

For neither of these purposes can the ordinary eyepiece
be used. It is provided with a mechanical means of
varying the distance between the eye and field lens for
the purpose of changing magnifying power.

Micrometer Eyepiece. A micrometer is a strip
of glass upon which a series of regularly spaced lines
are ruled having definite values in fractions of an inch
or millimeter and is used to measure the size of an
object. When used with the
eyepiece it must be situated in
the focus of the eye lens and
rests either upon the diaphragm
or in slots provided on opposite
sides of the mounting.

Another form is one in which
the micrometer is a permanent
part of the eyepiece and is pro-
vided with a screw adjustment to move it to different
points in the field of view, Fig. 45.

Binocular Eyepiece is one constructed to bisect
the beam of light which comes from the objective,
thus rendering the image visible to both eyes and pro-
ducing a stereoscopic effect. It is applicable to the
monocular microscope.

Erecting Eyepiece is composed of four systems
of lenses instead of two, some of which are sometimes

Fig. 45.

110

made achromatic. As the regular eyepiece of two
systems reverses the image, the erecting eyepiece again
reverses it, thus bringing the image back to the original
position* of the object. It is seldom used with the
microscope, but is necessary with small hand telescopes
and in the telescopes of surveying instruments.

Index Eyepiece is one which is provided with
an index or pointer to indicate a particular object in the
field or a certain structure of an object and is especially
valuable in class demonstration. The index can be
swung in and out of the field at will. Any of the
Huyghenian eyepieces may be converted into an index
eyepiece by cementing with shellac or Canada balsam an
eye-lash or sharp black paper wedge to the diaphragm
with the point toward the center of the field.

Compensating Eyepiece. In this place it is
also proper to speak of the new compensating eyepiece
as made by Prof. Abbe. This eyepiece compensates
for the residual errors in the apochromatic objectives
and is of no value except when used with them as
it is highly over-corrected, as can be seen at the edge
of the field, which has a strong orange color, whereas
in the Huyghenian it has a blue color.

In this series of eyepieces there is a searcher which
is of low power and intended to find objects. The
higher powers are for general work, some of them

being negative, the others positive. ' Another kind is
the projection eyepiece which is intended for projecting an
image on a photographic plate, or on a screen or wall.

Defects. As has been stated many eyepieces are
carelessly constructed and possess defects which inter-
fere with obtaining a distinct image. These defects
do not show easily with low power objectives, but can
readily be seen with high powers. The most frequently
occurring fault is the lack of perfect grinding and
polishing of the lens surfaces. If the former, it will
show itself as spots in the field, and if the latter, as a
series of streaks and shadows, usually circular in form
as if the lens had been wiped with greasy fingers.

Another defect may be in the glass itself, in the
so called striae, which will be indicated by dark and
light streaks across the field.

Care must be taken not to confound small particles
of dust which are apt to fall upon the field lens and
which at times are very prominent in the field, with
imperfections of the surface. These can be dis-
tinguished from other defects only by wiping, or using
a camel's hair brush, and even with the utmost care
some particles are liable to remain.

The eyepiece often fits too closely in the tube and
when making observations with high powers, a change
of eyepiece is apt to disturb the object. It should

enter without any friction and still tit so closely that it
drops slowly into its place from the compression of air
in the tube when the objective is attached.

In the course of time a film is apt to form on the
surfaces <sf lenses, which will cause indistinctness in the
image. It is advisable to periodically unscrew both eye
and field lens from the mounting and carefully wipe
the inner and outer surfaces. In conclusion it may be
said in a general way that the layman has rarely the
fundamental knowledge and sufficiently long experience
to judge critically of imperfections which may exist.

The idea is quite prevalent that there is a wide
variation in objectives of the same type from the same
maker. While this is quite true as between objectives
of different makers and also of makers whose pro-
cesses of production have remained the same as those
of former years, it is not so with the products of those
progressive firms who by common consent stand at the
head in their respective countries. Precise and delicate
as the various stages of production are, rigid control
and inspection removes all personal factors and elimin-
nates all chance in the final result. We do not mean to
argue that the work of the best should be unquestioned ;
on the contrary, it is advisable to examine all work
critically, and preferably by comparison, and we are
sure progressive makers will welcome all efforts in this
direction and will lend their aid to a more intelligent
use of their products.

HOW TO WORK.

How to Set Up the Instrument. Draw the
instrument from the case by grasping the base and
pillar, free it from dust with a large camel's hair brush
i inch or i 1-2 inch wide or by wiping carefully with
a chamois skin or old linen handkerchief. Place the
instrument on the work table, which should be of such
height that observations can be made with the utmost
possible comfort without straining the neck or com-
pressing the chest. Bear in mind always to sit as
upright as possible.

If the instrument is used in the upright position,
place the base close to the edge of the table; if
inclined, it may set farther in. Rest the arms, as
much as the height of the instrument will permit, upon
the table.

Bring the tube to the standard length for which the
objectives are corrected. To do this grasp the milled
edge of the draw-tube and give it a spiral motion while
holding the main tube with the other hand, page 50.
There is an objection, however, that in any but cloth-
lined sheaths the polished tube will soon be scratched,

114

especially if not kept very clean. In stands without
the graduated tube, a mark or ring is or should be
provided on it, which should be made to coincide with

Correct position at the microscope.

the upper end of main tube. Where the graduated
draw-tube is provided bring the proper figure, either

216 or 1 60, in line with the upper end of the main
tube, in accordance with the tube length for which the
objectives are corrected.

Attach low power eyepiece.
Attach low power objective.
Place object on stage.
Illuminate object.
Focus on object.

To Attach Eyepiece. The exterior surfaces of
the eye lens and field lens being exposed are apt to
become dusty and should always be carefully cleaned
before using. If there are two or more eyepieces,
always use the lowest power first. Eyepieces should
be so loosely fitted that they will drop into the tube as
far as the collar by their own weight. They do this
slowly when the objective is attached, as an airtight
compartment is formed and air to the extent of the
dimensions of the eyepiece must first be expelled from
the tube. This displacement of air may, however, be
hastened by gently pushing the eyepiece downward,
but not to such an extent as to push in the draw-tube,
or force down the coarse adjustment. In fact, care
must be used in applying the eyepiece, or sliding the
draw-tube, as the focus may be disturbed, or the
objective forced against the object and thus destroy it.

116

To Attach Objective. Using a low power
objective, remove from its box and see that its front
lens is clean ; elevate the tube of the stand by means
of the coarse adjustment so that the nose-piece shall be
at least fwo inches from the stage.

Proper manner of holding objective when attaching it to tube.

Grasp the upper knurled edge of objective between
thumb and forefinger of the left hand ; bring the screw
in contact with the screw of the nose-piece and, keeping
the objective in line with the tube and gently pressing
upward, revolve the objective with the thumb and fore-
finger of the right hand by the lower milled edge until
shoulder sets against shoulder.

117

To properly attach an objective is not always simple
and can not be done too carefully. One danger lies in
the fact that the objective may be dropped onto the
object and thus injure or destroy one or the other or
both, and another that the threads may be started wrong
by holding the objective sideways and the threads
injured.

In this connection we draw particular attention
again to the convenience of the double, triple and
quadruple nose-pieces. The convenience which is
obtained from their use, freedom from danger to objec-
tives and saving of time, commend them in all cases
where two or more objectives are used.

Finding an Object. The slide upon which the
object is mounted is placed upon the front of the stage
and slipped under the two spring clips to a point
where the object comes as nearly as possible in the
center of the opening of the stage. The slide should
pass easily under the clips, which, however, is not
the case when the forward ends of the clips are
too bluntly rounded or when they are too stiff. In
either case the clips may be bent so that the slide will
work easily. Some persons prefer to work without
clips, but this can only be done after considerable
experience has been acquired and only when the instru-
ment is in an upright position.

118

With the low power objectives, which are used on
coarse and large objects, it will be found after properly
focusing, that a portion of the object will show itself in
the field and by moving the slide it can easily be
brought, to the center. In this connection it must be
remembered that the image in the eyepiece is in a
reversed position from that of the object and that a
movement of the object to the left gives an apparent
movement to the right in the field. This will create
confusion at the outset, but after a little practice the
movement becomes involuntary. In the case of a small
object which is not found after the objective is known
to be in focus, as may be told if the mounting medium
or small particles of dust on the cover glass are visible,
move the slide about on the stage by grasping one end
with the thumb and forefinger, when the object can
usually be recognized by the shadowy outlines as it
flits across the field. The difficulty of finding an
object or a particular spot in it becomes more difficult
with the increase in power and even in experienced
hands becomes quite vexatious. Recourse may be had
to two methods :

By using a low power eyepiece,

By using a low power objective as a finder.

A large field is thus obtained in which the object
may be more easily found, and after moving to the
center of the field the objective is removed and the
high power attached ; or in case revolving nose-piece

119

is used, use the low power objective as a finder
then swing the high power objective into position,
care being taken not to touch the slide, and focus in
the manner to be described. The object may not be
in the field, due to a slight variation in the centers of
the objectives, but it will certainly be very close and
ought to be easily found.

The mechanical stage in either the fixed or attach-
able form will be found to greatly facilitate work in this
direction, particularly if the object is minute and if in
an important investigation one desires to be absolutely
convinced that the whole field has been covered, as for
instance in the search for bacilli.

To Illuminate the Object. This is an
extremely important feature and should always be
carefully done, as one may easily fail to obtain the
best results, may be led to wrong conclusions, or may
injure the eyes.

The mirrors of the microscope are usually plane
and concave and are provided with universal joint, so
as to reflect the light from any source in front or at
the side of the microscope.

The plane mirror, Fig. 46, reflects the light in the
initial intensity of its source and is used with low
power objectives. The concave mirror, Fig. 47, con-
centrates the rays on the object and thereby gives
intensified illumination and is used with medium and

high power objectives, except when substage condenser
is used, which subject is left for future consideration.

The sources of light are either daylight or artificial
light. In the former the light from a northern sky is
preferable and in the latter a flat-wick oil lamp, or a
Welsbach gas burner. An ordinary gas flame should
not be used on account of the difficulty of obtaining
equal illumination and the constant flickering which is
very injurious. When using the flat-wick lamp the
narrow edge of the flame should be used, as this is
more intense than the broad side.

Fig. 46. Fig. 47.

Illuminating object with plane mirror. Illuminating object with concave mirror.

When using daylight place the microscope, as
nearly as possible, directly before a window, and when
a lamp is employed have it on the table either in front
or at the right side of the microscope and within
easy reach.

Light is either transmitted or reflected. When the
former, it is used to illuminate transparent objects and
passes through the objects from below the stage into
the objective. In opaque objects this is impossible
and reflected light is required, when it is directed onto
the object from above and illuminates its upper surface.
In the following instructions it is assumed that trans-
mitted light is used unless otherwise stated.

The concave mirror converges the light and there-
fore has a focal point, and it is evident that if its focus
is of such length that with parallel rays (daylight) it
will fall on the object, the focus will be longer with
diverging rays (lamplight) and when no provision is
made in the instrument to adjust the mirror to meet
these two conditions, it becomes difficult and some-
times impossible in critical work to obtain the best
results. For this and other advantages an additional
illuminating apparatus, called a condenser, is now com-
monly used with medium and high power objectives.

Before lighting an object make certain that the
mirror-bar is in exactly central position and set the
mirror at such an angle to the light that it will be
directed upon the object, which can be done most
quickly at the outset by observing the object directly,
keeping the head at one side of the tube. Now
remove the eyepiece and observe the light through the
objective. It should be central and of equal intensity,
which with daylight is sometimes difficult to obtain as

the sash of the window may be reflected and show itself
in the field as dark bands, or in the case of lamplight
the blue portion of the flame may appear as a dark
spot. These are only preliminary directions but will
suffice for a beginning. There will be little difficulty
in obtaining proper illumination at the outset, if one
will observe the following :

Remove the eyepiece and, looking through the back
of the objective, have

Central illumination,
Even illumination over the entire field,
Mellow illumination.

Defects in illumination which may not be apparent
will show7 when the eyepiece is replaced.
Defective lighting is indicated,

When dark points or shadows appear in the

field,
When the outlines of an object are bright on

one side and dark on the other,
When the object appears to lie in a glare of
light.

In the first two cases the correction can be made
by suitably adjusting the position of the mirror ; in
the last by reducing the amount of light by the use
of a diaphragm.

It is now generally conceded that observations with
the microscope may be made to any extent without

123

any detrimental results to the eyes, provided, however,
that the conditions of light are just right. It is a good
rule to follow, to use as small an amount of illumination
as will comfortably show the structure which is being
studied, and it may also be safely accepted that if the
eye tires or feels uncomfortable the light should be
moderated.

Illumination is either :

Central or axial, when the center of the mirror
is in the optical axis, or

Oblique, when the mirror is swung to one side
which, in objectives of wide aperture, will disclose struc-
ture that cannot be seen with central illumination.

To Focus an objective is to adjust its relation to
the object so that a clear image is obtained. To
properly and safely focus an objective is a matter of
great importance and a certain line of procedure should
be followed, which in time should become habitual.
Focusing should involve no danger to the front lens
of the objective or to the cover glass by their coming
in violent contact. With the low power objectives, in
which the working distance is great, there should be
little danger ; with the higher power objectives, in
which the working distance is so small that the front
of the objective is very close to the cover glass, there
is considerable.

124

To focus low power objectives :

Attach objective to the nose-piece. Lower the head
to the level of the stage and, watching the front of the
objective, lower the tube by the coarse adjustment until
the front of the objective is within one quarter inch of
the object ; look through the eyepiece and slowly elevate
by the coarse adjustment until the image is distinct.
Use fine adjustment.

The upward movement should be slow so that if
the object be faint it is not missed and the adjustment
not run beyond its focal distance ; or it is possible that
in the case of a very minute object it may be out of
the center and thus out of the field of vision, in which
case the surface of the cover glass or the minute
particles of dust upon it should be distinguishable.

The object will first appear with faint outlines and
indistinct, then gradually more distinct and finally
sharply defined, and if the adjustment goes beyond
this point it will gradually become more dim, in which
case return to the point of greatest distinctness.

To focus high power objectives :

Attach objective to nose-piece. Lower the head to
the level of the stage and look between objective and cover
glass toward a window or flame. Slowly lower the
objective with the coarse adjustment until the front of
the objective is nearly in contact with the cover glass ;
look into the eyepiece and slowly elevate the tube vuith

I25

the coarse adjustment until the image appears. Use
fine adjustment.

It is also advisable while watching for the image
to appear to move the object slightly in different direc-
tions, as the flitting of shadows or colors across the
field will give indications that the objective is nearing
the focal point.

Always focus upward. In case a low power is
exchanged for a higher power objective or when the
low power has been used as a searcher, i. e., to find a
certain object in a collection or a certain locality on a
specimen, the tube should first be elevated as the
working distance in the high power is too short to
admit of screwing it into the nose-piece, then detach
the low power, attach the high power and proceed to
focus in the order given.

The method of procedure with the revolving nose-
pieces, either double, triple or quadruple, is different.
The writer believes he was the first to establish the
commercial possibilities of making two and more objec-
tives in revolving nose-pieces par focal, i. e., to so
adjust the focus of each objective that when either
one in swung into position it is nearly focused and
requires the use of the fine adjustment only. This
feature not only adds to the comfort of using objec-
tives, but facilitates work and removes the danger of
objectives coming in violent contact with the cover
glass. Very low power objectives vary from those

126

powers which make up the usual outfit, and it is some-
times impossible to make them par focal. The 2-3
and 1-6 or 1-8 on the double nose-piece, and the 2-3,
1-6 or 1-8, and 1-12 oil immersion on the triple
nose-piece can be so made. But in a combination of
a lower power with the 2-3 and 1-6 on the triple nose-
piece, or a lower power with the 2-3, 1-6 and 1-12 on
a quadruple nose-piece, it is impossible to make the
lower power par focal with the others on account of
the considerably greater working distance.

As the adjustment of the objectives to make them
par focal is quite delicate each screw in the nose-piece
should be marked to correspond with the power of the
objective which is to go into it.

To focus with double nose-piece :

Screw each objective into proper place in the double
nose-piece, with the 2-3 opposite the opening through
which the light passes.

Hold the nose-piece in the right hand, objectives down;
bring the revolving screw in contact with the screw in
the tube, square with tube ; with thumb and forefinger
of left hand turn milled edge of revolving screw until
it engages ; swing nose-piece toward the front and, hold-
ing it in this position, screw the ring home.

Focus with 2-j objective, then swing nose-piece
until 1-6 nears the cover glass ; lower the head to the
level of stage ; endeavor to slowly swing objective in

127

place. Should the front of objective come in contact
with the cover glass or with the ring of cement on its
surface the tube must be elevated and the objective
focused.

In the event of the objectives not being par focal,
the difference in their focal distance should be noted
for further use ; if little, by the amount of turn of the
micrometer screw, if considerable, by the extent of
adjustment required with the coarse adjustment.

The method of procedure with the triple and
quadruple nose-piece is very much the same as with
the double, and the same rules will apply. In the case
of oil immersion 1-12 objective apply oil to the front
of objective before the nose-piece is screwed on the
tube, and if after use the nose-piece is left on the micro-
scope carefully wipe the oil from the objective.

In case the coarse adjustment is by sliding tube
instead of rack and pinion elevate and depress the
tube by grasping the milled edge between the thumb
and forefinger, giving the tube a spiral motion. (See
page 50.)

To Focus with Fine Adjustment. After the
focus has been found with the coarse adjustment the
fine adjustment should be brought into action, in order
to obtain a more sensitive and reliable adjustment for
focusing through the different planes or depths of the
object. Its range of movement is necessarily short

128

and at one end the screw comes to a stop and at
the other goes beyond the limit of movement and
becomes inoperative. It should always be kept as
near as possible at the medium point of its range.
Grasp the milled head of the fine adjustment between
the thumb and forefinger of one hand (right) turning
the screw in either direction to focus in different planes
of the object, while the other hand (left) moves the
object.

" In the manipulation of the microscope it is not
uncommon to observe the operator rolling the milled
head of theflne adjustment instead of firmly grasping
it between the finger and thumb and governing, to the
minutest fraction of arc, the amount of alteration he
desires. It is undesirable and an entirely inexpert
procedure to roll the milled head, and cannot yield the
fine results which a delicate mastery of this part of the
instrument necessitates and implies. To use aright
the fine adjustment of a first-class microscope is not
the first and easiest thing mastered by the tyro. We
have already intimated that the fine adjustment should
never be resorted to while the coarse adjustment can
be efficiently employed. The focus should always be
found, even with the highest powers, by means of the
coarse adjustment." (Carpenter, The Microscope and
its Revelations.)

Use of Substage Diaphragm. The intelligent
use of the diaphragm is a great aid to good results,

129

and while a few hints can be given to guide the begin-
ner, practice will, after all, lead to the best results.
The purpose of the diaphragm is to modify the amount
of light and by its aid obtain results in the definition
of the object which without it are impossible. Much
will depend upon the density of the object, the intensity
of illumination, and the power of the objective. As
already stated the plane mirror should be used with
the low powers. The diaphragm of any form should
be close to the upper surface of the stage, and if
possible adjustable with respect to the object. This
is to prevent any amount of extraneous light from the
mirror reaching the object and not to shut out any of
the useful light.

Use an opening in the diaphragm of about the same
size as the front lens in the objective. As a rule this
will be found to give a super-abundance of light,
especially in the low power objectives, and by reducing
the aperture it will be found that there will be an
increased differentiation in the object. If in medium
and high power objectives the illumination is insuf-
ficient, it is not due to the size of the diaphragm,
unless this lies at a considerable distance from the
object, but to the insufficiency of illumination. The
diaphragm should be reduced to a point where the
amount of illumination will be perfectly comfortable to
the eye. Very often better results are obtained by
varying the distance of the diaphragm from the object

130

and it is easily recognized when better definition is
obtained in this way.

Do not use so large an opening that there will be
an uncomfortable glare, nor so small that undue
exertion is required to see structure.

When oblique light is used there should be no
obstruction to the course of light. If a cap diaphragm
is used with central illumination, it should be entirely
removed with oblique illumination. With the iris
diaphragm the full opening should be employed. In
the case of the revolving diaphragm which generally is
some distance from the object it should be removed if
possible, and if not, the largest opening should be used.

Which Eye to Use. The right eye is generally
used for observations, but while the manipulator may
from habit be inclined to use this, it may be possible
that in some cases the left can be used to better
advantage and with less fatigue. It is a fact well
known to oculists and opticians that many eyes are
defective, of which fact the possessors may not be
aware. Short or long sightedness has little or no
influence in viewing an object, except to require a
different adjustment, but so-called astigmatism, a defect
of the eye which makes it impossible to see in a certain
axis distinctly, may influence the best results. If this
error is corrected by wearing glasses and these are used
while making observations, either eye can be used.

13*

But in order to determine whether a defect exists, of
which the possessor is not aware, observe closely first
with one eye then with the other an object, preferably
one with fine striations such as a diatom, to learn
whether with one eye a better view is obtained than
with the other and use the one giving the best results.

Make it a habit at the outset to keep both eyes
open.

There is one point over the lens called the eye-point,
Fig. 39, e p, at which the rays cross within the smallest
compass, and this is the proper position for the eye as
the largest number of rays enter it. When above or
below this point the size of field will be reduced or
shadows or colors will appear in it. In low power
eyepieces the eye-point is some distance from the lens ;
in high powers quite close to it — in fact in some so
close that the eyelashes rest upon the lens and some-
times appear in the field as dark bars. Generally
speaking the best point is where the entire field is seen
and its margin (diaphragm) sharply defined.

What Objects to Use. Suitable objects for
preliminary work in leading the student to a skillful
use of the instrument and to give him proper judgment
in testing the capacity of objectives are also valuable.

Low Powers — Proboscis of blow-fly. This should
be flat and transparent. For i inch, 2-3 and 1-2 inch
objectives the scales from Lepisma saccharina.

132

Medium Powers — Pleurosigma angulatum, dry,
stained Bacteria and Micrococci.

High Powers — Oil immersion i-io inch and 1-12
inch objectives, Amphipleura peliucida, Surirella gemma
in balsam or sty rax. Stained Bacteria and Micrococci.

Test Plate. This will be an excellent acquisition
for all those who can meet the pecuniary outlay. It
consists of a series of twenty diatoms arranged accord-
ing to the coarseness of the lines. They are furnished
mounted both in balsam and styrax. Below is a table
giving the numbers, names of the various diatoms and
divisions on their surfaces in i-iooo inch. A specimen
of Eupodiscus argus begins and ends the series :

Striae in 1-1000
inch.

1. Triceratium favus, Ehrbg., 3.1 to 4.

2. Pinnularia nobilis, Ehrbg., 11.7 " 14.

3. Navicula lyra, Ehrbg., var., - 14.5 " 18.

4. Navicula lyra, Ehrbg., - 23. u 30.5

5. Pinnularia interrupta, Sm., var., - 25.5 " 29.5

6. Stauroneis phcenecenteron, Ehrbg., 31. " 36.5

7. Grammatiphora marina, Sm., - 36. " 39.

8. Pleurosigma balticum, Sm., 32. " 37.

9. Pleurosigma acuminatum (Kg.) Grun., 41. u 46.5
1.0. Nitzchia amphionys, Sm., - - 43. " 49.

11. Pleurosigma angulatum, Sm., - 44. " 49.

12. Grammatiphora oceanica, Ehrbg., G.

subtilissima, - - 60. " 67.

13. Surirella gemma, Ehrbg., 43. to 54.

14. Nitzchia sigmoidea, Sm., - - 61. " 64.

15. Pleurosigma fascicola, Sm., var.. 55. " 58.

1 6. Surirella gemma, Ehrbg., - - 64. " 69.

17. Cymatopleura elliptica, Breb., 55. kk 81.

1 8. Navicula crassinervis, Breb., Frustulia

saxonica. Rabh., - - 78. kt 87.

19. Nitzchia curvula, Sm., - 83. " 90.

20. Amphipleura pellucida, Kg., - 92. "95.

Whatever opinion one may have in reference to the
study of diatoms, the fact cannot be gainsaid that
they have been a great aid in the improvement of
objectives. They are used by opticians to judge the
various characteristics of objectives and offer a reliable
standard for testing resolving qualities, such as no other
object can. The writer particularly recommends that
the test Pleurosigma angulatum, dry, form a part of
every outfit, not only as a test for resolving power but
as an object for study with which to acquire skill in
manipulating the microscope. It may be used with
powers of 1-4 inch and higher and after it has served
its purpose may be put aside for work with objects
which come under the particular branch of study which
one is following.

The writer wishes to counteract as much as possible
the opinion, which is too prevalent, that the use of
diatoms indicates microscopical play and is unworthy
of consideration in histological and biological work ; but

the fact that the optician deems them necessary for
determination of optical qualities ought at least to
indicate that they are a valuable adjunct and certainly
will aid in giving greater manipulative skill.

At this point it is considered advisable to add some
suggestions from Carpenter.

"The correctness of the conclusions which the
microscopist will draw regarding the nature of any
object from the visual appearence which it presents to
him, when examined in the various modes now specified,
will necessarily depend in a great degree upon his
previous experience in microscopical observations and
upon his knowledge of the class of bodies to which the
particular specimen may belong. Not only are observa-
tions of any kind liable to certain fallacies arising out
of the previous notions which the observer may enter-
tain in regard to the constitution of the objects or the
nature of the actions to which his attention is directed,
but even the most practiced observer is apt to take no
note of such phenomena as his mind is not prepared to
appreciate. Errors and imperfections of this kind can
only be corrected, it is obvious, by general advance in
scientific knowledge; but the history of them affords
a useful warning against hasty conclusions drawn from
a too cursory examination. The suspension of the
judgment, whenever there seems room for doubt, is a
lesson inculcated by all those philosophers who have
gained the highest repute for practical wisdom, and it

is one which the microscopist cannot too soon learn or
too constantly practice. Besides these general warn-
ings, however, certain special cautions should be given
to the young microscopist with regard to errors into
which he is liable to be led even when the very best
instruments are employed."

Medium Power Objective. After sufficient
time has been devoted to study with the low power
objective, exchange it for the higher power and replace
the object with the slide Pleurosigma angulatum.
Focus upon this, being mindful of the suggestions
previously given and do not fail to observe what has
been said in regard to well illuminated field. Observe
now whether any lines can be seen upon the surface of
the diatoms. If not, vary the distance of the mirror from
the object, if adjustment is provided for ; or, if lamp-
light is used, bring the lamp closer to or remove it from
the instrument in one line so that the illumination will
not disappear. If this does not bring out the lines,
swing the mirror-bar from the central to an oblique
position on the side opposite to that of the light and
readjust the mirror. Grasp the ends of the mirror-fork
between the thumb and middle finger and move the
mirror with the first finger. If the field cannot be evenly
illuminated, it is evident that the mirror is beyond the
limit of angular aperture of the objective and must
therefore be brought back until the light appears

136

equally well over all parts of the field. It must also
be noticed here that if the diaphragm is still attached
to the instrument and does not swing with the mirror,
it may be the means of cutting off light. The largest
opening should be employed or, if the cap diaphragm
is used, »this should be removed. By means of the
micrometer screw carry the fine adjustment back and
forth beyond the plane of the object and observe
closely whether any lines can be distinguished. It is
very probable that they will show, but if not, the cause
should be determined. It may be that the magnifying
power is not sufficiently great to make the lines visible
and in this case a higher power eyepiece should be
used ; or the cover glass may be more or less than
the normal thickness, which would cause a spherical
over- or under-correction in the objective. In this
case the lines would appear when the outline of the
diatom is out of focus, and the structure will be
more readily apparent with oblique than with central
light. If the above directions have been followed, the
lines ought certainly to appear with a moderately good
1-5, 1-6 or 1-8 inch objective, but if they are not, the
trial should be repeated. Again, be careful to have
no obstruction in the course of rays from the mirror
to the stage ; have good illumination on the object ;
observe well, and keep the instrument in such a posi-
tion that the object is not illuminated from any other
direction than from the mirror.

When the diatoms are resolved in this manner the
lines will appear to be diagonal in some, longitudinal
or transverse in others, according to their position, and
if the resolution is very good, these lines will further
resolve themselves into minute beads of a hexagonal
form.

It will now be well to bring the mirror more nearly
to a central position. Do this by intervals of about
10 degrees and note the appearance at each decrease
of obliquity. It will be found that as the mirror
approaches the optical axis the lines will appear to
become more faint and may disappear before central
illumination is reached ; in this case it will be well to
begin again. An endeavor should be made to make
each attempt give better results than the preceding one.
Repeated trials will not only impress the various
phenomena upon the mind, but will cause a notable
improvement in manipulative skill and thus a better
performance in the objective.

To Judge Spherical Aberration. This is a
matter of experience based upon the knowledge of the
principles involved and after having been studied will
be found to be of the utmost value in utilizing the
capacity of a microscope. To judge sperical aberration
by the use of histological or biological objects, without
a previous knowledge acquired from objects which are
more suited, is extremely difficult. One may be aware

138

by the unsatisfactory appearance of the image that
something is amiss, but will probably not know how
to correct the deficiency.

Using a 1-5, 1-6 or 1-8 inch objective and the test
object Pleurosigma angulatum, select a diatom which
is flat and locate in the center of the field. Focus care-
fully so that the margin of the object will be sharply
defined and observe the markings. If they show in
the same plane without any further focusing, the
spherical correction may be accepted as being correct.
If the lines appear to lie in a higher plane and it is
necessary to focus upward, so that the margin of the
diatom is out of focus, it indicates spherical over-correc-
tion and the remedy is found in the contraction of the
tube length. This should be done progressively in
spaces of about one-half inch, and after each change
carefully focus again until proper correction is obtained.

When the lines appear to lie below the plane of the
object, it indicates spherical under-correction and can
be corrected by increasing the tube length. If there
are two or more eyepieces, results can be obtained
quicker with the higher powers.

If the markings cannot be seen, it may be due to
abnormally thick or thin covers, a not uncommon
occurrence, thus destroying the resolving power. This
may be judged by using slightly oblique illumination.
If too much is used the nice differences will be lost.

'39

In all objects other than diatoms it is generally
difficult to form an opinion as to spherical correction.
If from preconceived idea of what an object should
show it fails to meet expectations or is hazy where one
expects it to be distinct, and being certain that the
objective and eyepiece are properly cleaned, it may
generally be ascribed to lack of proper correction. By
focusing either above or below the proper focal plane
there will be an enlargement of the outlines of the
object or a coma which gradually enlarges as the objec-
tive recedes from the proper focal point.

If the expansion is greatest when the objective is
elevated, there is spherical over-correction and the tube
length should be decreased.

Should the expansion be greatest when the objective
is lowered, there is spherical under-correction and the
tube length should be increased.

Chromatic Aberration. This may be judged
as described under " Chromatic Aberration" in a
previous chapter, page 85.

Cover Glass. We have thus far not considered
the cover glass, except to show its influence on the
optical performance of objectives. In preliminary
examinations of solid objects with low powers it may
be dispensed with, but where fluids are used, whether
with low, medium or high powers, it should always be

140

used. A drop or small quantity of fluid placed upon
a slide assumes a spherical form and, on viewing it
with a low power, it will be found to give a distorted
field and cause disagreeable reflections and shadows.

In medium and high powers, the front lenses will
be so clcTse to the water, urine, blood, etc., that capillary
attraction will cause an adhesion to the front surface
of the objective ; besides this, there is such a consider-
able depth to the fluid that it obstructs the light,
requires a great change in adjustment for the various
planes and is usually in such vibration that sharp focus
becomes impossible. By merely dropping a cover
glass upon it all these objections are overcome.

Covers are commercially classified as No. i, No. 2
and No. 3, but there is a variation within the limits of
different numbers. The variation is about as follows :

No. i, 1-150 to 1-200 inch, or o.i 6 to 0.13 mm. thick.
No. 2. i-ioo to 1-150 inch, or 0.25 to o.i 6 mm. thick.
No. 3, 1-50 to i-ioo inch, or 0.50 to 0.26 mm. thick.

According to the 'prices of cover glasses, when
purchased by weight, the No. i gives the greatest
number and No. 3 the least. It may for this reason
be thought that the purchase of No. i is most advant-
ageous, but it must be considered that there is a
greater amount of breakage by cleaning, as they are
very thin and sensitive. Considered from the optical
standpoint the No. 2 covers offer a range in thickness

141

to meet the different standards as used by the makers
of objectives. Test objects which are prepared to test
the resolving power of objectives and consist of diatoms
are generally covered with these thicknesses. The
No. i are principally used with oil immersion objec-
tives as the working distance of these objectives is
very short and more working distance is gained by
using thin covers with them.

An excellent means of determining the thickness of
cover glass, as well as of studying spherical and
chromatic aberration, is the Abbe test-plate. This
consists of a series of cover glasses ranging in thickness
from 0.09 mm. to 0.24 mm., silvered on the under
surface, with lines cut through the film of silver, and
cemented to a slide, each cover being marked with its
correct thickness.

With this test-plate spherical aberration is corrected
when the bands or lines show distinctly without any
nebulous fringe, and thus indicate the proper thickness
of cover to use.

Chromatic correction may be judged by the character
of color bands which show with oblique light. But
correction is indicated when, on swinging the mirror to
the left, violet appears on the left and apple green on
the right side of the bands.

142

Dry, Adjustable Objectives. Adjustable objec-
tives, or objectives with collar correction are those in
which there is a mechanical provision to vary the
distance between the lenses, in order to make with
facility, proper correction for spherical aberration.
They involve a high degree of mechanical perfection
and are therefore more expensive than objectives with
fixed mountings. They may, however, be recommended
to microscopists who have acquired some experience
in handling objectives and even to beginners who will
use judgment in their use, as they certainly give excel-
lent results and quick means for obtaining the utmost
limit of efficiency in objectives, a fact best appreciated
by those who are expert in the use of them. In these
objectives, as at present constructed by the best
makers, a milled collar is provided, which when
rotated imparts a rectilinear motion to an interior tube
carrying the posterior system of the objectives, thus
varying the distance between them and the front
system which remains stationary. The screw collar is
graduated in such a manner that the figures indicate
the correct point for the proper thickness of cover
glass; thus 10 indicates proper correction for a cover
of o.io mm., 1 6 for 0.16 mm., and so on.

When set for thick covers, the lenses are closest
together and the adjustment is said to be closed : when
for thin covers, the lenses are farthest apart and the
adjustment is open.

'43

In objectives of older construction and in some
produced at the present day, the figures are abitrary
and serve no other purpose than an index for reference.

Close the adjustment before attaching the objective
as its front may otherwise come in contact with the
cover before the focus is reached. For practice with
this objective use P. angulatum. Focus carefully and
observe whether any lines can be seen ; if not, grasp
the milled edge of the adjustment collar between the
thumb and first finger of the left hand, keeping the
fingers of the right hand upon the milled head of the
fine adjustment ; turn the collar slightly toward its
open point and, as this will place the object out of
focus, move the fine adjustment correspondingly.
Continue to turn the collar little by little and do not
cease to observe closely ; also, after each movement,
focus above or below the plane of the object, so that
this will be distinct, and look for the lines. Possibly
after a little they will begin to appear faintly ; but,
if not, continue to bring the collar toward the middle
point. The lines must now soon make their appear-
ance, and when they do, it will probably be above the
plane of the diatom. This is an indication that the
objective is approaching its correction for the cover.
Now keep the lines in focus, while the correction collar
is being gradually turned, until the lines and the outline
of the diatom lie in one plane. The objective is now
said to be corrected for the cover used. Observe

144

which number corresponds to the index and, turning
the collar back to its closed point, go through the same
procedure as carefully as at first. When the best point
is again reached look for the number and see whether
it agrees with the first ; very likely it does not, which
is owing to a lack in the faculty of perception, due to
a too slight acquaintance with the phenomena. These
trials should be repeated until the proper sensitiveness
of feeling in making the adjustments is acquired and
until they can be made to correspond with certainty
to at least within two divisions. When it is found
after repeated trials that sufficient skill has been
acquired, mark the number upon the slide. For future
examination of the same slide, this will facilitate work
and give the assurance that the best results are thus
obtained without further trial by simply referring to the
recorded number.

On stained Bacteria and Micrococci focus briskly
with the fine adjustment to either side of exact focus.
There will be an expansion of the outline of the object
both when within and without the focus.

If the greater expansion or coma is within the focus,
or when it is necessary to raise the objective, there is
spherical over-correction and the adjustment must be
closed.

If the greater expansion is below the focal plane,
there is sphericdl under-correction and the adjustment
should be opened.

145

When the proper point of correction is reached the
expansion of outline is the same in both directions.

Immersion Objectives. As has been stated
before, immersion contact between the objective and
cover glass is made with either water or homogeneous
fluid. With the former distilled water only should be
used and kept in a suitable bottle. .Cedar oil is used
for the homogeneous fluid but is specially prepared, the
commercial cedar oil being too thin and volatile and
not of the proper refractive index. A small bottle is
generally supplied with each oil immersion objective.
Great care should be used in keeping it free from dust,
as it often happens that an objective fails to give satis-
faction because of a small particle of dust which may
float in the fluid in front of the hemisphere. Great
care should also be exercised in applying oil to the
front lens and after the application it is strongly
recommended to examine it with a magnifier, that
there may be no air bubbles present. Air bubbles as
well as dust seriously interfere with obtaining satis-
factory results. If bubbles are present the oil should
be removed and a fresh quantity applied. An air
bubble will entirely destroy the clear definition of a lens
even when not directly in front of the lens itself. Care
should also be taken not to apply too great a quantity.
After the stopper has been withdrawn from the oil,
allow the oil to run down the rod or brush until the

146

last natural drop has separated from it and apply the
remainder, or less than a drop, to the front of the
objective.

Attach the objective and lower it until the fluid
comes in contact with the cover, observe this by lower-
ing the 'head to the level of the stage. Focus as with
dry objectives. The use of immersion fluid in itself
involves a certain amount of inconvenience, but the
observance of fixed rules will materially help to over-
come some of the . disagreeable features. Extreme
cleanliness should be observed with it. After the
work has been completed the objective should be
removed from the stand and its front as well as the
slide should invariably be cleaned. The fluid may be
removed by a moist piece of soft linen and the front
then cleaned with a dry piece or with lens paper.
Chamois skin is not suitable, as it does not absorb
the fluid.

Immersion Objectives on Test Plate. Oil

immersion objectives are not so sensitive to variations
in thickness of cover, although many of the most expert
manipulators prefer adjustable mountings in order to
obtain the highest results.

To determine the highest capacity on test objects,
ordinary daylight is not sufficient ; a flat-wick oil lamp
is best suited. If the right hand is used on micro-
meter screw, place the lamp at the right side of the

147

instrument, about ten inches from it, with the edge of
the flame turned toward the mirror.

Place the test plate upon the stage and, as the
diatoms in balsam are very transparent and therefore
very difficult to find, a low power objective may be
used as a finder ; bring No. i, Triceratium favus, into
the center of the field and after the low power objective
has been removed attach the immersion objective,
which we assume to be a 1-12, in the manner
prescribed. Get the best possible illumination with
the mirror at the central point and move the test • plate
from diatom to diatom until it reaches No. n, Pleuro-
sigma angulatum, but observe closely the structure of
each one as it comes into the field. Next, see whether
the objective is spherically corrected. If the lines
and outlines, or middle rib, do not appear to be in
the same plane, adjust the collar in adjustable or
the tube length in non-adjustable objectives until they
are, then continue the advance toward the higher
numbers until one is reached on which no lines can
be seen. Swing the mirror-bar to an obliquity of
20 degrees to the left side and, readjusting the mirror,
observe the effect. It is very probable that the
lines will show and if so, continue the advance ; if
they do not, increase the obliquity of the mirror-bar
10 degrees or 20 degrees and after the structure
comes out, again go forward. A ppint may thus be
reached where with the greatest obliquity which can

148

be given and with the best possible illumination the
objective seems to have come to the limit of its per-
formance. From the claims which have been made
for it, it ought to do better. What is the cause of
failure ? Possibly the mirror is not correctly focused,
or the adjustment collar may not be correct for oblique
light ; perhaps the eyepiece does not give sufficient
magnifying power to distinguish the striae. It may be
any one of these causes or all combined. As to the
eyepiece, the manipulator must remember the amount
of separation of lines in the last object which was
resolved and from the gradation in the coarser spec-
imens must judge whether the power is sufficient ; it
should be added that for any over No. 14 and under
No. 1 8 a i inch eyepiece should be used and for those
above No. 18 a power of 3-4 inch for the long
tube and of 1-2 inch for the short tube will be
necessary. After this condition has been complied
with, look to the correction collar of the objective. To
obtain the highest results it very often occurs that a
different adjustment is required for oblique light from
that for central light. Note the number at which the
collar stands and then work it back and forth, watching
carefully for results. If this has no influence, return
it to its number or to a point where the outline of the
object appears most sharp. Now look to the illumina-
tion ; vary the distance of the mirror from the object,
or if this cannot be done, vary the distance of the lamp

149

from the instrument and watch the effect of the change
through the eyepiece. If neither of these changes give
any improvement, recourse must be had to another
expedient. Place a bull's-eye between the lamp and
mirror with the plane side of the lens toward the lamp
and close to it, so that the light is thrown on the mirror.
It should be properly concentrated, so that the circle
of light will not be larger than the mirror, which can
be determined by placing the hand or a piece of paper
back of it. Adjust when necessary by moving the
lamp or bull's-eye. Keep the mirror a little below the
line of the top of the stage, so that the beam from the
bull's-eye will not illuminate the object on its upper
surface. If the direct light from the bull's-eye reaches
the object, it destroys to some extent the effect of
the oblique illumination from the mirror. Great care
should be given to this point as it is very important.

If all of these suggestions have been followed, a
great difference will undoubtedly be noticed in the
performance of the objective ; but if it still does not
come up to the standard, patience must not be lost.
The slightest change in the position of the mirror, or
bull's-eye, or lamp, or a touch to the correction collar
or micrometer screw, is sometimes followed by astonish-
ing results. The beginner should sit down with the
expectation that he will fail at the first trial. At each
succeeding trial he can easily notice his improvement
in manipulation and a corresponding gain in the results.

He should be able to bring the performance of the
objective up to the claims made for it, if it has come
from the hands of a reliable optician, and should not
rest until this is accomplished.

The writer has often recommended sunlight with
generally * successful results where ordinary means of
illumination have failed. The light is of course intense
and great care will have to be used to modify it by
properly using the mirror, but success is often attained
and then creates confidence. It is, however, only
recommended for this purpose and not for general use.

Stained Bacteria and Micrococci also make excel-
lent objects for immersion objectives. The mode of
illumination is the same as with dry objectives. Great
care and judgment should, however, be exercised in
forming an opinion as to resolving power from such
specimens. Only the most carefully stained and suit-
ably selected specimens are of any value, among the
best of which may be mentioned the beaded form of
Bacillus tuberculosis, if clearly and deeply stained.
The enveloping structures of many bacteria and the
diffusion of stain from them to the surrounding sub-
stances in which they are imbedded render them
entirely useless for test purposes.

Opaque Objects are so dense in their structure
that the light from the mirror below the stage will not
pass through them. They generally consist of plants,
minerals, shells, etc.

Place the object on a slide and slip under the
clips.

In this case the low power objective is used
for two reasons ; because a general view is sought,
involving low magnification and large field with light-
giving power and because a higher power cannot be
used on account of its short working distance. _The
light may and undoubtedly will be found insufficient to
distinguish the object clearly. If the instrument is of
the American type, swing the mirror-bar upon its axis

around the stage to a point
,,-.---"** above it so that it will be

at an angle of about 45
~~~ degrees to its surface. If

a lamp is used and in the
-•"* *- same position as when

used for transmitted light,
it is probable that the tube
of the instrument will
^ " ' * obstruct the light and it is

Fig. 48. Illuminating Opaque object then well to move it to-
with mirror above the stage. f .

ward the front. Using the

concave mirror, adjust so that the light will be con-
centrated upon the object, by watching it directly, and
then observe through the tube. If it is not sufficiently
illuminated continue to adjust the mirror; also vary
its distance from the object and swing the mirror bar
to a higher or lower point.

In the Continental form of microscope where there
is no provision for swinging or placing the mirror above
the stage, one is dependent upon the direct source of
light when, in the case of lamp light, the lamp should
be raised to a higher position. When the light so
obtained is not sufficient for proper examination,
recourse must be had to a separate apparatus, the
bull's-eye condenser. This is a plane convex lens of
strong curvature fixed to a stand giving adjustments in
different directions. It is interposed between the light
and the object, with its plane side toward the object
in such a manner that the lens will concentrate the
light upon its upper surface.

'53

ILLUMINATION WITH SUBSTAGE
CONDENSER.

Up to this -point the matter of illumination has been
treated in its most simple form as being given with
mirror only, but we must now consider the substage
condenser, which is a most valuable adjunct to the
microscope. While skillfull treatment of the mirror
only will go far toward obtaining good results, it will
not suffice except in the most simple investigations.

Purpose of the Condenser. The purpose of
the condenser is not only as its name implies, to con-
dense light, and thus give an amply illuminated field
when the illumination is otherwise insufficient, but is
more especially to illuminate the object with a cone of
light having an angular aperture equal to that of the
objective, which is absolutely unattainable with a mirror
only, as well as to provide means for controlling the
amount and character of the illumination to suit the
various conditions of work.

Abbe Condenser. The history of the substage
condenser is very unique and interesting and shows
how, from having been the subject of no end of con-

demnation, which for many years it received, it is now
generally accepted as a necessary adjunct to a com-
plete outfit, should in fact be part of an equipment in
which there is a medium power dry or an oil immersion
objective. From single lenses, compound and achro-
matic lenses, the use of eyepieces and objectives as con-
densers and any number of devices for regulating the
light, the generally accepted forms at the present time
have come to be those" devised by Prof. Abbe. One of
them with a 'numerical aperture of 1.20 consists of a
combination of two lenses, Fig. 49, and the other with
an aperture of 1.42 of three lenses, Fig. 50. A third
is made achromatic with an aperture of i.o. It is,
hqwever, considerably more expensive.

Fig. 49. Fig. 50.

The particularly distinguishing feature of these
condensers is that they will transmit a beam of light as
large as can pass within the limits of the substage ring.

The one of 1.20 N. A. is that in most common use
as it meets the conditions for all except the most
critical requirements.

The condensers are mounted in a variety of forms
offering greater or less facility for vertical adjustment,

the amount and direction of light, the displacement of
condenser when it is not used, etc.

The most simple form, largely used for instruments
for laboratory and everyday work, is one which has
attached to its lower side an iris diaphragm for regulat-
ing the amount and angle of light and to which is
attached a swinging arm to receive blue glass for use
with artificial light, or stops for dark ground or oblique
illumination. A vertical screw motion gives a service-
able means of adjustment and when at its lowrest limit
of adjustment it may be swung out of the optical axis.

The most complete
form is that shown in
Fig. 5 1 a, b and c, .whfch
has adjustments for obtain-
ing every modification and
character of illumination,
with rack and pinion for
vertical adjustment and
swinging the condenser
and iris diaphragm out of
the way if it is not desired
to use them.

The condenser should not be used on very low
power .objectives as it is distinctly harmful and the
mirror alone provides ample illumination.

The following description of the parts of the com-
plete substage will facilitate its use.

Fig. 5 la. Complete Substage.
Front view.

156

The Complete Substage consists of a vertical
bar fixed to the microscope in place of the mirror bar
and having an adjustable slide x operated by rack
and pinion a, carrying supporting attachments for upper
iris diaphragm, condenser and lower iris diaphragm
respectively, and at its lower end the plane and concave
mirrors m.

s 1

, r d

Jft

j ^ C ^

_n_

1 1 V n

p '

m g)

Fig. 51b. Complete Substage. Front view, with condenser and lower
iris diaphragm swung out of optical axis.

The Upper Iris Diaphragm e is carried in the
upper substage ring s. In use it is opened and closed
V^y the lever / and serves to regulate the amount of light
when an object is viewed without the condenser and
to limit the volume of light without reducing its angle
when the condenser is used with extremely transparent
objects, such as unstained bacteria, moulds, etc. It is
raised and lowered by the pinion a which moves the
whole substage.

'57

The Condenser c is carried in the lower substage
ring mounted on a lateral axis by which it may be
swung to the left, out of the optical axis. It has a
centering adjustment operated by the milled heads n n
by which its axis can be made to correspond with
that of the instrument with the greatest facility.

Fig. 51c. Complete Substage. Top view, with condenser and lower iris
diaphragm swung out of optical axis.

The Lower Iris Diaphragm d is mounted
on the plate o attached to a lateral axis by which it can
be swung to the right out of the path of light frcm the
mirror. It has a lateral motion by the rack and pinion r b
on the plate o and rotates on its own axis so that when
oblique illumination with the condenser is desired a
small aperture of the diaphragm may be brought the
proper distance from the optical axis and rotated to
bring it opposite to the direction from which the light

158

comes to the mirror .or at right angles to the striae of
striated structures. The diaphragm d is operated by
the lever v.

Use only plane mirror with the condenser.

A condenser is so constructed that, parallel rays
of lighfr are brought to a focus above the upper surface
of its uppermost lens and in the plane of the object,
Fig. 52. If the concave mirror is used the convergence
of light is more rapid and the apex of the cone of light
is within the condenser, Fig. 53, and its effectiveness
depreciated.

Fig. 52. Illuminating object with con-
denser and plane mirror.
The right way.

Fig. 53. Illuminating object with

condenser and concave mirror.

The wrong way.

Centering the Condenser is the act of bringing
its optical axis coincident with the optical axis of the
objective.

In the simple forms of microscopes in which the
work is not critical, the condenser as sent out is

sufficiently centered by the maker for all ordinary
requirements but there should be provision to center
it in case of disarrangement.

In the more complete apparatus adjustment screws
should be supplied. These provide a ready means of
centering and when once the correct position with one
objective is determined it will not require any further
attention.

To verify correct centering two easy methods may
be followed :

I. Use a 2 inch objective and focus through the
condenser onto the diaphragm, which is reduced to its
smallest opening.

II. Use a 2-3 or I inch objective; focus upon
upper surface of condenser or upon an object which
should then be removed; elevate objective with coarse
adjustment until a dimly defined dark spot appears in
the field and with proper focusing is about i-j of the
diameter of the field.

Centering of the condenser does not imply that the
cone of illumination is also centered and it is fully as
important to secure the correct conditions in one as it
is in the other.

Centering the Illumination. The mirror may
be so adjusted that the light will be directed toward the
periphery of the condenser and when lamplight is used

1 60

the light may be so placed, as to give all gradations
of oblique illumination from the central to the limit of
aperture, although the condenser may be centered ;

With daylight have evenly illuminated field.

With lamplight attach 2-3 inch objective; open
diaphrfigm to full extent and focus upon the minute
image of flame; adjust mirror so that the image will
be in the center of the field.

To Focus Condenser. Since the condenser is
nothing more than a combination of lenses similar to
those in an objective, but used in a reversed position,
it has the same properties of angular aperture, focus
and working distance. But, as it must work through
the thickness of slide, its working distance is propor-
tionately long. Owing to the variation in thickness of
slides, its focus falls in some slides above and in others
below its effective position, in relation to the object,
and an adjustment is therefore necessary to make it
most effective.

In all the various forms of mountings the condenser
is so mounted that at the uppermost limit of adjustment
its upper surface is just below the surface of the stage
so that it cannot come in contact with the slide.

With all objectives having a numerical aperture
less than i.o the condenser may be used dry, i. e. with-
out oil.

161

In the use of the condenser with oil immersion objec-
tives the custom prevails of using the condenser dry.
It is convenient especially in changing specimens and
meets the requirements of every day work. It is well
to point out, however, that both the condenser and
the objective lose in their efficiency when the former
is used dry, and for critical work the condenser should
be in immersion contact with the slide.

To make immersion contact between condenser and
the slide place a drop of oil on the top of condenser,
drop the slide upon the stage, first throwing the clips to
one side.

With immersion objectives the proper focusing of
the condenser becomes a matter of nice distinction to
obtain best results and can only be reliably accom-
plished by considerable practice and experience.
To obtain best position :

Use a 2-3 objective ; focus upon the object ; adjust
condenser until image of window-sash or flame is in
the same plane with the object.

Relation of Aperture of Condenser to
Objective. In the study of Bacteria and other micro-
organisms the objectives used being of wide aperture,
it is sought to have them stand out boldly in a bright
field. This is accomplished by bringing the diaphragm
to its full aperture. On all other objects, however,

162

too much illumination decidedly injures definition by
obliterating detail.

Little experience is required to judge when the
condenser has its proper opening. When correct, the
image will stand out sharply defined without any
appearance of fogginess and as the diaphragm aperture
is reduced it will be noticeable by the decrease in the
amount of light. By removing the eyepiece and
looking at the back of the objective the relative aper-
ture of the condenser to that of the objective may be
easily seen, as the outlines of the diaphragm are
sharply defined. In testing for this, start with the
smallest aperture of the diaphragm and gradually
increase its diameter. If the opening in diaphragm
appears to have the same opening as the back of
objective, the condenser has the same angular aper-
ture. In the following instructions for the proper use
of light from the condenser the size of opening of its
diaphragm as it appears by viewing the back of the
objective is called apparent aperture. By experience
the following conditions have been found to give most
satisfactory results :

With oil immersion objectives on bacteria use the
full opening of diaphragm.

For diatoms reduce the apparent aperture to about
two -thirds opening in objective.

For histological and other dense objects the apparent
aperture should be equal to about one-half the opening
of back lens in objective.

With dry objectives the aperture of the condenser
should always be less than that of the objective,

Oblique Light with Condenser. Oblique light
may be obtained by setting the mirror alone in such a
position that the light reflected from it shall enter the
condenser only at one side, leaving the balance of it
unused. This, however, is only advisable when the
condenser mounting has no other provision for obtaining
oblique light. In the mountings having such provision
oblique illumination may be obtained by two methods :

/. Focus objective; reduce
the apparent aperture to that
of the rear lens of the objective,
swing the plate o, Fig. 51 a,
carrying the lateral adjust-
ment around so that the pinion
button is at the front. Turn
the pinion button so that the
opening will move from the
center to the periphery of the
condenser, Fig. 54.
II. Proceed as above with this difference: Remove
eyepiece and view the bright circle of light as it passes
from the center to the periphery of the rear lens.

Fig. 54. Illuminating object by
oblique light with condenser.

164

When the circle of light has passed beyond the
limit of aperture of the objective the field will become
dark. The amount of illumination may be modified,
but in a general way it may be said that the best
results with oblique illumination are obtained by
reducing the amount of illumination to its minimum
practicable amount.

In objects with striated structure, the illuminating
rays should be brought to a position at right angles
to the striae, either by rotating the object to the proper
position, or by swinging the diaphragm plate.

In using process II the circle of light should be
bright. As it nears the edge, colors become apparent
until finally at the edge of the objective the violet
and red are quite pronounced. With proper disposi-
tion of the mirror the colors may be so equalized
that after a little practice the illumination will be found
at its best after the eyepiece is applied.

With either method lamplight will be found to
give best results but care must be taken that, as the
diaphragm passes toward the oblique point, the mirror
is also turned so that the illumination will not be lost.

In the Abbe condensers the chromatic aberrations
are quite apparent with extreme oblique illumination —
more so with that of 1.40 N. A. than with 1.20 N. A.
The field ceases to be equally illuminated and all the
colors of the spectrum from the violet to the red are

165

plainly evident within the field of view. Under these
conditions it is of course impossible to view large
specimens without slightly shifting the mirror so as to
move the lightest portions of the spectrum to different
parts of the field. With small specimens the mirror
should be so directed that the light or yellow portion
of the spectrum is in the middle of the field.

166

HOW TO DRAW OBJECTS.

To be able to make a correct drawing of the
enlarged image of an object is very important and
while for some lines of work the photographic camera
is called into use, drawings are nevertheless largely
relied upon. The apparatus requisite for this purpose
is the camera lucida and while there is a variety of
forms, all are based upon the principle of causing the
image of the object to appear projected upon the
paper, where it may be drawn.

The image of the object, the paper and the pencil
point are viewed at the same time and, with proper
regulation of light, are equally distinct. The micro-
scope image seems to be projected
upon the paper and it is only neces-
sary to draw the outlines and fill out
details to obtain an exact picture of it.

The most simple form is that
shown in Fig. 55. With this the p. 55

microscope must be inclined to a
horizontal position or nearly so and the camera lucida
attached to the eyepiece. The pencil of light from the

microscope is reflected by the thin film of glass into
the eye and the image is apparently projected beyond
the glass upon the paper where the pencil point may
trace out its details.

The best form and that which has nearly superceded
all others which have made pretense to giving good
results is the Abbe camera lucida, Fig. 56.

While in the original construction it gave superior
results it has been gradually improved, so that in its

Fig. 56.

complete state it leaves nothing to be desired. Its
price is necessarily high but it is also offered in simpli-
fied form to meet more modest demands and will with
proper use do very good work although not as good
nor with equal facility as the more complete.

The optical construction, Fig. 57, is the same in all
forms. Two 1-4 inch rectangular prisms of which one
has a silvered diagonal surface with a central opening of
one half the diameter of the pupil of the eye, are cemented

1 68

together at their diagonal surfaces, thus forming a
cube. At one side of the cube about 3 inches from
it is a mirror set at an angle of about 45 degrees.
The optical parts are mounted in various degrees of
mechanical completeness to be clamped to the tube in
such a manner that the glass cube is situated above
the eye l£ns.

Fig. 57. Optical construction of Abbe camera lucida.

A very simple form is one with a small fixed mirror
near the tube and arranged to swing out so that the
eyepiece is unobstructed.

Another form has large mirror which is adjustable
in its inclination to the table and distance from the eye-
piece. This form is provided with dark glass discs at
the side and below the prism to control the amount of
illumination from the object and the paper. The best
form gives full control of the mirror, has rotating discs

169

carrying a series of dark glasses of different shades,
has centering arrangement to bring the opening of the
cube into the optical axis and swings out so that the
object can be viewed without disturbing the camera
lucida.

To obtain satisfactory results with any camera
lucida several fundamental rules must be observed :

Use sharp pencil point. If spectacles are necessary
o read they must be used with the comer a lucida to see
the pencil point.

The light in the field of view and from the drawing
surface should be of about the same intensity.

The drawing surface should be at right angles to
the axis of the projected image, otherwise there will be
an elongated picture and distortion at one end.

Draw the outline of the image and indicate the

details of structure in light lines and complete the

drawing without the camera lucida by reference to the
microscope.

Illumination. The relative illumination of the
field in the eyepiece and drawing paper varies with the
magnifying power and distance of paper from the eye-
piece. The control of the illumination in the better
class of camera lucidas provided with glass shades is
comparatively easy, but in the simple forms several
expedients must be resorted to.

170

With low power objectives the light in the field in
the eyepiece is stronger than that from the paper. It
may be modified

by using plane mirror,

by covering the mirror with tissue paper,

by using a small opening of the substage diaphragm.

With the higher powers the conditions are reversed
and the field of the paper appears brighter than that
in the eyepiece.

In this case reduce the light on the paper by using
a tissue paper or opaque screen.

Use substage condenser and modify light with the
iris diaphragm.

Arrangement of Drawing Surface. As has
been stated this should be at right angles to the axis
of the projected image. In all camera lucidas the
reflecting mirror is quite close to the tube of the
microscope which is supposed to be in upright position
and if set so that the axis of the cone of projected rays
is vertical, there will be no distortion, or in other words
the field of view will be perfectly circular. It will
be found, however, that the stage and base of the
microscope will project within this circle and so
prevent obtaining a complete drawing of the object.
The mirror of the camera lucida must therefore be so
tilted as to throw the cone of rays farther away from the

171

microscope which would form an elongated or distorted
image and to correct this the drawing surface must be
sufficiently tilted to bring it at right angles to the axial
ray. An easy method to determine whether the surface
is at a right angle is to mark the outlines of the field of
view on the paper at the right and left and at the front
and back extremes and measure the distance. The
surface should be so tilted that the distance in both
directions is the same.

If the microscope is tilted another complication
arises inasmuch as the drawing surface must be tilted
with it.

Field and Magnification. The size of the
drawing of an object depends upon

the initial magnification of the microscope,
the distance of the drawing from the eyepiece.

Drawings should be made with such objectives and
eyepieces as would be used to examine structure. As
the distance of the drawifig surface from the eyepiece
is increased the image becomes larger. The size of
drawing must therefore be regulated by varying the
height of the drawing surface.

To Use the Simple Camera Lucida. Focus
the objective upon the object ; incline microscope to the
horizontal position, raise microscope by underlaying with
blocks or books, readjust mirror and attach camera

172

lucida, place paper upon the table or board, view
through camera lucida for correct position and fasten
with drawing tacks ; modify light so that image and
pencil point are equally distinct.

Use of the Abbe Camera Lucida. Care must
be observed in attaching this to the 'microscope tube.
The opening in the silvered surface of the glass cube
must be so placed that it will be coincident with the
focal point and axis of the eyepiece. If too high or
low the field will be reduced or one side of the field
will be cut off.

Focus upon object ; attach camera lucida and adjust
so that field is round and not cut off.

Place drawing board with paper attached at right
hand side of microscope ; look into camera lucida and
adjust reflecting mirror ; place paper in proper position
and tilt board to proper inclination ; place pencil upon
the paper and regulate light from both fields until image
and pencil are equally distinct.

To Measure the Amount of Enlargement.

This is accomplished by means of a stage micrometer
(which as has already been stated consists of a very
fine scale which is ruled upon the glass slide) after the
drawing is made.

Remove the object and replace with stage micrometer.
Place so that one of the lines coincides with the outline

of the image at one side and indicate each line of the
micrometer on the drawing until the outline at the
opposite side is reached. The computation is made by
dividing the diameter of the drawing by the value of a
division of micrometer multiplied by the number of
divisions which are projected on the drawing. Thus
if the ruling on the micrometer is o.oi mm. and 10
divisions cover the outline of the drawing, while the
actual measurement of the drawing is 30.0 mm. the
formula would be3o-^-(i-ioox 10) = 300.

If a standard of 10 inches is maintained in all
drawings and the amount of magnification with certain
objectives and eyepieces be previously determined by
means of the micrometer, a standard is established for
each and further measurement will not be required.
If, however, variations from this standard distance are
made, the actual magnification should always be
determined.

The magnification is sensitive to slight variations
in the distance. The standard distance of 10 inches
is determined by measuring from the optical axis of
the microscope to the axis of the mirror of the camera
lucida and from this to the drawing surface.

Drawing Table. It will be noticed that great
stress has been laid upon the fact of keeping the draw-
ing surface at right angles to the axis of the projecting
cone of rays and that the drawing board must be tilted

to accomplish this. In order to maintain a standard
distance of 10 inches or to vary this distance to meet
requirements the drawing surface must be raised or
lowered. This can only be done by blocks or books
and the tilting by underlaying or by cutting a block
with properly inclined surface, which, however, is fixed
in height. All of these methods are crude and cumber-
some and easily liable to derangement. An excellent
substitute is offered in the drawing table, Fig. 58,

Fig. 58.

which while simple in construction and inexpensive is
very serviceable. Upon its base, the upper part of
which is hinged, the microscope is clamped. The
drawing board is fastened to a hinged attachment which
is adjustable for height. The pawl at the right provides
the necessary amount of inclination and the extent of
this is indicated by a quadrant at the hinge. An arm
rest is provided which may be moved along the front.

To Determine Magnifying Power. While
the magnifying power may be known from tables
accompanying the microscope, these are only approx-
imate, as there is more or less variation in eyepieces
and objectives and furthermore the microscope may be
used under different conditions from those under which
the original determinations were made.

To determine magnifying power three requisites
are necessary :

A camera lucida.

A stage micrometer ruled in divisions of inches or
millimeters.

A pocket or foot rule in inches or millimeters,
according to the stage micrometer which is used.

The stage micrometer is a glass slip having a very
fine scale ruled upon it. The lines are often so fine as
to be nearly invisible except under the microscope.

Place the stage micrometer with divisions ofo.oi mm.
upon the stage and focus. Attach the camera lucida,
place the microscope in exactly the same position as for
drawing, maintaining the standard distance of 10 inches
from optical axis to drawing paper ; mark the spaces
of the micrometer as projected upon the paper and
determine how many of the divisions of the rule are
contained within one or more spaces on the paper. If
the values are in millimeters and it should be found
that 25.0 mm. on the rule are contained in one space

on the paper, the magnification would be 2500. If 18.6
on the rule are contained within three spaces on the
paper, the magnification would be 620 times.

To Measure the Size of an Object. One of
the most valuable possibilities of the microscope is to be
able to accurately measure the actual size of a minute
object. Computations may be made in inches or milli-
meters by figuring 25.4 mm. equal to i inch. It may
be done by several methods, either of the following
being generally employed :

The first of these gives satisfactory results on
coarser objects and wherever the most accurate results
are not required, although it is somewhat inconvenient.
The requisites are :

A camera lucida.
A stage micrometer.

The object is placed upon the stage and after
focusing, the camera lucida is attached and the instru-
ment set up exactly as for drawing. On the drawing
paper the outlines of object, or those portions which are
to be measured, are marked. Without in the least dis-
turbing any of the conditions of tube length or distance
from the paper, remove the object and replace it by the
stage micrometer, focusing only with the fine adjust-
ment and, it may be added, there should be very little
variation in thickness between the two slides. Move

177

the micrometer so that one of its .lines shall exactly
coincide with one end of the drawing on the paper and
then measure off how many spaces are covered by
the object. Thus if o.ooi inch is the value of the
micrometer spaces and the object covers one space,
its size will be o.ooi inch, or covering seven spaces
will be 0.007 mch.

A variation of the distance of the camera lucida
from the paper, or a change of power in eyepiece or
objective does not vary the results so long as object
and micrometer are used under exactly the same
conditions.

The second method is with the eyepiece micrometer
or micrometer eyepiece. The eyepiece micrometer
consists of a circular disc of glass of suitable size to
just fit inside the tube of the eyepiece, resting upon the
diaphragm at the focus of the eye lens, scale side up,
or mounted in an oblong holder to be slipped into a
slot in the eyepiece mounting just above the diaphragm.
In case the lines on the eyepiece micrometer do not
show plainly, adjust the eye lens until they do. The
micrometer eyepiece, Fig. 45, in which the eyepiece
and micrometer form a complete apparatus and a
lateral adjustment of the scale across the field is given
by a screw, is much more convenient.

In either of these forms the ruled lines appear to
lie directly on the image of the object, but while we

have on the one hand the actual value of the micro-
meter we have on the other only the image of the
object. The value of the eyepiece micrometer in the
value of the stage micrometer must be first determined.
While the optician can do this, it should be done by
the manipulator on account of the varying conditions
of tube length, power of objective, etc. A stage micro-
meter ruled in the same values as the eyepiece micro-
meter is necessary.

Focus the eye lens
on the eyepiece micro-
meter and the objec-
tive on the stage mi-
crometer, being care-
ful to bring the first
line of the former
coincident with a line
of the latter, using
care to see that they
are parallel. As the
lines of the stage mi-
crometer will appear
to hare a certain

amount of thickness, make the first line of the eye-
piece micrometer correspond with one edge of a line on
the other. Now read off how many of the lines are
contained in one space of the stage micrometer and note
this. We will assume that there are eight divisions.

The eyepiece micrometer compared with the
stage micrometer.

I79

Replace the stage micrometer by the object to be
measured and bring one edge of the object coincident
with the first line of the eyepiece micrometer, being care-
ful to leave all the conditions unchanged. Note how
many divisions are required to cover the object and
divide by the figure first obtained with the stage micro-
meter.

Thus if an object
covers forty spaces
of the eyepiece mi-
crometer its real size
will be (40 ™ 8) x
o.oi mm. = 0.05
mm.

If measurements
are made under ex-
actly the same condi-
tions of tube length,
with same objectives,
it will be unneces-
sary to repeat the

operation with eyepiece and stage micrometer, as the
ratio remains constant and maybe marked on a card
for reference.

The most efficient apparatus, however, for obtaining
accurate results is theyf/^r or screw micrometer, Fig. 59.
This consists of a metal case to the upper surface of

Measuring an object with the eyepiece
micrometer.

180

which is fitted an adjustable Ramsden eyepiece. Within
is a frame carrying one or several delicate spider lines,
which are moved across the space to be measured by

Fig. 59.

an accurate screw of either 0.5 mm. or 1-50 inch pitch,
which has at its end a graduated disc divided in 100
or more spaces, thus giving a definite value for each

Field of Filar Micrometer.

space. An adapter is also provided for attaching to
the tube of the microscope.

181

TO SELECT A MICROSCOPE.

When one has concluded to obtain a microscope,
a suitable selection is a matter of considerable import-
ance to him. The varieties are innumerable, prices
without end, all sorts of claims made for them.

The variety of special lines of investigation involves
nearly as great a variety of requirements. The amount
of money to be expended ; what shall be the stand ;
what the objectives ; shall the entire outfit be purchased
at one time or little by little ; are all questions of par-
amount importance which the writer does not expect
to solve, but hopes to give sufficient information so that
a more intelligent selection may be made than might
probably be done otherwise.

If one has a friend or teacher, who is generally
accepted as an authority, it will be well to consult him
or her and obtain suggestions as to the most suitable
selection for the intended work, and such advice will
always be gladly given. Or, if advice is asked of a
reputable manufacturer, the writer is convinced that it
will be honestly and disinterestedly given.

182

When means will permit, the outfit for immediate
requirements should be obtained complete and as
Prof. Gage says, " the best that can be afforded should
be obtained," and further, " even when all the optical
parts cannot be obtained in the beginning it is wise to
secure a stand upon which they may all be used when
they are finally secured." The writer agrees entirely
with this advice. Even though the stand be plain, it
should be good, with the necessary adjustments and
capable of receiving and fully utilizing such further
accessories as may be obtained later on.

Stand. While one's sense of the aesthetic may be
a factor it is mainly the practical utility which must
govern the decision. \Vhether large or small must
largely be determined by the future use to which it is to
be put. One rule may apply to all, however, and that
is, that the instrument shall be so balanced that it will
be absolutely steady during manipulation in the upright
or inclined position. In general the low Continental
stand is preferred as it permits of resting the arms
upon the table while moving the object and a more
comfortable position while looking through the tube
whether the instrument be upright or inclined.

Tube Length. In. the matter of tube length the
optical results are the same in both, so that tube length
must be considered only in so far as it affects the

183

height of the instrument. The short tube length of
1 60.0 mm. is at the present time in general use.

Base. The base is an important feature and while
it should not be over heavy, should insure steadiness
by the proper form and disposition of metal ; it should
not rest on more than three points, with the rear one
fairly distant from the pillar.

The Joint for Inclination. This, without
question, is an advantage and while it is an inex-
pensive addition it will add considerably to the comfort
of working and should invariably be present, if
pecuniary considerations do not absolutely prohibit it.

Coarse Adjustment. Almost all reliable instru-
ments are provided with both fine and coarse adjust-
ments. The choice of the latter lies between the
sliding tube and the rack and pinion. The former has
only the advantage of economy and is a decided disad-
vantage in the hands of students who almost invariably
injure objectives and preparations with it. Further
than this, it is almost impossible for the maker to center
the nose-piece to the tube, so that a change of objective
usually throws an object out of the field and requires
that it be looked for anew with each change. With
the rack and pinion the nose-piece has an unvarying
relation to the tube and is not liable to this difficulty
and offers a steady and agreeable adjustment. The

184

advantages of the rack and pinion seem to be generally
appreciated in this country and there are few instru-
ments sold and used without it. Dr. Stokes speaks of
the sliding tube adjustment as follows :

" This is a very inconvenient and undesirable
arrangement. It is awkward, since the friction is often
so great that the whole stand will move out of position
before the body will budge, and frequently, more
frequently than not, even when the foot is heavy
enough to keep the instrument firmly on the table,
both hands are needed to manipulate the body. It is
dangerous too, since under certain circumstances the
body has the obnoxious habit of suddenly slipping
farther than the microscopist intends, stopping only
when it crashes against the slide, where it usually
grinds and crunches cover glass and objective with
apparently fiendish glee. A stand without a coarse
adjustment by rack and pinion is a good stand to be
permanently left with the optician. No fine micro-
scopical work can be done with an instrument whose
body slides through a friction collar. That arrange-
ment may be cheap, but it is also a torment and a peril."

Rack and Pinion. This should be absolutely
smooth with no back-lash or lost motion throughout its
entire length, which can be determined by holding the
main tube and working the pinion buttons very slightly
but quickly back and forth. It should be perfectly

185

fitted in its bearings, so that there will not be the least
side motion and this should be tested under the magni-
fying power of an objective. There should be no
sensation of the individual teeth coming in contact.
It is safe to assume that if the rack and pinion shows
either of the above defects, the instrument is faulty in
other directions as well.

Fine Adjustment. Nothing in the microscope
will cause more aggravation than a faulty fine adjust-
ment. It should work with absolute smoothness and
with no side play in the screw. The body should
respond promptly, when moving the milled head
rapidly forward and backward and should not cause
any swaying of the image during observation. The
micrometer screw should be back of the pinion, not at
the front of the tube nor under the stage.

Metal. Whether an instrument shall be of
japanned iron or lacquered brass is probably largely
determined by the amount of money to be expended.
So far as the intrinsic suitability of the metals is con-
cerned there is no difference. Brass, however, offers
the maker a better opportunity for displaying his
mechanical skill and while it is no doubt true that
many highly finished instruments are of poor work-
manship in their working parts, it is also a fact that a
well made instrument is always nicely finished.

1 8.6

Size and Weight. The size of instrument is
worthy of consideration. If an instrument is to remain
stationary in a practitioner's office or laboratory, it may
be large without being cumbersome. If, however, it is
intended to be carried about it should be of the smaller
and more contracted pattern.

WoHdng Space Below Stage. Another im-
portant consideration is the space between the stage
and base, or table. While it is advisable to have the
stage low on account of the convenience in manipulat-
ing a slide, there should still be sufficient space for the
convenient attachment of substage accessories. In this
respect the American instruments are superior as they
are built for the better accommodation of accessories.

Stage. A variety of stages is offered on instru-
ments of similar construction. The plain, flat stage,
while preferred by some, offers no advantages over the
ordinary round one, unless specially made for examining
specimens on larger slides than the standard 3 by i
inch. Those stages in which the upper surface is
covered with vulcanite offer many advantages. The
spring clips are usually of similar construction, although
varying in detail and curves. Properly constructed
clips should have such elasticity as to allow specimens
to be brought under them without resistance and keep
them properly in pkce, without too much pressure and
consequent friction.

The Mechanical Stage, while an absolute neces-
sity in petrographical and other work where a systematic
search, as for bacilli or in blood counting, over the
entire surface of object is required, will also be found
a most useful accessory. The obstacle of considerable
cost which formerly prevailed is now removed and
good mechanical stages may be obtained at a very
reasonable cost. They are supplied in two forms :

Fixed mechanical stage, in which the mechanical
movement is an integral part of the stage.

Attachable mechanical stage, which can be attached
to the Continental stands having plain stages. This
has advantages, since it may be removed, leaving the
stage plate free, but it cannot be revolved. Either
form involves the most delicate work and while the
parts are necessarily small should be built with a view
to strength and durability. They should work with the
utmost precision and smoothness and with absolutely no
lost motion.

Revolving Stage. While this is also a great
convenience in all work, it is a necessity in some
directions, and when provided with centering screws
may be used to some extent as a mechanical stage with
only a limited movement, however. It should work
freely with the rolling motion of one finger, without
any side play and without throwing the object out of
the plane of focus during revolution.

1 88

Substage. This is an absolute necessity in a
modern microscope, except perhaps for students' use
in primary work. It should have a vertical adjustment
and preferably with rack and pinion. If possible select
the complete sub stage attachment.

Substage Condenser. If means will permit,
purchase this, as it is in all work most convenient and
in some, bacteriology, etc., absolutely necessary. The
Abbe condenser is the cheapest form giving good
results and one with numerical aperture of 1.20 is
sufficient in all cases unless oil immersion objectives of
the greatest aperture are used.

Objectives and Eyepieces. It is hoped that
the information given of the various qualities in an
objective will aid to make a suitable selection of the
optical parts. Since the stands have been classified as
of long and short standard tube lengths, the first quality
to look for is, after the stand has been selected, the
suitability of objective and eyepiece to it and to the
work. A variety of powers is obtained by a suitable
combination of eyepieces and objectives and while
a considerable increase in power can be obtained by
short focus eyepieces this is not advantageous.

Eyepiece. Select the Huyghenian eyepiece and
no higher power than 3-4 inch. In catalogues many
outfits are made up of one eyepiece and two objectives,
but this is only for economy ; it is always advisable to

189

select two eyepieces, preferably the 2 inch and i inch
and insist that they be parfocal, as this will be found
extremely convenient and will not disturb the optical
standard length. If for any work 1-2 inch or higher '
powers are desired, the solid eyepiece may be recom-
mended. With the apochromatic objectives use the
compensating eyepieces only. Every eyepiece should
be marked with its equivalent focus.

Objectives. For all ordinary student and pro-
fessional work, not involving bacteriological investiga-
tions, the 3-4 inch 0.22 N. A., and 1-5 inch 0.62 N. A.
for long tube ; and 2-3 inch 0.24 N. A., and t-6 inch
0.85 N. A. or 1-8 inch 0.93 N. A., for short tube have
generally been accepted as best suited.

Bacteriological investigations absolutely require an
oil immersion objective in which the 1-12 inch of 1.32
N. A. is generally employed.

Botanical work necessitates a 2 inch or 3 inch in
addition to the regular outfit.

For urinary work and blood counting where a
considerable working distance is an absolute necessity
the 1-5 or 1-6 inch of reduced aperture and long working
distance are recommended.

Objectives of Wide Aperture. It will be
noted that objectives of the lowest price and lowest in
the scale of efficiency have been recommended as
ample for ordinary use, but it is wrell to bear in mind
or study the advantage which is obtained by objectives

190

of larger angular aperture. These advantages are
absolute and unquestionable, but whether commensurate
with the additional pecuniary outlay must be left mostly
to the judgment of the purchaser. That he may be
somewhat guided, we may say that the selection of
higher or highest grade objectives is not by any means
exceptipnal, but general, and would undoubtedly be
more common but for the barrier of expense.

In these days of competition, prices alone are too
often made the inducement without any reference to
quality. Be distrustful of all such objectives and if
contemplating their purchase, always reserve the right
of having them examined by an expert. Have a dis-
trust especially of all " nameless " objectives. It is
safe to assume that if the maker cannot attach his
name he is doubtful of their quality.

It is sometimes found that dealers offer the same
objectives of different quality at different prices. Too
great care cannot be observed in such cases, as the
very fact of the admission of a difference in quality
indicates that they are made by an unreliable maker.
This mode of offering objectives was in vogue many
years ago when the principles of optics and the facilities
for making were limited and when a higher price was
asked for those which might be termed a happy com-
bination. There is no excuse, however, at the present
day, for anything of this kind, because every con-
scientious optician has his standard for every objective.

191

In purchasing a microscope a beginner may be
easily misled by the enticing appearance of an object,
which may be due not so much to the instrument as to
the object itself and if the optical parts are inferior, it
will require but a short experience to become convinced
of it — usually as soon as a comparison can be made with
reliable work. The investment in one of these objec-
tives is not only a source of disappointment, but usually
proves to be a pecuniary loss, as it is generally followed
by a fresh outlay in responsible work. It is of ordinary
occurrence that such objectives have been sent to the
writer's firm with the request to examine them and
rectify the faults ; but an examination almost invariably
proves that the cost of doing so is considerably greater
than that of a new objective of the same power and it
would not even then be equal to the latter.

Accessories. We have already stated in the
body of this book which kinds of accessories are con-
sidered useful. Some of them are absolutely necessary
in some special lines of work, in which case, however,
the student is generally conversant with the require-
ments and may make a suitable selection, but for all
general purposes some accessories are necessities where
others are only conveniences and we append a list of
such which, unless prohibited by necessity, should
accompany each outfit.

Abbe substage condenser, preferably the complete
substage attachment giving all adjustments.

192

Double, triple or quadruple nose-piece according to
the number of objectives accompanying the microscope.

Abbe camera lucida.

Revolving or attachable mechanical stage.

Eyepiece micrometer and stage micrometer.

Mounted objects, Proboscis of Blow-fly and P.
angulqtum, dry.

Pocket magnifier, preferably Aplanatic or Hastings
triplet.

Cover glass gauge.
Flat-wick oil lamp.
Dissecting stand or dissecting microscope.

Besides these there are other requirements such as
slides, covers, mounting media, forceps, etc., the neces-
sity of which, however, will be suggested by teachers
or can be determined from books devoted to this
purpose. There are other articles which in some
directions are necessities, but are general conveniences,
among which may be mentioned :

Adjustable drawing table.

Polariscope.

Bull's Eye Condenser.

Photomicrographic camera.

Live box or compressor.

Turn table.

Revolving microscopical table.

Cabinet for objects.

CARE OF A MICROSCOPE.

Besides acquiring the ability to properly use an
instrument with its accessories, it is important to know
how to keep it in the best working condition. It may
be said without reserve that an instrument properly
made at the outset and judiciously used should hardly
show any signs of wear either in appearance or in its
working parts, even after the most protracted use ; and
further than this, every good instrument should have a
provision for taking up lost motion, if there is a likeli-
hood that this may occur in any of the parts.

Especial care should be given to the optical parts,
in fact such care that they will remain in as good con-
dition as when first received, after any amount of use.
Accidental injury may occur, but is quite unlikely if a
systematic manner of working is followed, if a special
receptacle for each part is provided.

Do not allow any person except your teacher to
manipulate your microscope or accessories. One
person may be expert in the manipulation of one
instrument and still find it difficult to work with
another. The fine adjustment particularly causes the

194

greatest difficulty, as in some instruments the move-
ment of the fine adjustment is in a direction opposite
to that of the micrometer screw, and thus the objective
as well as the object is endangered.

If the microscope is to be carried any distance it
should be done in its case.

Avoid exposure of the microscope to direct sunlight
and extreme or sudden changes in temperature. If by
chance the microscope should have become very cold,
as during transportation in winter, allow it to warm
gradually.

Care of the Stand. Keep free from dust is one
of the first rules to be observed. When not in use place
the microscope in its case or cover with a bell jar or
close mesh cloth such as cotton flannel or velvet which
should reach to the table. If the case will not receive
the entire outfit, remove the double or triple nose-piece,
if these form part of it, and place objectives in their
cases. If dust settles on any part of the instrument
remove it first with a camel's hair brush and then wipe
carefully with a chamois skin, wiping with the grain of
the finish of the metal and not across it, as in the latter
case it is likely to cause scratches.

When handling the stand, grasp it by the pillar or
stage. While the arm is the most convenient part it is
at the same time the most dangerous to the fine
adjustment.

Avoid sudden jars, such as placing upon the table
or into the case with force.

Remove any Canada balsam or cedar oil which may
adhere to any part of the stand with a cloth moistened
with benzole and wipe dry with chamois.

Use no alcohol on any part of the instrument as it
will remove the lacquer.. As the latter is for the
purpose of preventing oxydization of the metals, it is
important to observe this rule.

To use the draw-tube impart the spiral motion.

To lubricate any of the parts, use a slight quantity
of soft tallow or good clock oil, or paraffine oil.

If the pinion works loose from the jar incident to
transportation or long use, which sometimes occurs to
such an extent that the body will not remain in position,
increase its tension by tightening the screws on pinion
cover.

Occasionally withdraw the tube from the arm, wipe
clean and lubricate both slides. This is highly important
as the slides being constantly exposed become dusty
and the lubricant is inclined to gum.

Apply a small quantity of soft tallow or good clock
or paraffine oil to a cloth, wipe well over the surfaces
and remove the superfluous amount with a dry cloth or
Japanese paper. If the lubricant becomes gummy,
remove by wiping with a small quantity of benzine or
benzole applied to cambric.

196

Do not apply oil or grease to the rack or pinion
as this will act as a dirt catcher and wear out the teeth
at the points of contact.

/;/ inclining the stand always grasp it by the arm
and never by the tube, as in the latter case it may
loosen the slide or tear off some of the parts.

In using a screw driver grind its two large surfaces
so that they are parallel and not wedge-shaped, so it
will exactly fit in the slot of the screw-head. Turn the
screw with a slow steady motion pressing the screw-
driver firmly into the slot. No screw-head will ever
be injured if these points are observed.

When repairs or alterations are necessary, always
have these made by the manufacturers who can, from
a system of duplicated parts, do it not only cheapest,
but best.

Joint for Inclination. If the joint should
become loose so as to prevent the arm being set at
any angle of inclination, it should be tightened by
drawing up the nut at one or the other side. If the
nut has screw slot use a properly prepared screw
driver, but if two holes a suitable key should be
obtained from the maker. In high grade instruments
the axle is generally tapering, and to determine which
nut is to be drawn up can only be done by trial.

'97

Care of the Coarse Adjustment. Special
care should be given to keep the coarse adjustment
free from dust as its effect is particularly pernicious.
The slides and rack and pinion are necessarily exposed
and the lubricant is apt to catch dust and also to gum.
The tube should be occasionally withdrawn from the
arm and the slides carefully wiped with a cloth
moistened with benzole. Lubricate by applying a
small quantity of soft tallow or paraffine oil to a cloth
and wiping well over the surfaces, removing the super-
fluous amount with a dry cloth. The teeth of neither
rack nor pinion should ever be lubricated. An
occasional cleaning of the teeth with an old tooth
brush is advisable.

It is advisable occasionally to lubricate the pinion
shank on both, sides of the arm with a very minute
quantity of paraffine oil.

If the pinion works loose from jar incident to trans-
portation or long use, which sometimes occurs to such
an extent that the body will not remain in position,
increase the friction upon it by tightening the screws
on the pinion cover.

Fine Adjustment. In a general way it may be
said that if the fine adjustment ceases to work satis-
factorily the instrument had better be returned to the
maker, as it involves the most delicate working and
few people are conversant with its construction. There

198

is very seldom any occasion for this, however, if used
with reasonable care.

If the fine adjustment does not respond to the
turning of the micrometer screw, or if it comes to a
stop, it indicates that the adjustment screw has come
to the limit of its motion at either end. It should by
no means be forced ; it should at all times be kept at
a medium point.

The micrometer screw should never be removed
unless after long use it works with a pronounced gritty
feeling. In this case unscrew from its bearing, wipe
clean with a cloth moistened with benzole, and after
wiping dry apply good tallow, being careful to start the
threads properly. If they are not properly started
much mischief may be done. In some instruments the
threads are left handed. In removing the screw
observe whether there is a small steel pin in a recess
in it, and if so be careful that this is in proper position
when returned or else the fine adjustment will be
inoperative.

Screw- Driver. Ordinary screw-drivers are not fit
for use on the microscope. A properly made screw-
driver should be ground on its two large surfaces so
that they are parallel and not wedge shaped, so that it
will exactly fit in the slot of the screw head. In using,
turn the screw with a slow, steady motion, pressing the
screw-driver slowly into the slot. No screw head will
ever be injured or marred if these points are observed.

199

Care of Objectives and Eyepieces. Every
outfit should be provided with a camel's hair brush
and a well washed piece of linen. On account of its
fine texture chamois skin is desirable, but only after it
has been repeatedly washed. No dust should be per-
mitted to settle upon nor should the fingers come in
contact with any of the surfaces. Occasional cleaning
is desirable even when they (o and e p ) are not used,
as a film settles upon the outer as well as the inner
surfaces of the eyepiece and the rear surface of the
objective, and creates a cloudiness in the image.

When not in use objectives and eyepieces should
be kept in their receptacles. If objectives are left
attached to the microscope either singly or on revolv-
ing nose-pieces, leave the eyepiece in the tube so that
no dust can enter and settle upon the rear lens of
the objective.

Objectives especially should be kept where they
are not subject to extreme and sudden changes of
temperature as the expansion and contraction may
cause the cement between the lenses to crack. Also
avoid direct sunlight, as the cement may soften
sufficiently to ooze out.

Eyepiece. Visible defects in the field are always
traceable to impurities in the eyepiece, not in the
objective, and are easily recognized by revolving it.
Indistinctness in the image or loss of light may be due

to soiled or coated surfaces in either eyepiece or
objective.

Dust if on either the eye-lens or field-lens is apparent
as dark, indistinct spots.

To dean the surfaces, breath upon them and,
giving a revolving motion to the eyepiece, wipe with
well washed linen and finally blow upon the surface, or
use camel's hair brush to remove particles of lint.

At regular periods unscrew the eye-lens and field-
lens and clean the inner surfaces.

Objective. This should be used with the utmost
care. The systems should never be separated, even if
they can be unscrewed, as they are liable to become
decentered and dust may enter.

Avoid all violent contact of the front lens with the
cover glass. The oil immersion objectives are partic-
ularly sensitive and easily ruined.

Screw into the nose-piece and unscrew by grasping
the, knurled edge and keeping in line with the tube.

Occasionally examine the rear surface of the objec-
tive with magnifier and if dust be present remove with
camel's hair brush.

Clean an immersion objective immediately after it
has been used by removing the fluid with moist cloth
and wiping clean with dry cloth or lens paper.

While cleaning give the objective a revolving motion.

201

If the immersion oil should have become thick, or
any substance adheres to the surface, which cannot be
removed by wiping, apply a small amount of benzine
to a cloth and wipe carefully but quickly, so that the
fluid will not affect the setting of the lens. Wipe clean
with dry cloth.

Do not apply alcohol to objectives under any con-
dition.

If any part of the microscope cannot be brought to
a satisfactory working condition by the foregoing
instructions, or any part is injured by accident it should
invariably be sent to the maker or to a well known
manufacturer of microscopes.

INDEX.

Abbe Camera Lucida, 168—173, 193

Optical Construction of, 168

Abbe Condenser, 154

Abbe Test Plate, 142

Aberration, Chromatic, 10, 85

Correction of, 12

Spherical, 9, 12, 88, 138

Accessories to Microscope, 192

Achromatic Combination or Lens, 13

Adjustable Objectives, 64, 93, 143

Adjustments, Coarse and Fine, 32, 45, 48

Care of, 198

Closed and Open, 93, 143

Amplification, 100

Angle, 70, 72

Angular Aperture, 70, 80, 101

How to Measure, 80

Anterior System of Objective, 66

Apertometer, 82

Aperture, 72, 162

Numerical, 77

Of Condenser, 162

Aplanatic Lens, 13

Aplanatic Triplet, 16, 23

Hastings', 16, 23

Apochromatic Objectives, 102

Apparent Aperture, 163

Areas Magnification, 24

Arm of Microscope, 29

Astigmatism, 131

Attachable Mechanical Stage, 41, 188

Axis, Optical, 34

Principal, 5

Barnes Dissecting Microscope, 20

Base of Microscope, 29, 51, 184

Binocular Eyepiece, 110

Binocular Microscope, 44

Blood Counting, 40, 190

Bodies or Tubes, 44

Body of Microscope, 29

Bruecke Lens, 17

Bull's Eye Condenser, 153, 193

Burning Point, 7

Cabinet for Objects, 193

Camera Lucida, 167 — 177

Camera Lucida, Abbe, 168—173

Drawing Table for, 175

Field and Magnification with, 172

How to Use, 172, 173

Illumination with, 170

Measuring Magnification with, 176

Measuring Size of Object with, 177

Optical Construction of, 169

Simple Form, 167, 172

Cap Diaphragms, 54, 131

Care of Microscope, 194

Cedar Oil, 61, 75, 146

Centering Screws, 33, 39

Centering the Condenser, 159

Centering the Illumination, 160

Chromatic Aberration, 10, 85

Correction of, 86

To Judge, 86

Classification of Microscopes,

Clips,

Closed Adjustment, 93, 143

Coarse Adjustment, 32, 45, 184

Care of, 198

Coddington Lens, 15

Collar, 31

Collar Correction, 143

Collective or Field Lens, 58, 107

Compensating Eyepiece, 103, 111

Complete Substage, 156—158

Compound Microscope, 1, 27

Concave Lenses, 4, 6

Concavp Mirror, 121, 152

Focal Point of, 122

Use with Condenser, 159

Condenser, 122, 154—166

Aperture of, 162

Bull's Eye, 153

Centering of, 159

Focusing of, 161

Illumination with, 164

Oblique Light with, 164

Purpose of, 154

Substage, 156

Use with Immersion Objectives, 162

Continental Type Eyepiece, 107

Continental Type Microscope, 35

Convex Lenses, 4, 5

Focal Length of, 5, 6

Correct Position at the Microscope, 115

Corrected Lens, 13

Cover Glass, 34, 74

Abbe Test Plate for Measuring, 142

Correction for, 88

Influence upon Objective, 140

Influence upon Angular Aperture, 74

Influence upon Spherical Aberration, 88

Sizes of, 141

Standard, 91

Thickness to Use, 93

Cover Glass Gauge, 93, 193

Crown Glass in Achromatic Combination, 13

Defects in Eyepieces and Objectives, 112

Diameters Magnification, 24

Diaphragm, 10, 33, 129

Cap, 54

Eyepiece, 107

Iris, 54, 157, 158

Revolving, 53

Substage, 53, 129, 157, 158

Use of, 129, 163

Diatoms as Test Objects, 133

Dispersion of Light by Prism, 11

Dissecting Microscopes, 18 — 21, 193

How to Use, 21

Double Nose-piece, 43, 118

To Focus with, 127

Double System of Lenses, 66

Doublet Magnifier, 15

Draw-Tube, 31, 49

To Operate, 50, 114

Drawing Objects with Camera Lucida, 167

Drawing Table, 174, 175

Dry Adjustable Objectives, 143

Dry Objectives, 60, 75

English Type Eyepiece, 107

Erecting Eyepiece, 110

Equivalent Focus, 62

Excelsior Dissecting "Microscope, 20

Eye, Which to Use, 21, 131

Eye-lens, 58, 107

Eyepiece or Ocular, 31, 55, 107—110, 189

Binocular, 110

Care of, 200

Compensating, 103, 111

Continental Type, 107

Eyepiece or Ocular, Defects in, 112

English Type, 107

Erecting, 110

How to Attach, 116

Huyghenian, 107

Index, 111

Kellner, 109

Micrometer, 109, 110, 178, 181

' Negative, 107

Orthoscopic, 109

Parfocal, 106

Periscopic, 109

Projection, 109, 112

Rams den, 109

Rating of, 104

Searcher, 111

Selection of, 189

Solid, 108

Eyepiece Micrometer, 179, 193

Eyepoint, 132, 105, 106

Field of View, 16, 17, 95, 101, 104

Field, Flatness of, 95

Field-lens, 58, 107

Filar Micrometer, 180

Finding the Object, 118

Fine Adjustment, 32, 48, 186

Care of, 198

Focusing with, 128

Fixed Mechanical Stage, 41, 188

Flatness of Field, 95

Flint Glass in Achromatic Combination, 13

Focal Length, 5

To Determine, 6

Focal Point, 5

Of Mirror, 53, 122

Focus, Equivalent, 62

Focus, Principal, 5

Virtual and Real, 6

Focusing the Condenser, 161

Focusing the Objective, 124

Focusing with Fine Adjustment, 128

Glass, Crown and Flint, 13

Jena, 57

Glass Stage, 39

Graduated Head, 48

Graduated Stage, 38

Hastings' Aplanatic Triplet, 16

Head, Micrometer Screw, 48

High Power Objectives, . 63, 99

How to Focus, 125

Test Objects for, 133

Holders, Stands and Dissecting Microscopes, 18

Homogeneous Immersion Fluid, 61

Homogeneous Immersion Objectives, 60, 75, 146

Huyghenian Eyepieces, 107

Illuminating the Object, 23, 120, 124, 159, 164

Illuminating Objectives, 64

Illuminating Opaque Objects, 152

Illumination with Substage Condenser, 154

Image, Magnified, 27

Real and Virtual, 8, 59, 105, 106

Immersion Fluid, 61

Immersion Objectives, 60. 75, 146

Applying Oil to, 146

Test Plate with, 147

Index, 48

Index Eyepiece, 111

Iris Diaphragm, 54, 129, 157, 158

Jena Glass, 57

Joint for Inclination, 52, 184, 197

Kellner Eyepiece, 10

Lens Holders or Stands, 18

Lens Systems, 66

Lenses, How Made, 67

Magnifying, 14 — 24

Optical Properties of, 1

Simple, 4

Stopped down, 10

Linear Magnification, 24

Linen Tester, 19

Lister's Method for Measuring Angular Aperture, 81

Low Power Objectives, 62, 99

How to Focus, 125

Text Objects for 132

Lower Iris Diaphragm, . 158

Magnifier, Tripod, 18

Magnifiers, 14-24, 193

Caution in Purchasing, 16, 24

How to Use, 21

Magnification with Camera Lucida, 172, 173

Magnifying Power. 7, 21, 24, 60, 84, 99

To Determine, 25, 176

Measuring the Size of an Object, 177

Mechanical Stage, 40, 120, 188, 193

Medium Power Objectives, 62, 99, 136

Test Objects for, 133

Meniscus Lenses, 4

Metal for Microscope, 186

Micrometer, Eyepiece, 178

Filar, 180

, Stage, 177

Micrometer Eyepiece, 109, 110, 178, 181

Micrometer Screw Adjustment, 32, 48, 199

Microscope, Binocular, 44

Care of, 194

Caution in Purchasing, 24

Classification of, 34

Microscope, Compound, 1, 27, 59

Continental Type, 35, 183

Correct Position at, 115

Dissecting, 18 — 21

How to Set up,

How to Work with, 114

Jackson Model, 34

Material of, 28, 186

Monocular,

Parts of, 29

Purpose of,

Ross Model,

Selection of, 182

Simple, 1, 14

Size and Weight of,

Microscopical Table, Revolving, 193

Middle System of Objective, 66

Milled Heads, 32

Mirror, 33, 52, 120, 157

Focal Point of,

Use with Condenser, 159

Mirror Bar, 33, 52

Negative Eyepiece,
Nose-piece, 31, 42, 193

Revolving,
Numerical Aperture,

How to Compute,

How to Measure,

Of Condenser, 155

Object, 34, 132

Finding under the Microscope,

How to Draw,

Measuring Size of,

Opaque,

To Illuminate, 120

Test, 132, 192

Object, What to use as, 132

Objective, . 31, 55, 60, 99, 189, 190

Adjustable, ' 64, 143

Angular Aperture of, 70

Apochromatic, 102

Care of, 69, 200, 201

Caution in Choosing, 190

Color Correction in, 86

Construction of, 67

Defects in, 112

How to Focus, 124

How to Attach, 117

Immersion, 60, 75, 146

Illuminating, 64

Influence of Cover Glass on, 93

Photographic Microscope, 64

Powers of, 62, 99

Projection, 65

Quality of, 70

Rating of, 62

Searcher, 126

Special, 64

Systems of, 65

Variable, 64, 143

Wide Aperture, 190

Oblique Light with Condenser, 164

Ocular, 31, 55, 104, 200

Oil Immersion Objectives, 60, 75, 99, 146

Opaque Objects, 151

Open Adjustment, 143, 145

Opening or Aperture, . 73

Optical Axis, 34

Optical Glass, 57, 67

Optical Properties of Lenses, 1

Orthoscopic Eyepiece, 109

Over-Correction of Objectives, 86

Over-Correction of Objectives, To Judge, 87

Parfocal Eyepieces, 106

Penetration 94

Periscopic Eyepiece, 109

Photographic Microscope Objectives 64

Pillar, 29

Pinion, 32, 198

Plane Mirror, 52, 121

Use with Condenser, 159

Plankton Work, 40
PleurosigmaAngulatum, 100,133,134,136,139,144,148,193
Pocket Magnifiers, 14—24, 193

Positive Eyepiece, 107

Posterior System of Objective, 66
Powers of Objectives, 62, 99, 101

Primary Colors, 11

Principal Axis, 5

Principal Focus, 5

Prism, Dispersion of Light by, 11

Refraction of Light by, 2

Projection Eyepiece, 109, 112

Projection Objectives, 65

Quadruple Nose-piece, 43, 118

Quadruple System of Lenses, 66

Rack and Pinion, Substage, 157, 158
Rack and Pinion Coarse Adjustment, 32, 46. 183

Rack and Pinion Mechanical Stage, 41

Radius of Curvature, 5

Ramsden Eyepiece, 109

Rating of Eyepieces, 104

Rating of Objectives, 62

Reading Glass, , 17

Real Focus, 6
Real Image, 8, 58, 105, 106

Refraction, 2

Resolving Power, 82, 101

Test Objects for Determining, 132

Revolving Diaphragm, 53, 131

Revolving Nose-piece, 43

' To Focus with, 127

Revolving Stage, 38, 188

Screw Driver, 199

Screw Micrometer, 180

Searcher Eyepiece, 111

Searcher Objective, 126

Secondary Spectrum, 85, 102

Simple and Compound Microscopes, 1, 14, 27

Simple Lenses, 4

Magnifying Power of, 25

Single System of Lenses, 66

Size and Weight of Microscope, 187

Slide or Slip, 34

Slide Carrier, 39

Sliding Tube Adjustment, 45, 184

To operate, 128

Slow Motion, 48

Society Screw, 31

Solid Eyepiece, 108

Special Objectives, 64

Spectrum, 11, 85, 102

Spherical Aberration, 9, 88, 96

Influence of Cover Glass on, 88

To Judge, 138

Spring Clips, 33, 187

Stage, 18, 32, 187

Construction of, 37

Glass, 39

Mechanical, 40, 120, 188

Revolving, 38, 188

Working Space Below, 187

Stage Micrometer, 176, 177, 179, 193

Stand, 18, 28

Care of, 195

Selection of, 183

Striae, 112

Substage, 33, 156

Complete, 157

Substage Condenser, Illumination with, 154

Substage Diaphragm, 53

Use of, 129

Systems of Objectives, 65, 99

Table, Drawing, 175, 193

Test Objects, 132

Test Plate, 133, 147

Judging Spherical Aberration with, 142

Use with Immersion Objectives, 147

Triple Nose-piece, 43, 118

Triple System of Lenses, 66

Triplet Magnifiers, 16

Tripod Magnifier, 18
Tube Length, 35, 36, 61, 183

Tubes or Bodies, 44

Turn Table, 193

Under-Correction of Objectives, 86

To Judge,

Universal or Society Screw, 31

Upper Iris Diaphragm, 157

Variable Objectives, 64

Virtual Focus, 6
Virtual Image, 8, 58, 105, 106

Water Immersion Objectives,

Working Distance, 17, 96

To Measure,
Working Space below Stage,

14 DAY USE

RETURN TO DESK FROM WHICH BORROWED

This book is due on the last date stamped below, or

on the date to which renewed.
Renewed books are subject to immediate recall.

QCT101960

QCT 3 1 19D0

General Library

University of California

Berkeley

UNIVERSITY OF CALIFORNIA LIBRARY

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "Manipulation of the microscope"

See other formats