,%m UC-NRLF

Manipulation
of the Microscope

EDWARD BAUSCH

LIBRARY

UNIVERSITY OF CALIFORNIA.
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^Accessions No. //&> ~ QMS No.

MANIPULATION

. OF THK

MICROSCOPE

BY

EDWARD BAUSCH

THIRD EDITION
FIFTEENTH THOUSAND

PUBLISHED BY

BAUSCH & LOME OPTICAL COMPANY,
ROCHESTER, N. Y.

GA\

Aim. Bit*

.

Copyright
KAI'SCII & LOMB OTTICAL COMPANY

PREFACE TO FIRST EDITION.

It may seem to some persons an act of presump-
tion for a maker of microscopes and microscopic
accessories to enter the field of authorship and
attempt to supplement the valuable labors which
in recent years have made the use of the micro-
scope 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 method of technique,
even among owners of microscopes. Indeed, with
so many complications, with almost unlimited pow-
ers and uses of the instrument, the beginner can-
not 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 supposi-
tion 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.

OF

UNIVERSITY

PREFACE TO SECOND EDITION.

The demand for this book having considerably
exceeded the expectations of its author and the
comments on its utility having been so favorable
leads 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 sub-
jects 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 considered it beyond the province of his inten-
tion 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 experi-
enced 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
appearing 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
accessories and while it is not expected to be a
complete guide, it is, nevertheless, hoped that it
will make the labor of the beginner more easy.

Since its first issue there have appeared two
books covering the same purpose: " The Micro-
scope and Microscopical Methods " by Prof. S. H.
Gage of Cornell University and " Microscopical
Praxis " or " Simple Methods of Ascertaining the
Properties of Microscopical Accessories " by Dr.
A. C. vStokes, both of which are heartily commended
to the microscopist. 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 microscope
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 1, 1897.

CONTENTS.

Purpose of the Microscope; Simple and Compound Micro-
scopes; Lenses; To Determine the Focus of a Lens;
Magnifying Power; Spherical Aberration; Chromatic
Aberration.

SIMPLE MICROSCOPES.

Coddington Lenses; Aplanatic Triplet; Hastings' Triplet;
Reading Glass; Bruecke Lens; Stands and Holders;
How to Use Magnifiers and Dissecting Microscopes;
Magnifying Power; To Determine Magnifying Power.

THE COMPOUND MICROSCOPE.

Description of Parts; A, Base; B, Pillar; C, Arm; D, Body;
E, Nose-Piece; Society Screw; F, Objective; G, Eyepiece
or Ocular; H, Draw-Tube; I, Collar; J, Coarse Adjust-
ment; K, Milled Heads; L, Fine Adjustment; M, Stage;
N, Clips; Centering Screws; O, Mirror; P, Mirror-Bar;
Q, Substage; R, Substage Bar; S, Diaphragm; Optical
Axis; Object; Slide or Slip; Cover Glass; Classification
of Microscopes; Tube Length; Stage; Revolving Stage;
Glass Stage; Mechanical Stage; Nose-Piece; Double
Nose-Piece; Bodies or Tubes; Coarse Adjustment; Fine
Adjustment; Draw-Tube; Base; Joint for Inclination;
Mirror and Mirror-Bar; Diaphragm.

OBJECTIVES AND EYEPIECES.

Objectives; Tube Length; Nomenclature, or Rating of
Objectives; Powers; Systems; Angular Aperture ; Cover
Glass; How to Measure Angular Aperture; Resolving
Power; Chromatic Aberration ; Spherical Aberration and
Cover Glass; Penetration; Flatness of Field; Working-
Distance; To Measure Working Distance; Magnifying
Po\ver; Apochromatic Objectives; Eyepiece or Ocular;
Huyghenian Eyepiece; Solid Eyepiece; Ramsden Eye-
piece; Periscopic, Orthoscopic and Kellner Eyepieces;
Compensating Eyepiece; Rating or Designation; Flat-
ness of Field; Size of Field; Defects.

HOW TO WORK.

To Attach Eyepiece; To Attach Objective; Finding an
Object; To Illuminate the Object; How to Focus; Which
Eye to Use; What Objects to Use; Test Plate; How to
Work; Opaque Object; Medium Power of Objective;
Chromatic Aberrations; Cover Glass; Dry Adjustable
Objectives; Immersion Objectives; Immersion. Objec-
tives on Test Plate.

ILLUMINATION WITH CONDENSER.

Purpose of the Condenser; Abbe Condenser; Centering;
Centering the Illumination; To Focus Condenser; Rela-
tion of Aperture of Condenser to Objective; Oblique
Light with Condenser.

HOW TO DRAW OBJECTS.

To Determine Magnifying Power; To Measure the Size of
an Object.

TO SELECT A MICROSCOPE.

Stand, American or Continental; Tube Length; Base; The
Joint for Inclination of Arm; Coarse Adjustment; Rack
and Pinion; Fine Adjustment; Metal; Size and Weight;
Working Space Below Stage; Stage; A Glass Stage and
Slide Carrier; The Mechanical Stage; Revolving Stage;
Substage; Substage Condenser; Objectives and Eye-
pieces; Eyepiece; Objectives; Objectives of Wide
Aperture; Accessories.

CARE OF A MICROSCOPE.

To Take Care of a Stand; To Take Care of Objectives and
Eyepieces.

INTRODUCTORY.

The knowledge which this volume attempts to
convey in the proper use of the microscope, may
be gleaned by following several methods.

First. — By reading carefully all contents from
first to last. This is strongly recommended, as an
effort has been made to make the contents progres-
sive and it is hoped that in this manner a general
knowledge of terms and optical principles involved
will be acquired, which will greatly aid in intelli-
gently and with greater facility and pleasure
carrying out the instructions in the chapter " How
to Work."

Second. — By reading the general principles and
studying the various terms and omitting some of the
instructive processes, as " How to Judge Chromatic
and Spherical Aberration," " How to Measure
Angle," " How to Measure Working Distance,"
etc., which may be carried out after some famili-
arity with manipulation of the instrument has been
acquired.

Third. — By studying the parts of the stand and
then referring to chapter " How to Work." This
is not in any case advised, but may be done when

14

there is a pressing- requirement for the use of the
instrument, or great anxiety to operate it. The
previous chapters should then be studied later on.

Fourth. — If the reader does not possess a micro-
scope and desires to purchase one, study the parts
of the stand, general principles and " How to
vSelect a Microscope."

OPTICAL PROPERTIES OF LENSES.

Purpose of the Microscope. — The microscope
is an instrument which magnifies near 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: With the simple microscope the object
is viewed directly and the magnified image shows
the object erect, or in its real position. It must
consist of one lens, but may consist of more, which
however are in close contact. With the compound
microscope a magnified image is observed and this
in a reversed position from the true one of the
object, so that what is right in the object is left in
the image. This must consist of at least two
lenses with suitable distance between them.

Lenses. — As microscopes depend upon the
action of lenses it seems fitting that these, as
well as the action of light passing through them,

16

should receive attention. Every person has un-
questionably 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 great principle of actually
bending 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 different direction than their
real one. The amount of this deviation is absolute
and depends for the one factor upon the density of
the glass composing the prism and for the other
upon its shape.

Fig. i.

Fig. 1 represents a cross section of a prism
and shows how the ray, as it touches the first
surface of the prism, undergoes refraction and on

17

its emergence from the prism, its second refraction
takes place; it will also 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 its course will be upward, always
toivard its base.

Now a lens in either of its two principle forms is
nothing more in effect than the prisms showing
as in Fig. 2, where the two bases are placed

together, how the light is refracted toward the
bases and thus converges; or how, as in Fig. 3,
where the bases are above and below, the action of

18

the prisms on the light still remains the same,
refracting the rays of light toward the bases, and
in this case causing them to diverge or separate.

If the combinations of prisms be imagined with
curved surfaces instead of flat ones, the action of
light through them will be precisely the same and
we have the two great classes of lenses, converging
or convex and diverging or concave, as shown
in Fig. 4.

Fig. 4.

These two classes are again subdivided, as
shown in Fig. 5, showing the three forms which

Fig. 5.

are made in each of the convex and concave types
and which are called in rotation' as drawn, double

convex ', plane convex, convex meniscus and double
concave, plane concave, concave meniscus.

In the convex lenses, parallel rays are converged
to one point as shown in Fig. 6, in which c is

Fig. 6.

the principal focus, or focal point and the distance
from b to c the focal distance. , With the same
lens, if a flame be placed at c, all the rays which
strike the lens will be emitted in a parallel direc-
tion 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 line a c' the
radius.

The radius determines the power or converging
quality and as it becomes longer, reduces the
power by making the lens longer focus. If in a
double convex lens the radius of each surface is
1 inch, the lens has a focus of 1 inch and if
in a plane convex lens, the convex surface has the
same radius, the focus is twice as long, or 2 inches.

In a concave lens the action of the lens is oppo-

so

site; instead of converging the light toward the
axis, it diverges the light from the axis as shown
in Fig. 7. The imaginary extension of the

,£•

diverging rays should meet at c and the distance
e d indicates the virtual focus, in contradistinction
to the real focus in the convex lens.

To Determine the Focus of a Lens. — The

focus of a lens may be quite accurately determined
by the following methods: Taking the sun as an
object, hold the lens toward it and bringing a piece
of paper under it, move it 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 minute
and as the paper is brought still closer to the lens,

21

the image will be found to enlarge again. Return
until the most intense point is reached again. By
holding sufficiently long, especially if the paper be
dark, it will be found to burn, due to the concen-
tration of heat rays and hence is called the burn-
ing point vr focal point. Or, in a room opposite a
window or lamp, hold a rule against a white wall
and placing the edge of a lens upon it, move it
slowly toward the wall until a greatly reduced, but
bright image of either object appears and read off
the distance between the wall and edge of lens,
which is its focus.

A — ^_

i
A

/'\
A

Fig. 8.

Magnifying Power. — Magnifying power of a
convex lens depends also upon its convergence.
In Fig. 8 a b represents the object, c d the
lens, e the pupil of the eye. By following the

22

course of rays from the object it will be noted how
they are converged and refracted by the lens and
intercepted by the pup.il of the eye. If now the
lines between c and e be considered elongated,
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-raz/ image is one which
can be accurately seen and projected upon a sur-
face, as in the magic lantern, or in the picture
which is given by the photographic camera. The
virtual image cannot be so shown. In a lens
of less convexity or longer focus, there is less
convergence of rays and the length of the virtual
image is consequently reduced and thus the mag-
nification is less.

Spherical Aberration. — In considering the
refraction of light by a lens up to this point, we
have purposely avoided mentioning another quality
which is incident to it. In magnifying an object
by 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 unequal refrac-
tion between those rays passing near the margin
and those passing through the central portion, so
that the rays instead of combining at the focal

point, come tog-ether at different intervals along
the central line .or axis. By reference to Fig. i)
it will be seen that the outer, or marginal rays are

refracted at c and f so that they will combine at
g and the inner, or central rays are refracted at
j and k so that they will meet at /. In the same
manner will the rays which enter between c li
and i d come together at intermediate points
between g I and those of the central portion
between // and i will fall beyond /. The amount
of spherical aberration depends upon the shape of
the lens and in different lenses of the same focus,
is greatest in the double convex form, less in the
plane convex and least in the so-called crossed
lens, in which the two surfaces are of different
radii and in the proportion of 1 to 6, on condition
however, that the surface of short radius is directed
toward the object. This form of lens however, is
seldom used, as it shows the greatest amount of
aberration if used in the reversed position. The

24

most common form is the double convex and when
considerable magnifying- power is desired, the
defects of spherical aberration are partially over-
come by the interposition of an opaque plate with
round opening of such size that it will shut out the
marginal rays. This 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 is fringed with colors, predomi-
nantly violet and red, or if objects are viewed
through a pri^m, we have not only an apparent
change of position, but a decided appearance of
so-calied rainbow colors. This appearance is
called chromatic aberration and is a result of
refraction. It is caused by the dispersion, or
dispersive quality, the separation of light into its
primary colors, violet, indigo, blue, green, yellovv,
orange and red in the order given. As the light is
refracted by a lens or prism on its emergence, the
violet end being more refrangible will be princi-
pally affected and be brought to a focus within,
and the red to a focus beyond the principal focus.
This band of colors is called the spectrum and is
nicely illustrated in the diamond. This has a very
high degree of refractiveness and is polished or
cut to make a many-sided prism in which each
face or facet creates refraction with its consequent

25

dispersion and play of colors. To avoid to the
greatest possible extent the spherical and chro-
matic aberration, has been for many years the
study of opticians.

Fig. 10.

If in Fig. 10 the beam of light a b is followed
it will be found to separate at the first surface of
the first prism and still more at its emergence from
the second surface. The violet ray undergoing
the greatest amount of refraction, takes the direc-
tion of v v ', and the red ray being refracted to
a lesser extent, follows the course of r r . To
neutralize or correct this dispersion a second
prism, identical with the first, but in11 a reversed
position, may be placed to intercept these rays, so
that they will be recombined, as shown in r c and
v c and finally emerge as white light. Now while
the dispersion can be corrected in this manner, it
will be noticed that the emergent ray c d from
the second prism takes the same direction as the
incident ray a b and that this course corresponds

26

to the diverging rays of a concave lens. It will be
remembered however, that in order to have image
forming rays they must converge.

As dispersion depends upon two conditions, the
angle of the prism or convexity of the lens and
dispersive power of the glass, it will be seen that
in two prisms of the same angle but made of glass
of different dispersive power, the separation of the
red and violet rays will be greatest in that having
the greatest dispersive power. Or, in two prisms of
the same density but one having one half the
angle of the other, the dispersion will be one half
as great in the prism of smaller angle. Again, in
the prism of twice the angle but one half the dis-
persive power of another the dispersion will be the
same. We also know that in a prism having a
greater angle the refraction will be greater than
in one of lesser angle and as a refraction must be
maintained which will create convergence while at
the same time neutralizing the dispersion, a solu-
tion will offer itself in using in conjunction with a
prism of ascertain angle and dispersion, another
of lesser angle and greater dispersion. If the
same principle be carried out in lenses, a convex
lens of low dispersion would be combined with a
concave lens of high dispersive power. Such a
lens is shown in Fig. 11, and is called an
achromatic or corrected lens. If the chromatic and
spherical aberrations are both corrected it is called

aplanatic. The convex lens is made of crown glass
and the concave of flint glass.

Fig. 11.

The corrected lens shown in Fig. 11 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.

SIMPLE MICROSCOPES.

Simple microscopes are usually termed magni-
fiers and whether consisting- of one or several
lenses in close contact, always remain simple.
They are made to be held in the hand or at'e fixed

Fig. 12. Fig. 13.

on a stand or mounting, with provisions for adjust-
ing for focus, thus to give steadiness and leave the
hands free for dissecting or moving the object

29

during observation. They 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 and mounted in hard rubber or vul-
canite, as shown in Fig. 12 and Fig. 13. Those
containing several lenses are preferable, since they
offer a variety of magnifying powers and in com-
bination give the greatest magnification admissible
with single lenses.

Coddington Lens. — A lens of greater efficiency
than the ordinary magnifier is the so-called Cod-
dington lens, Fig. 14. While this is also a single

O

Fig. 14.

double convex lens it will be noticed that it has con-
siderable 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.

30

Aplanatic Triplet. — The best type of magnifier
is the Aplanatic Triplet, Fig. 15, being composed
of three lenses cemented together in such a man-
ner that they are virtually one. For many years

Fig. 15,

this form was, on account of cost, used only to a
limited extent, but an increased demand has
brought about a greatly reduced price. While it
corrects both aberrations it gives a large and flat
field and can be commended to those wishing a
good magnifier.

Hastings Triplet. — This form of lens has been
recently computed by Prof. C. S. Hastings of Yale
University and is a modification of the Aplanatic
Triplet, giving not only the highest spherical and
chromatic corrections, but a considerably flatter
and larger angular field and longer working dis-
tance; that is, the distance between lens and
object is greater and is an important feature in
higher powers, as it admits of better illumination.

In all of 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 arid 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 pro-
vided with a handle. They are used to enlarge
small printed matter, or to determine detail in
engravings and photographs.

Bruecke Lens. — This is a combination of lenses
named after the inventor which, while at one time
a popular construction went almost into disuse,
but in recent years has been found to have valu-
able properties and is now very generally used. A
combination of achromatic lenses is superposed by
an achromatic concave lens and gives a very satis-
factory image. Much higher power can be ob-
tained than would be possible in any other form of
simple microscope. While it has the disadvantage
of a small field, this is more than compensated for
by a much larger focal distance than would be the
case in the ordinary forms of simple or compound
microscope, thus making dissection possible during
observation and a further advantage over the
compound microscope, in that the image is erect
as in the simple form and not reversed as in the
compound. A change of powers is made conven-
ient either by changing the lenses or varying the
position of the eye lens.

32

Stands and Holders. — There is a great variety
of mechanical contrivances 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 is added, upon which the object can
be placed, and a mirror for illuminating the object
properly, it is called a dissecting microscope.

There are two forms of magnifiers which contain
within themselves some properties of stands, one a
Tripod Magnifier, Fig. 16, rests upon three legs

Fig. 16.

and has a screw for adjustment of focus. Its
optical parts are two convex lenses, usually having
a power of 10 diameters, and which are separated

33

by a diaphragm, giving a large, fairly flat field.
It is especially used in primary botanical work.

Another is the Linen Tester (Fig. 17). It is
made in various sizes, but the ordinary form has a

Fig. 17.

lens of 1 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. Its principal purpose is however, as its
name indicates, its use in the textile industries for
counting the number of threads which appear in
the standard opening of \ inch.

The most simple form of holder is a base to
which is fixed a stem or rod upon which the
magnifier may be adjusted. Fig. 18 shows one
with a white opal glass plate for placing the object
upon. In this or other similar forms the advantage
of holding the lens is counterbalanced by disad-

35

vantages, such as the necessity of holding the base
when adjusting the magnifier and the danger to
the eye from the projecting stem.

The Excelsior Dissecting Microscope (Fig. 19)
can be recommended when portability is a
desideratum and for school use in primary work,
the dissecting microscope designed by Prof. C. R.
Barnes. This meets many conditions of a dissect-
ing microscope in a simple manner.

There is a variety of more complex forms,
giving a number of advantages, such as stability ?
convenience of varied illumination, delicate means
for and long range of adjustment, jointed arms
for moving the lens conveniently over a large
field, etc., to the various descriptions of which the
reader is referred to catalogues.

How to use Magnifiers and Dissecting
Microscopes. — It is generally admitted that the
intelligent use of a magnifier is a great aid in
microscopical 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 microscopes, 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.

37

Keep the eye comfortably near to the upper
surface of the lens, as the angular view is in-
creased and there is the least spherical aberration
and the focal distance is the greatest. This can
be easily tested by gradually increasing the dis-
tance 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 chro-
matic aberrations become more pronounced and
the field smaller.

As magnifiers which are used on opaque objects
— those which are not transparent and which are
illuminated by reflected light not transmitted
through the object — a position should be chosen
toward a window or flame, which will allow the
greatest amount of light to reach the object. If a
hat is worn, place this back on the head so that
the rim or shield will not cut off the light.

In holding the object in one hand, take the
magnifier between the thumb and forefinger of
the other and place the fingers of hand holding
the lens in such a manner that they shall rest
upon the other hand ; this will insure steadiness
between the lens and object and will add consider-
ably to the comfort of working.

While it seldom occurs that magnifiers are made
with other than double convex surfaces, single
achromatic lenses are sometimes used which are
plane convex. In these the plane side should

always be toward the eye. In the case of two
plane convex lenses, they should be used with
their convex surfaces toward one another.

In magnifiers containing several lenses, when
these are used together, the one of highest power
should be nearest the object. When reversed the
angular field is greater, but presents considerable
spherical and chromatic aberration, which it is
advantageous to limit.

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 light, such as daylight,
is always preferable to any artificial illumination.

It should always be sought to modify the light
as much as possible and still have enough to see
easily as the eyes are much less fatigued.

While in some classes of work it is perhaps un-
necessary to have the very best magnifiers, such
as the Aplantic or Hastings Triplets, the latter
can always be recommended when the means will
permit on account of the higher results and
greater degree of satisfaction and comfort derived
from them.

Magnifying Power. — Unless a microscope,
whether simple or compound, is known to come
from the hands of a reliable firm, any claim as to
magnifying power should be accepted with re-
serve. In former years, when the country was

39

overrun with cheap foreign productions, 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 articles of worthless
or at best doubtful value.

Some precaution should also be used in refer-
ence 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 show-
ing small pit-holes or undulating surfaces. This
is common among cheap Coddington lenses and
naturally prevents obtaining a distinct image.

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, so that if an object
is magnified four times in the linear, it is sixteen
times in area. The commonly accepted term to
express magnifying power of simple, as well as
compound microscopes, is in diameters (linear).
A single lens of 1 inch focus magnifies about ten
diameters; one of -2 inch focus, about five diameters ;

OP THE

TJNIVERSmr

40

one of | inch focus, twenty diameters and so on.
In a lens of high magnifying power, the focus is
ordinarily about twice the diameter, so that if
a lens is \ inch diameter its focus is about 1 inch.
To Determine Magnifying Power. — While
the determination of focus in single lenses gives
approximate magnifying power, it cannot be done
in some of the forms which have been described.
The following method, if carefully followed, will
give very accurate results and is withall simple
and interesting. Upon the upper or farther edge
of a sheet of white paper, which rests upon a table
with the light at the right or left hand, place a
series of books of such height that when the mag-
nifier is placed upon the inner edge or that nearest
the body, of the upper book with the lens project-
ing over it, the distance to the upper surface will
be exactly 10 inches and weigh down the mount-
ing with an additional book or weight. Now
place a pocket rule between the leaves of upper
book so that when the edge is close to the magni-
fier the divisions on the rule will be exactly in
focus, place the rule so that it shall not project
much over the lens. It is immaterial what the
divisions on the rule 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 projected upon the paper. Take a

41

pencil and outline upon the paper one of the
spaces. By dividing the actual number of divi-
sions on the rule into this enlarged space the exact
magnifying power will be determined. Thus, if it
is found that five spaces are contained in the one
space on the paper, the magnifying power is five and
the focus of the lens 2 inches, or if ten spaces, it is
ten with a focus of 1 inch. One or several lenses
in conjunction may be examined in this way.
Some difficulty may or probably will be experi-
enced in seeing the divisions on the rule 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 difficulty may be experienced,
it will be found that after a little earnest effort,
both eyes unconsciously remain open and the
prominence with which objects have appeared to
the unoccupied eye is less, 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,
one of short, the other of long focus, placed suffi-
ciently far apart, will attain this object and this
was for years the method of its construction.

In any microscope, whether simple or com-
pound, the difficulty of holding it or the object
steady during observation, increases with the
increase in magnifying power and in the com-
pound form with only a moderately high power,
it is utterly impossible to retain sufficient steadi-
ness to make any reliable observation. Mechanical
contrivances therefore became a necessity 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 fulfill the following conditions :

A platform for holding the object.

A means of adjustment for properly focusing on
the object.

Provisions for suitably illuminating the object.

From what may be called a crude attainment of
of these three purposes, the construction gradually
became more complex. Many additions have

43

been made which have proven useful and have
remained, while others have been discarded. As
the first microscope was constructed kf 1590 itfhas
required nearly three centuries to bring the in-
strument up to its Ipresent 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 predominates, although bronze and
iron are used to a considerable extent. The first
two metals are usually highly finished and as they
easily tarnish, are protected by lacquer, which is
not only serviceable in this direction when well
done, but offers a means of ornamentation. Iron
is covered with a heavy coating of japan and be-
ing dark is on this account often recommended by
instructors 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. Some people call it a "machine,"
but we earnestly protest against this harsh term
as applied to an instrument of such precision as
the microscope is.

M

Fig. 20.

45

Description of Parts. — As it is necessary for
the student to become conversant with the terms
of the various parts and to understand their use,
we give an illustration (Fig. 20) with letters
and append a list giving the names and recom-
mend that they be impressed upon the memory,
as they are the basis of microscopical language.

A. Base. — This is the foundation of the instru-
ment. It usually rests upon three points (or
should do so) and is of such a weight that it keeps
the instrument firm when it is in an upright or
inclined position. The two principal forms are
the tripod and horseshoe. The revolving plate,
when this is provided, by means of which the
upper portion of the instrument is revolved, with-
out changing the position of the base, is considered
a part of it.

B. Pillar.1 — It is the vertical column which is
fastened to the base and carries upon its upper
end the joint or axis which is provided for inclin-
ing the instrument. It generally consists of one
piece either round or square but is often, in larger
instruments, made in two columns.

C. Arm. — This supports all the upper working
parts of the instrument and carries the provisions
for adjusting for focus.

D. Body. — This is the tube portion to which
the optical parts are attached.

46

E. Nose-Piece. — This is an extra piece which
is attached to the lower part of the tube.

Society Screw. — This is a standard screw
which is cut into the nose-piece and is called so
from the fact that it was first established by the
Royal Microscopical Society of London. It is
also called the universal screw and is in general
use in this country and England ; it has lately
been adopted by some firms on the continent of
Europe.

F. Objective. — This is screwed into the nose-
piece and is called so because it is nearest the
object. It is the most important of the two opti-
cal parts (of the microscope proper) and upon its
perfection the distinctness of the image and
therefore the value of the instrument almost
entirely depends.

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

H. Draw-Tube. — This is that portion of the
body which moves in the outer sheath and which
receives the eyepiece. It is provided for the pur-
pose of attaining different lengths and variations
in magnifying power.

47

I. Collar. — This is a ring which is attached to
the draw-tube and is usually provided with a milled
edge.

J. Coarse Adjustment. — This is a provision
for moving the body quickly back and forth for
adjusting the focus approximately. It is done by
a sliding rack and stationary pinion (not shown in
cut) or a sliding body in an outer sheath.

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

L. Fine Adjustment. — This is slow moving
and serves to get an exact focus. It is attained by
a fine screw, provided with a milled head and acts
upon the body, either directly or by levers. 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 have it, is evidence of poor workmanship.

M. Stage. — This is the portion on which the
object is placed for examination and is usually
attached to the arm, although the arm is sometimes
attached to the stage.

48

N. Clips. — These are two springs which are
attached to the upper surface of the stage and
serve to hold down the object.

Centering Screws. — These are provided for
moving the stage in different directions to bring
the center of its revolving motion in the center of
the field.

O. Mirror. — This is used for reflecting and con-
densing light upon the object. As a rule two
mirrors are used, one plane and the other concave.
The first gives a comparatively weak light, while
the second concentrates it and gives it more
intensity.

P. Mirror Bar. — This carries the mirror and
swings in a circle around the object in order to
illuminate it from various directions.

Q. Substage. — This is a ring or attachment
below the stage to receive various accessories
which may be required. It is sometimes fixed to
the stage but in the best instruments it is separated
from it and is provided with an adjustment to
vary its distance from the object.

Substage Bar. — This receives the substage
and permits its adjustment. In modern American
instruments this, as well as the mirror-bar, is on
an axis in the plane of the stage, so that whatever
position they may be in, relative to the object, the

49

distance from this to the substage or mirror does
not vary, except when made to do so.

S. Diaphragm. — This is a provision for stopping-
down or regulating the amount of light which
illuminates the object.

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

Object. — That which is examined.

Slide or Slip. — This is a thin plate of glass
upon which the object is placed or mounted, the
prevailing standard being 3 inches long by 1 inch
wide.

Cover Glass. — This is an extremely thin piece
of glass, round or sqtiare, which is placed upon
the object, either for flattening or preserving it, or
both.

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 adjust-
ing were provided, as near as consistent with the
construction, to the body or tubes, whereas in the

r>o

Ross they are 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 certain form of instrument which at the
present is very popular and called the Continental
pattern, from the fact that it was made originally
by the manufacturers on the Continent of Europe,
is a combination of both the Jackson and Ross
models. Whereas the coarse adjustment, when
consisting of a rack and pinion, is placed closely to
the tubes, the fine adjustment is placed in the arm.

There is another direction, however, in which
microscopes are divided into two classes, which is
of far more importance, and affects their utility in
a much higher degree. The writer does not know
that instruments have been so classified by others,
but knows that they can be with perfect propriety.
The distance between the eyepiece and the objec-
tive is one of vital importance in the optical results
and as there are several lengths of tube, the optical
qualities of an objective are injuriously affected
if it is used with a different length than that for
which it was originally intended. It is therefore
proposed to classify microscopes according to their
tube length as long tube ancLs*/w/ /W;r instruments.

Tube Length. — In the Continental form (Fig.
-20), a short tube from 160.0 to 170.0 mm. (6.8 to 6.7
inches) is used, whereas in the English form, this

51

Pts. included
in Tube-length.
See Diagram.
a

Tube-length
in
Millimeters.

203

rz

/^

-X\

-H£ rr»

f Grunow,

E. Leitz,

170

-

~c Nachet et -Fils,

146 or 200

«-*/•{ Powell and Lealand,

254

C. Reichert,
Spencer Lens Co. ,

160 to 180
235 or 160

^

— -^

X

1 W.Wales, -

254

Bausch & Lomb Opt. Co.,

216 or 160

Bezu, Hausser et Cie.,

220

f , Klonne und Muller,
o-a +

W. & H. Seibert,

160 -180 or 254
190

Swift & Son,
[C. Zeiss,

165 to 228£
160 or 250

tf

$

a \ Gundlach Optical Co.,
( R. Winkel,

254
220

I*

*H -^ o</ Ross & Co.,

254

•N

^— —
^-_

' -v

— -*

x-1

„ e c-e R. & J. Beck, -
c-f J. Green,
Hartnack,

254
254

160-180

v —

^

_ Verick,
«s
Watson & Sons,

160-200
160-250

Fig. 21.

52

being largely followed in America, the length is
from 8J to 10 inches (216.0 to 250.0 mm.) The
short tube of the European makers offers no
optical advantages, but is mainly used to contract
the height of the instrument to as great an extent
as possible, as this is the vital point throughout its
construction.

Until recently this subject was given little atten-
. tion, each maker following a standard which he
had adopted for himself. The pernicious influence
of this diversity was not appreciated by the public,
as it was not acquainted with the products of dif-
ferent makers, until Prof. S. H. Gage made it the
subject of a paper before the American Society of
Microscopists and as a result of his inquiries,
tabulated the standards as followed by the different
makers, which is published herewith.

As a result of his reports a committee was
appointed to consider this subject, as well as that
of eyepiece, objective and thickness of cover glass,
to which we will recur farther on and reported in
favor of the adoption of two standards for tube
length.

Short standard 160.0 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 tiibe where the ocular is inserted
and the lower end of the tube where the objective
is inserted. While the European makers have paid

53

little heed to this standard, we believe that all of
the American firms follow it and if it has accom-
plished nothing with the foreign producers, it is at
any rate a mean between the standards of the
leading makers and with the publicity which the
matter has received, gives the user an opportunity
to use his intelligence to obtain the best results.
There are no optical advantages in the one or the
other. The short length is almost a necessity
however, in the Continental pattern of micro-
scopes as compactness is the special desideratum,
but while this subject will be given more extended
attention and optically considered farther on, it
might be here stated that when an objective,
except perhaps in the very low powers, is con-
structed 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 being the plate or platform on
which the object is placed, it should be of a
strength to stand the weight of the finger under
considerable magnification. This depends upon
the material of which it is made and its thickness,
but since the material is virtually the same in all
instruments, we must depend upon the thickness
for rigidity. Absolute rigidity is practically impos-
sible when considerable force is exerted, as can
easily be determined in the best instruments and

54

it is a mistake to condemn an instrument for this
cause as is sometimes done. If the object will
remain in focus under a high power with a fair
amount of exertion above that which is required in
moving the object about, the stability may be con-
sidered ample. In fact in this, as well as other
directions, the writer considers it unwise for a
user of a microscope with, in most cases, a neces-
sarily limited knowledge of its construction, to
make assertions, as is often done, as to supposed
defects against the long experience of the micro-
scope maker, although it is the purpose of this
book to aid the reader in judging of defects when
such exist. In older instruments the fault often
occurred of making the stage unnecessarily thick,
which interfered with the proper use of substage
accessories and the mistake has in recent years
been made by some makers of having it too thin,
so that a slide could not be moved except with
the utmost care without depressing it and thus put-
ting the object out of focus. At the present time,
however, the writer believes that in instruments of
the best makers the thickness is ample, although
in cheap foreign ones he knows that it is some-
times not the case. It is of the greatest impor-
tance however, that the surface of the stage
should be square with the tube in all directions.

The lower surface is generally blackened and
the upper surface brightly polished and lacqiiered

or blackened. From the constant friction of mov-
ing the slide around, the lacquer or artificial
blacking becomes worn and in time the stage
assumes a shabby appearance. To overcome this,
glass has been used with more or less success, but
in recent years vulcanite has come into use and
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 affected by acids or alkalies
and will therefore retain its neat appearance
almost indefinitely.

Revolving Stage. — While in the largest num-
ber of instruments the stage is fixed and either
round or square, there are others which are revolv-
ing, that is, may be revolved around the optical
axis. These are absolutely necessary in the
examination of crystals and rock sections and are
then arranged with graduations around the edge —
a series of divisions reading to degrees or fractions
of them — by means of which the angles of the
objects are measured. As a slight deviation of
the center of revolving motion from exact coinci-
dence with the optical axis will cause the object to
swing out of the field, centering screws are pro-
vided by which this error can be quickly corrected.
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

56

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 direc-
tions with the body ; in either case a serious
defect.

Glass Stage. — This with the slide carrier
(Fig. 22) is a device for moving the object more

Fig. 22.

steadily and smoothly than can be done directly
on the stage. It is made detachable and consists
of a glass plate in a metal frame upon which rests,
on four points, the slide carrier. At its ends the
spring arms are passed around the glass plate and
press against its lower surface, thus offering the
minimum of friction, with sufficient resistence to
make an easy movement.

Mechanical Stage. — This is a very important
form in which the movements are mechanical in
two directions and at right angles to one another,

57

being transmitted through %the milled heads by a
rack and pinion or screw movement. If pecuniary
means will permit it, it is a most useful addition
and by it work can be done systematically and
with the assurance that every portion of the field

Fig. 23.

has been covered and withall with a degree of
comfort which must be experienced to be appre-
ciated. For instance, in a bacteriological or
urinary specimen, where one is searching the field

5s

for certain appearances, 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 as a toy by many scientists, 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 movement must be smooth and
easy and on reversing the milled heads, must not
show any lost motion or dead point.

There is a variety of forms, but two principal
types, one of which is built onto the stage, usually
the revolving one, the other attachable (Fig. 23)
which may be removed when not wanted, leaving
the ordinary stage intact. They are made with
graduations, usually divisions in millimeters, by
which one may read off the amount of space which
is covered.

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
lateral movement make the object pass across
the field. Move the object forward with the other
movement the amount of space which has been
previously determined and by a return motion

of the lateral movement, bring- it across again and
thus through the entire specimen, or until the
object is found or the specimen searched over.

Nose-Piece. — This being the lower end of the
body, to which the objectives are attached, it is
important in so far as it must be accurately made.
As 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 36 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 are
not 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 a diversity in the products of
different makers, so that it often happens that the
objectives of one maker will not enter the stand
of others. The writer in 1884 read a paper on this
subject before the American Society of Micro-
scopists 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

60

being the expense involved, so that we must con-
trive to suffer until some concerted action is taken
by the manufacturers themselves, which we trust
will not be too far distant.

Double Nose-Piece. — In changing- one objec-
tive for another to obtain another power, time is
consumed and it is often inconvenient ; besides,
there is danger of dropping the objective or dis-
turbing or destroying the object. To avoid this
the double (Fig. 24) the triple and quadruple nose-
pieces are offered, to the first of which two, to the

Fig. 24.

next three and to the quadruple four objectives
may be attached in such a manner that, when fixed
to the noseTpiece of the tube, each objective may
be in turn brought into use by swinging the nose-
piece and in such a manner that each is centered
and will be in focus, if not exactly, at any rate very
closely. Of all the conveniences in accessories
these are the most useful and in most common
use, the writer knowing from his experience as

manufacturer that 80
per cent, of the in-
struments sold for
personal use are sup-
plied with a double
nose-piece when two
objectives are used,
and the triple when
three are taken.

Bodies or Tubes.

These are divided
into two classes, mo-
nocular, having one
body with which
observation is made
with one eye, which
may contain one or
two draw-tubes and
binocular for observa-
tion with both eyes,
in which the tubes
are fixed together at
the nose-piece and
gradually separate
until they reach the
pupillary distance.
The first would be a
monocular microscope,
the second a binocular
microscope.

Fig. 25.

62

While the methods for transmitting the rays
from the objective to the binocular tube vary, the
construction in most common use is that introduced
by Mr. Wenham.

By reference to .Fig. -25 it will be seen that the
rays from one-half the objective are transmitted
uninterruptedly to the vertical tube, while the
prism intercepts the rays from the other half and
by reflection induces them to pass into the oblique
tube. The result is an image in each eyepiece
thus giving stereoscopic vision. This gives a per-
ception of depth, a peculiar faculty of being able
to look into an object and conveys to the mind the
impression of roundness or separation of the
object into different planes which it is impossible
to obtain with monocular vision. Its use, how-
ever, is limited to the lower power objectives.

Coarse Adjustment. — In providing this adjust-
ment, two methods are followed. The most simple
form is that by a sliding tube in which the tube
which carries the nose-piece at the lower end and
draw-tube at the upper end, is moved up and down
in an outer sheath, which is fastened to the arm.
The milled ring, which is provided, is ^grasped by
thumb and fore and middle fingers and pushed
down or drawn up by 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

63

objective from the liability of sudden or jerking'
motions and does not well permit the application
of the double nose-piece. While the clamping-
ring is provided in some instruments, which will
fasten the tube in a fixed position, 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 &&& pinion adjustment is by far prefer-
able 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 adjust-
ment, 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 of 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 the slide, which
also has its bearing in the recessed vertical length
of the arm, as shown in Fig. ^(>.

This adjustment must meet the following con-
ditions and if it does not, the instrument may be
safely condemned as faulty.

64

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

lii

pi, « 4J

!

65

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.

Fine Adjustment. — While this is constructed
in numerous ways it depends in all upon a screw
for the propelling power. It is sometimes called
the slow motion, as one revolution of the screw
seldom gives more than -£$ inch motion. This
screw is also called the micrometer screw. The
brass button is called its head and when it is pro-
vided with equal divisions upon its upper surface,
it is the graduated head. In this case a stationary
index is fastened to the arm. A fine adjustment
which will not fulfill the following conditions may
be considered as faulty.

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

The adjustment should act promptly in the
forward and backward motion 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 is worked
forward and backward.

While the fine adjustment, even more than the
stage, will show displacement with moderate
magnifying power by a slight exertion of force

66

against the tube, it should return to its position
when released.

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

Draw-Tube. — While this part of the instrument
may be an advantage when judiciously used, it
may have a pernicious influence when abused. It
will give both short and long tube standards and
should be provided with a mark to indicate each
length, or should have divisions by which the
standard can be read off. It should not be over-
looked, that when a double nose-piece is used the
heighth of this is added to the optical length and
suitable allowance should be made for it. In the
cheaper instruments the draw-tube slides in the
inner surface of the outer tube, but in the better
instruments, a special 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. 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 downward, may force the
objective onto the object or by the compression of

67

air in the tube, may force out the eyepiece. To
slide it properly, hold the main tube with one hand
and with the other grasp the draw-tube and move
it up or down by spiral movement.

The draw-tube may be used to vary the magni-
fying power, but unless used judiciously may be
the cause of more harm than good. While this
feature will again be touched upon 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 when with high
powers, unly under well defined conditions.

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

Base. — The judicious form and weight of the
base adds greatly to the stability of the micro-
scope and it is a too common fault that in many
instruments, even from reputable makers, this
essential feature is sacrificed from wrong motives
of economy, portability or compactness. While it
can hardly be expected that when the arm is
inclined to the horizontal position it shall be firm,
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, or a great expansion of
the resting points with extreme thinness is little
better. There should be a combination of both
qualities and if suitable proportions are not main-
tained, an excess can hardly be called a fault,
whereas too little would certainly be.

A favorite and good method to obtain increased
weight within reasonable dimensions is, to load the
base with lead and while some dealers, from inter-
ested motives, point this out as a defect, a pur-
chaser need not hesitate on this account if the
instrument is in other respects acceptable.

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 over a
small space.

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

69

Mirror and Mirror-Bar. — The proper illumina-
tion of an object is an important feature and
although there are numerous accessories for
properly accomplishing this, which will be spoken
of later on, the mirrors alone are effective agents
when properly constructed and applied, particu-
larly when no high magnification is used. The
plane mirror is generally used with very low
powers and reflects light in about its original inten-
sity. 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 is usually arranged to
coincide with the opening of the stage, when
parallel rays such as from the sky are used. . When
the source of light comes considerably nearer to
the mirror, as for instance from a lamp and the
rays are diverging, the focal distance becomes
considerably longer. Some of the intensity is lost
in consequence, as well as the degree of conver-
gence. 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,

70

with the result that, under certain conditions, the
utmost effectiveness of the microscope is not
obtained.

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

Besides the revolving diaphragm there are other
forms which maybe said to be better — for instance,
the so-called cap diaphragms, which require a sep-
arate piece for each aperature and which are held
by a special substage receiver; then the dome
diaphragm, which is a new application of the ordi-
nary revolving diaphragm. It consists of a sub-
stage fitting having a dome, to which is fitted a
curved revolving diaphragm.

The ideal regulator of light is the iris diaphragm
consisting of a series of overlapping blades placed
around a central opening, size of which may be
varied by means of a lever or milled edge. In the
ordinary revolving form the aperture is of neces-
sity at some distance from the object and does not
fully control the light on account of the stray rays,
which the other three forms accomplish. The

71

distance of diaphragm from the object is one of
considerable importance. The best position is just
below the surface of the stage, but as this is not
always possible, it should be as near as conditions
will permit. The nearer it is, the more intensity
will be maintained with suitable moderation by
the limit of opening. Very recently it has been
possible, to so construct the iris diaphragm, that it
passes up through the opening of the stage and is
flush with its upper surface.

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 explained by mathematical formulae,
but as it is our purpose to give intelligible explan-
ations to those who may not be conversant with
algebraic expressions, many of the statements and
descriptions will appear rather dog-matic. We can
but advise those who wish to study the subject
further, to consult such books as contain more
explicit information.

As has already been said, the compound micro-
scope is composed of two lenses, the upper one of
which magnifies the image which is formed by the
lower one. We know that the purpose is to. give a
greater magnification than can be obtained with
the simple microscope. The defects of chromatic
and spherical aberration will however become more
pronounced than they would in the simple form,
to such an extent as to nullify the benefit which
might be derived from the magnifying power only.
In fact, magnifying power in itself is of very little
value without the attributes obtained from the

73

chromatic and spherical corrections and their
qualities which will appear as we proceed. The
advent of achromatic lenses was the first decisive
step in advance and has been the foundation of all
later improvements and the high standard of the
best productions of the present day.

It is a matter of pride to Americans to note that
two of our countrymen, now deceased, were influ-
ential in furthering- the progress to a considerable
extent and that their memories should always be
honored by the microscopical world. They should
be remembered with feelings of gratitude, particu-
larly as the compensation for their efforts was
extremely limited. The pioneer in microscopical
optics in this country was C. E. Spencer, who was
followed by R. B. Tolles, and while both men did
a great amount of original advance work, it is the
latter particularly who, by his wonderful achieve-
ments, created a great discussion in European
circles, by enabling results to be obtained which for
a long time it was claimed could not be accom-
plished.

Of inestimable value to the scientific world have
been the labors of that most capable and genial
gentleman, 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 and Abbe camera lucida, we are
much more indebted for his profound disclosure of

74

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. In this con-
nection 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
consists mainly of the large variety of glass with
different ratios of refractive index and dispersive
power.

As defects of the objective have thus far been
specially mentioned, it will at this point be well to
state that the single lens of the eyepiece has also
been found unsuited and wrhile an achromatic lens
is unnecessary, a supplementary lens below and
near the upper lens has been beneficial in so
affecting the image, by collecting the rays, that it
can be viewed at one glance and without spherical
and chromatic aberration which the single lens
would show. It is for this reason called the field
lens or collective and the upper lens the eye lens.
Both of these lenses are mounted so as to form one
part of the microscope with fixed relations and is

Fig. 28,

76

then called eyepiece or ocular. In the diagram
(Fig. 28) the course of rays from the object through
the objective and eyepiece is shown. O G repre-
sents the objective, F L the field lens and E L
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 E L. 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 several
changes with consequent results can be noted.

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

If the distance between objective and eyepiece
is increased, an enlarged real image at c d
results.

If the eyepiece is of higher power, an enlarged
virtual image is formed at e f.

In the same manner a reduced magnifying
power may be obtained by reversing these con-
ditions.

As has already been stated a 1 inch lens with a
distance of 10 inches between it and the image
gives a power of 10 diameters, and the eyepiece

77

multiplies the virtual image by the extent of its
power. From this it can be easily computed that
with a 1 inch objective used with a tube length of
10 inches and a 1 inch eyepiece, a magnifying
power of 10X10 = 100 will be obtained; or the
same combination with a tube length of 5 inches
will give one-half this power or 50.

Objectives. — These are divided into two classes,
dry and immersion. In the dry there is no inter-
vening medium other than air between the bottom
lens of the objective and the top of the cover-glass.
In the immersion a liquid fills up this space. From
this fact it is easily seen that liquid can only be used
in objectives which are quite close to the cover and
therefore short focus or high power and so on the
other hand can objectives of long focus or low
power not be immersion. We will see later that
the purpose of the immersion is to obtain higher
optical results, is in fact a necessary condition, and
an objective which is constructed 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 cedar oil was discovered and as by means
of this better results are obtained, it has largely
taken its place. Its optical properties are almost

78

identical in refraction and dispersion with those of
crown glass and it is for this reason often termed
homogeneous immersion fluid, but for brevity ob-
jectives 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 will 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 pro-
gressive firms and it is hoped that this will become
a universal custom in time. 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, or rather the
mountings, were marked arbitrarily by the makers
and differently by each maker, it is now customary
to mark the objectives so that the figures shall
indicate the true optical value ; on the continent
of Europe in millimeters and in England and this
country in inches. The obj ectives are rated accord-
ing to a single lens, which the combined value or
equivalent focus of the lenses contained in the
objective, shall equal. This is also true of eye-
pieces. So, if two combinations equal in magnify-
ing power a single lens of 1 inch focus they are

79

marked 1 inch, or if a collection of four lenses will
be equal to a lens of T^ inch focus it is so marked.
The powers increase in proportion to the decrease
in focal length, so that a ^ for instance will give
a real image twelve times larger than 1 inch or a
real magnification of 120 -times.

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

Low powers, 3 inch, 2 inch, \\ inch, 1 inch, f
inch, |- inch.

Medium powers, \ inch, -fa inch, J- inch, \ inch.

High powers, \ inch, \ inch, -fa inch, y1^ inch, T^
inch, -fo inch.

It might be stated that such powers as -fa and -fa
inch are very rarely constructed at the present
time and that the -^ inch may be considered the
maximum, although seldom used. The -fa 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 detract from the optical qualities.
A modern objective of the highest capacity may
be considered a work of art and there are few pro-
ductions of the human hand which exact so much
untiring application, ingenuity and skill.

Systems. — An objective is said to consist of
systems which may vary in number from one to

OF THB

I UNIVERSITY
V —

82

extent of which is expressed in degrees and of all
the qualities in an ideal objective, this is the most
important. Thus in Fig-. 30. D is considered the
point of focus, and c D E the angular aperture.
The above definition has its limitations however.
While in objectives of proper 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 in excess of those which combine to form an
image and it is evident that as they do not aid in
forming an image, they serve no purpose and
therefore have no value in consideration of ang-
ular aperture.

As there are many objectives of the same power
but of different angular aperture, there are again
others of varying power but of the same angle.

Light is radiated by an object equally in all
directions aiid the more of the rays which can be
collected to form an image, the more distinct will
it become and to a greater extent can we see detail.
If in two objectives one receives on its front sur-
face and transmits a larger number of rays than
another of equal 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 objec-
tives in which one has one-half the power of the

88

other, but which takes in 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 they in
reality show equally well.

Aperture, without the defining- word angular,
indicates a very important feature in an objective
and designates 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. Many objectives are made in which the
rear lens is larger in diameter than the beam of
rays which, coming from an object, can be trans-
mitted through it and while not particularly
detrimental, has no value except perhaps to lead
to a wrong conclusion in reference to angular
aperture when this is measured, as the excess of
image forming rays, called stray rays, may indi-
cate a greater angle than the objective really
possesses.

Cover Glass. — At this point we must introduce
another feature which we have not yet considered
but which is important in understanding this, the
most important property of an objective. As we
have shown, in describing the principle of refrac-
tion, rays which fall from air upon a surface of
glass, are bent out of their course or refracted.
The cover glass which is used on the object during
examination, or for its preservation, although

84

extremely thin, has a marked influence on the
optical performance of an objective and this influ-
ence increases as the magnifying power of the
objective increases.

For the purpose of illustration, we will imagine
a cover glass of considerable thickness (Fig-. 31).

gf-\ a

f

altf,

Fig. 31.

o represents the source of light, or in this case the
object: While the rays from it are emitted in all
directions, we need only consider those coming
from the upper half, and for simplicity we will
select from them two pairs. As o a and o a' strike
the lower surface of the cover glass they are
refracted toward the axis o i and on their emer-
gence from the upper surface of the cover glass
are again refracted away from the axis in the
direction of b c and b' c', which is the original direc-
tion of the rays. The same action will take place
with the extreme rays o d and o d' , which will
emerge as shown at e f and e' f .-

AIR.
I.

WATER.
II.

OIL.
III.

Fig. 32.

If we now bring the front lens of an objective
close to the cover glass we can study its influence
and for this purpose will use the simple but intel-
ligible illustrations of Prof. Gage to show the
same phenomena.

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

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

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

From what we have 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 differ-
ence 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 homogeneous
fluid has the same density as glass, or as we have
stated, is virtually fluid glass, no refraction between
these two media takes place.

If we now refer to the diagrams and will bear
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 some of

the emerging- rays coming from the top of the
cover glass are so refracted that they fail to touch
the front lens of the objective and are therefore
lost. In the water immersion, however, the rays
being less bent, more of them reach the front lens
and are passed through the objective. With the
oil immersion the only refraction is that which
takes place at the lower surface of the cover and
from that point the rays are carried uninterruptedly
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 lost might be utilized, but as the radius
of the front lens would have, to be increased we
know that its focus and magnifying power would
be decreased. This, however, would be detrimen-
tal, inasmuch as, according to the law which has
been determined and promulgated by Prof. Abbe,
the capacity of an objective is determined by the
ratio between its focal length and the diameter of
the emergent pencil at the point of its emergence,
from the back of the objective, or in other words,
it is " the sine of half the angle of aperture multi-
plied by the refractive index of the medium
between front of objective and cover." This has
been called by Prof. Abbe the Numerical Aperture,
and is now commonly used to designate the effici-

ency of an objective. When applied to objective
mountings or used in tables it is abbreviated to
N. A. The formula by which computations are
made is N.A.=(« sine u) in which N. A. is numerical
aperture, n the index of refraction of front
medium and u one-half the angle of aperture.
Since the media are air, water or oil it is necessary
to know the refractive index of each, which is 1.0
for air, 1.33 for water, 1.5 for oil and for the ordi-
nary crown glass the index is the same as for oil.

To illustrate this formula better we will take an
example. A dry objective £ inch focus has an
angular aperture of 100 degrees. By reference to
a logarithmic table we find that one-half of the
sine of 100 degrees is 0.766 and we know that n,
the medium in front of the objective being air,
has a value equal to 1.0. The formula then is, in
figures, N. A.=n or 1.0 X u or 0.766=0.766.

To make this computation we have all figures
except that of angular aperture at hand. This
must be determined and the method for accomp-
lishing it 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
handi-work and such variations as do occur, have
little influence on the optical capacity. It is mani-

festly 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
determined, 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 catalogue of the Bausch & Lomb Optical Co.

Before the time when the numerical aperture
was so thoroughly elucidated, it was known that
an increase of angular aperture gave higher
results, but just why this was so was not appre-
ciated. 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
had the same effective aperture.

How to Measure Angular Aperture.— In in-
struments of the American type in which the axis
of the mirror-bar is in- the plane of the stage and
in which the circular part is graduated, the mat-
ter of measuring angular aperture is quite simple.
It was first recommended by Mr. Tolles and has
been carefully worked out by Dr. George E.
Blackham of Dunkirk, N. Y. After the object
has been focused upon, incline the body of the
microscope to a horizontal position, remove the

90

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 such a
point where the definition of the object shall
appear impaired without regard to the illumina-
tion of the field. These limits will mark the
efficient beam of light which passes through the
rear system of the objective. As we have stated
before, in some objectives the rear system is larger
in diameter than the effective beam of light trans-
mitted 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. The following is called
Lister's method.

After the objective has been focused, place the
body of the microscope 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. Indi-
cate the position of the center of the lamp at each

91

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 aperto-
meter, 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 can be read off on the
scale as well as the corresponding numerical
aperture.

In the foregoing we have gone to some length in
stating the importance of angular and numerical
aperture 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.

Resolving Power. — First of all qualities in an
objective is the resolving power, which is the
ability 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

92

aperture may be, its effectiveness may be injured
by the presence of chromatic or spherical aberra-
tions or defective 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 aper-
ture 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
show one-half as many lines as one with a numer-
ical aperture of 1.0. This, it will be seen, refers
only to the resolving power of an objective and
makes absolutely 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 certain structure which is not
visible, how can we reconcile this with the fact
that numerical aperature only does it? Why, if
this is true, should we use a higher power object-
ive with a certain aperture in preference to a lower
one of the same aperture?

The normal eye can see about 200 lines to the
inch, or structures which are ^-^th of an inch
apart and it is therefore evident that any structures
under the microscope 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 power of objective,
by increasing power of eyepiece-,
by increasing tube length,

we are limited in the last direction mainly by
such length of tube as has been found conveni-
ent 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 indistinctness, 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 such separation
to structure and fine detail, that it will be visible
to the eye without any undue strain.

94

Chromatic Aberration. — Up to this point we
have spoken of the correction of chromatic aberra-
tion 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 cor-
rection is for only two colors of the spectrum, the
red and violet, leaving as a residue the other colors
which appear as green and purple. These are
called the secondary spectritm and on bright objects
can easily be discerned. For all ordinary purposes
it is not prejudicial to the performance of an
objective. It becomes more pronounced in dry
objectives of large aperture and when high power
eyepieces are used. In properly corrected low
and medium powers of medium aperture and high
power immersion objectives it is most noticeable,
but certainly not to any extent to be objectionable,
except when oblique light is used.

Great care should be used in judging an object-
ive 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 had 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

• 95

involves a greater amount of color, that an object-
ive showing the proper colors of green and violet
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 violet are not suffi-
ciently 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 for an object a coarse diatom
mounted dry. If the mirror is swung to the left
the object will be fringed on the right side by
yellow-green and on the left by violet 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.

Under-correction may be judged by central
light, after the object has been focused, by slightly
increasing the distance between objective and
object, when the latter will show a blue color and
by decreasing the distance an orange color. Or,
will show over-correction, if on increasing the dis-
tance the orange will appear and by decreasing the
distance the blue. The appearance will also
become more pronounced by using oblique light.

94

Chromatic Aberration. — Up to this point we
have spoken of the correction of chromatic aberra-
tion 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 cor-
rection is for only two colors of the spectrum, the
red and violet, leaving as a residue the other colors
which appear as green and purple. These are
called the secondary spectrum and on bright objects
can easily be discerned. For all ordinary purposes
it is not prejudicial to the performance of an
objective. It becomes more pronounced in dry
objectives of large aperture and when high power
eyepieces are used. In properly corrected low
and medium powers of medium aperture and high
power immersion objectives it is most noticeable,
but certainly not to any extent to be objectionable,
except when oblique light is used.

Great care should be used in judging an object-
ive 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 wrriter to have had 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

- 95

involves a greater amount of color, that an object-
ive showing the proper colors of green and violet
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 violet are not suffi-
ciently 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 for an object a coarse diatom
mounted dry. If the mirror is swung to the left
the object will be fringed on the right side by
yellow-green and on the left by violet 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.

Under-correction may be judged by central
light, after the object has been focused, by slightly
increasing the distance between objective and
object, when the latter will show a blue color and
by decreasing the distance an orange color. Or,
will show over-correction, if on increasing the dis-
tance the orange will appear and by decreasing the
distance the blue. The appearance will also
become more pronounced by using oblique light.

Under-correction will show when the mirror is
swung to the left and when the object is fringed
with orange on the left and the blue-violet on the
right. Or, over- corrected when the colors are
reversed.

Another method is that recommended by
Naegeli & Schwendener in their work on the
microscope. The right half at the front or back
of the objective is covered by black paper or tin-
foil 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 produced by blacken-
ing one surface of a slide with chimney soot 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 and the right red or
orange.

Spherical Aberration and Cover Glass. — As

we have 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. If we
refer to Fig. 33 it will be remembered that the
rays from the object do not uninterruptedly reach

97

the surface of the objective front 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 e c, f d, f d, e c until they meet
at the axis, these points will be the apparant location
of the object and will appear to meet in the planes
£ a and d b instead of at o. To neutralize this con-
dition the objective will require to be spherically

Fig. 33.

under-corrected, by which is understood that the
marginal rays will eminate from a point near the
objective and the central rays at a greater dis-
tance from the 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 c, h d, h d and g' c.

If a thicker cover is used the objective will re-
quire to be more under-corrected, or if a thinner

98

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, as first condition, upon the proper cor-
rection of the two aberrations, it will be entirely
lost if the variation from the normal thickness is
considerable.

It will be well to bear this phase in mind, as it
has a great influence on the efficiency of the objec-
tive and will appear repeatedly in the instructions
to be given later on.

There are several methods by which corrections
may be made. In objectives with fixed mountings,
such as are ordinarily used, in which the lenses
have fixed relations, which correct the spherical
aberration for a definite thickness of cover glass,
no change can be made in them and therefore
cover glass should be used, which is very close in
thickness to that which was used as a standard.
Or, 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 also constructed in which the dis-
tances between lenses may be varied and then are
called adjustable.

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

99

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

It is a misfortune that cover glasses are not
made of the same thickness, but the difficulties in
producing them of equal thickness are almost un-
surmountable, and it is also to be deplored that
opticians have not agreed upon a definite thickness
to which they correct the objectives ; but since
this is not the case, all aid should be given to the
microscopist to obtain the best possible results.
The list herewith given, which has been prepared
by Prof. Gage will show the variations in thickness
used by different opticians as standard.

J. Green,

J. Grunow,

Powell & Lealand,

Spencer Lens Co.,

W. Wales.

Watson & Sons,

Klonne & Muller,

E. Leitz,
R wink'elj

Ross & Co,

Bausch & Lomb Optical Co,

c- Zeiss>
C. Reichert,
Gundlach Optical Co,
W. & H. Seibert,
R. & J. Beck,
J. Zentmayer,

Nachet et Fils,
Bezu^ Hausser et Cie?

Swift & Son.

mm.

mm.
mm.

mm.

TV6o-T2oV mm.
Y1^ mm.

To5o-T2(ro mm-
mm.

mm

mm.

i o _ 1 5
VTJO- TO o

100

The writer may be pardoned for introducing at
this point extracts from a paper which he read a
number of years ago.

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

" It must be remembered that the majority of
objectives will always be dry, and especially so
when improvements, which we hope are still to
be made, are accomplished. 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, we should, if
possible, provide an agency which, eliminating the
personal factor of efficiency, will give, under all
conditions, results closely equal to those under
which the objectives were originally corrected.

" 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

101

influence, still, in a general way, say it is of little
moment. * * *

"The system which I have devised to aid in
overcoming these difficulties depends in the first
instance upon a micrometer for measuring the
thickness of cover glass. (Fig. 34.)

Fig. 34.

102

" In objectives provided with cover correction
the graduation is so arranged as to read to T^ ^ mm.
No matter what the power of objective, the num-
ber gives proper correction for a thickness corres-
ponding to it. Thus, with a cover glass of 0.20 mm.
the collar of such an objective need merely to be
set at 20 to give the proper correction and, conse-
quently, the best results.

All the other scales give the correct tube
length in inches and millimeters for covers corres-
ponding to them, and in this manner offer a ready
and definite means of correction. The tube lengths
required for the thinnest and thickest covers are so
extreme that probably no convenient means for
obtaining them can be practically arranged, but
they can be so, approximately if not entirely. At
any rate, the micrometer will detect the require-
ments before using the covers, and those deviating
considerably from the normal can be used on
objects for use with low powers only, in which
case the effect will not be very appreciable.

" Iri this system I do not overlook the fact that
variation in tube length involves a variation in
magnifying power ; but, except in cases where
micrometers are used, I consider this of secondary
importance, as it always is in comparison to results
obtained in resolving power.

103
" This system involves four conditions :

First — That all cover glasses be measured before
using them, and % that the thickness be noted on the
preparation.

Second — That for convenience all draw-tubes be
marked in inches or millimeters, or both.

Third — That adjustable objectives be corrected
according to this scale.

Fourth — That the same tube length and cover
glass thickness be used in all original corrections
of objectives."

Penetration. — Penetrating power is the quality
which enables us to look into an object — to observe
different planes at one time. In the mind of the
writer, it is of no special importance, or at any rate
not as much as is claimed for it, and if desired is
easily attained. It depends upon magnifying
power and angular aperture, and decreases with
the increase of either of these. Objectives are
generally not constructed with any reference to it ;
it is a natural consequence of certain conditions.

Penetration and resolving power are antago-
nistic, or at any rate in an inverse ratio, and can
only be combined to a certain extent. In two
objectives of the same power and aperture, one
cannot have penetration as a special feature and
the other resolving power ; they will be almost
similar in these qualities provided that they are

104

similarly corrected. However, if they are not
similar in their angular aperture the one of small
aperture will have more penetration than the
other. In objectives of the same angle but differ-
ent power, the one of low power will have in itself
more penetration ; it will be similar in its action
to the eye, which, when an object is close to it, can
distinguish but one portion of it distinctly, while,
as its distance to the eye is increased, can dis-
tinguish various parts of it lying at different dis-
tances, and will finally see other objects outside of
it. By looking at an object at 5 feet distance, only
this can be seen plainly ; but, at 10 feet, others
quite a distance in front or back of it can be seen
as well.

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 with-
out focusing on them and thus obtain an apprecia-
tion of form which is impossible in the higher
powers, as in these the 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 in a microscope
is that portion which is observed in the eyepiece,
and its flatness may be observed when focused on

105

a flat object — preferably a micrometer. It is said
to be flat when all portions of the object are seen
over the entire field at once without further focus-
ing. When not flat, it will be found that as the
image approaches the edge of the field it becomes
more and more indistinct, and that the objective
must be correspondingly adjusted ; in many cases
it remains indistinct or blurred, and this may be
considered the most serious fault. In the case of
looking at straight parallel lines, such as in a
micrometer, they will appear to become more
curved as they near the edge, as shown in Fig. 35,

Fig. 35.

Flatness of field mainly depends upon the cor-
rection of the spherical aberrations and as under
the best conditions the latter cannot be entirely
eliminated, it is impossible to attain 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 beginners, especially, it is usually
most complained of, owing probably to the fact

106

that it is most easily noticeable. 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.

Working Distance. — This term, strictly con-
sidered, is an invariable quality of the objective
and is the distance between the front lens in the
objective and the object, when the objective is in
focus and is corrected for that object. All objec-
tives require a certain amount of projecting metal
to protect the front lens and this, with a certain
thickness of cover glass, lessens the working dis-
tance. In objectives with fixed mountings this
may be2 and with thick cover glasses is, consider-
able. It is comparatively unimportant however,
for the working microscopist to know the working
distance .per se of his objectives, but of considerable
moment to know what is the actual space between
the objective and cover glass.

In objectives of low and medium power, it is of
little consideration ; but where it must be ex-
pressed in y^-Q or yo^-Q mcn> it becomes a matter of
importance.

Working distance is spoken of as being long or
-short and varies with the power and angular aper-
ture. Generally the working distance decreases
with the increase in numerical aperture and be-

107

comes 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 considered to be the
case. While in objectives of the same aperture it
may vary considerably, it may in others of differ-
ent aperture be so that the higher one may have
the greater working 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 dis-
tance 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 single
lens of the same magnifying power.

As may be imagined, there is a variety of
opinions as to what constitutes long or short work-
ing distance in a certain objective. No definite
rule can be laid down for this, as it is conditioned
by the skill and requirements 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 window glass or a slide had been used
for a cover.

To Measure Working Distance. — The actual
or available working distance is of little moment
except in objectives of medium or high power and

108

then for two reasons. As we will show later how
to focus an objective, it may be of value to the
student to take no risk in endangering- the object
and in the case of oil immersion objectives, where
the working distance is exceedingly small, to know
what thickness of cover glass may be used. In
instruments having a graduated micrometer screw,
bring the front of the objective just in contact
with the top of the cover glass. This can best be
done with no danger to either objective or cover
glass, by grasping the instrument beneath the
base and raising it so that the cover glass is level
with the eye. By looking toward a window the
slightest space can be seen. In the case of a heavy
instrument lower the head to the level of cover
glass. Note the division and by an upward focus-
ing turn of the screw, bring the object in focus and
read off the distance traversed. In the case of an
oil immersion objective follow the same process by
bringing the objective in contact with the cover
glass dry, then separate slightly and inclining the
instrument allow some of the oil to drop into the
space, then focus and read off.

In the case of a micromenter screw without
graduations the matter becomes more difficult, but
can be done in the same manner although not so
accurately, by marking the milled edge of the head
of screw with a wax pencil or ink, in the two posi-
tions, by taking the middle of the arm as a fixed

109

point and by knowing or determining" the pitch of
screw ascertain the value of space traversed.

It must however, be borne in mind that cover
glass of normal thickness be used, particularly in
'oil immersion objectives, since if unusual thickness
is used, working distance may be entirely elimi-
nated.

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 definitions are
under a certain magnifying 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 procured 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 from 200 to 250
lines to the inch and in a microscope such magni-
fying power should be used as will apparently
bring the structure which is sought after at least
up to this figure. To illustrate take a J inch objec-
tive of 0.77 N. A. and a 2 inch eyepiece. An
objective of this kind properly corrected, resolves
the test-object Pleurosigna angulatum, in which the

110

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 f inch or 1 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
to such an extent that the lines have apparently
been separated and become visible to the eye.

Beginners as a rule are apt to use too much
magnification 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 de-
creased 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.

In objectives of the same power, but of different
angular apertures, the magnifying power and field
^v^ll always be the same.

The following table will probably be of assist-
ance 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 \ inch diameter.

A power of 50 'diameters will show a surface of
about 1^ inch diameter.

Ill

A power of 100 diameters will show a surface of
about -g*Q inch diameter.

A power of 500 diameters will show a surface of
about yJo- inch diameter.

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

This table is approximately correct with a
Huyghenian eyepiece.

As we have already shown, magnifying power
may be obtained by increase of power in objective,,
eyepiece, or increase of tube length and have also
pointed out that the objective should be relied upon
to obtain this increase.

Objectives of the same angular aperture, but of
different povvers, will give identical results by bring-
ing them up to the same magnifying power, unless
the difference is considerable. In both objectives
and eyepieces the lenses decrease in size with the
increase in power and consequently give 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 axisr
which at a long sitting, becomes fatiguing.

Choice of eyepiece should be determined by
requirements and individual preference. All re-
sponsible manufacturers and dealers make up such
outfits of stands, objectives and eyepieces, as ex-

112

perience has taught them are most generally
useful.

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

Apochromatic Objectives. — All objectives of
what may be termed the ordinary corrections have
a residual chromatic and spherical aberration, the
former being called the secondary spectrum, but
Prof. Abbe has in this direction also effected a
notable improvement, which with a uniform cor-
rection 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 con-
ditions, results in a higher resolving 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 cor-
rected. This is balanced in the eyepiece, which is
correspondingly over-corrected and is called the
.compensating eyepiece. It has therefore been
necessary to make the low powers under-corrected
as well, so that they might be used with the same
eyepieces, for it is evident that neither these
objectives with the Huyghenian eyepiece, nor the
compensating eypiece with the achromatic objec-
tives will give satisfactory results.

113

The proper combination of apochromatic objec-
tive and compensating eyepiece gives a beautiful
image with the maximum of resolving power,
unapproached by any of the achromatic combina-
tions. They furthermore have the advantage that
they are exactly suited to photomicrography,
there being exact coincidence between the photo-
graphic and visual image, which in the achromatic
objectives is not the case, as these must be either
corrected for photography, when they are not sat-
isfactory for ordinary purposes, or an allowance
must be made for the difference between two
images in the camera, which is very difficult.

One objection to the high powers has been the
liability of deterioration in some of the materials
used, which has happily been overcome by Prof.
C. S. Hastings of New Haven, Conn., in the
apochromatic objectives, which have been com-
puted by him for the Bausch & Lomb Optical Co.

Eyepiece or Ocular. — The purpose of the eye-
piece is so to refract the diverging rays coming
from the objective that they will reach the pupil
of the eye and while the most simple part of the
optical combination is, withall, 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 com-
mensurate with the quality of the objectives.

114

They are divided into two classes :

Negative eyepieces in which the focus is within
the eyepiece itself (at the diaphragm).

Positive eyepieces, in which the focus is outside
and below the field lens and which can be used as
magnifiers.

Huyghenian Eyepieces. — This is named after
Huyghens, who is said to have first used it. It is
a construction which is in most general use,
although made up in a variety of mountings. It
is negative and consists, as we have already stated,
of an eye lens, nearest the eye and field lens or col-
lective which is the large lens nearest the objec-

Fig. 36.

tive. Between them and placed as the focus of
the eye lens is a perforated, blackened disk, called
a diaphragm, which limits the size of field and

115

shows it within a sharply defined border. It is
made up in two forms :

The English type as shown in Fig. 36, 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 has a straight tube which
drops entirely into the tube of the microscope and
rests upon the mounting of the eye lens.

Solid Eyepiece. — This is also a negative eye-
piece and is the invention of the late R. B. Tolles.
It is called solid, from the fact that instead of
being composed of two lenses, it consists of one
piece of glass, 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 surfaces, which is
then filled up with an opaque pigment. (Fig. 37.)

11

/^ A

Fig. 37.

These eyepieces are only made in high powers,
as optical glass is usually not of sufficient homo-

116

geneity to make low powers, and their cost would
be too considerable, without a corresponding ad-
vantage. They are usually only made in powers-
of ^ inch and stronger and for this reason have
but a limited use.

Ramsden Eyepiece. — This is a positive eye-
piece and is constructed of two plane convex lenses
with the convex surfaces toward one another.
They are especially useful as micrometer eyepieces
in microscopes and telescopes, as they are used as
a magnifier on the fine divisions of micrometer and
at the same time give the virtual image.

Periscopic, Orthoscopic and Kellner Eye-
pieces.— These are also positive eyepieces which.

c

Fig. 38.

are achromatized by making the eye lens a doublet,
or triplet and by its correction permits the use of
a larger and flatter field. (Fig. 38.)

117

There are a large number of other eyepieces
called by a variety of names which can be found
in catalogues, but for which there is very limited
use. They, therefore, require no special mention
in this connection.

Compensating Eyepiece. — In this place it is
.also proper to speak of the new compensating eye-
piece as made and suggested by Prof. Abbe.
These eyepieces compensate the residual errors in
the apochromatic objectives and are of no value
except when used with them and for projection, as
they are highly over-corrected, as can be seen in
the edge of the field, which has a strong orange
color, whereas the Huyghenian has a blue color.

In this list 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 others positive. One more kind is
the projection eyepiece which is intended for pro-
jecting an image on a photographic plate, or on a
screen or wall.

Rating or Designation. — Until within recent
years the rating of eyepieces was arbitrary, each
maker following a system of his own and while
this is still done, the general method followed is to
name them as in objectives, according to a single
lens, which the equivalent focus of the eyepiece
lenses will equal and do this in inches or milli-
meters. This conveys an idea of the extent to

118

which an eyepiece magnifies the real image. Thus
a 2 inch or 50.0 mm. eyepiece magnifies five times,
one of 1 inch or 25.4 mm. focus ten times and so on.

Flatness of Field. — Although this depends
mainly upon the objective, the absence of it may
be owing to a faulty construction of the eyepiece.
If it is so prominent as to be easily noticeable and
to the same degree with a number of objectives it
may be ascribed to the eyepiece. It must, how-
ever, be remembered that an absolutely flat field
has not yet been obtained ; it may be closely
approached by decreasing the diameter of field to
less than its normal size.

Size of Field. — Quite a general but erroneous
idea prevails that the size of the tube has an influ-
ence on the size of the field. Except in eyepieces
of very low power, or with tubes with smaller than
usual dimensions, this is not true. . It must be
remembered that a Huyghenian eyepiece admits
of a definite size of field and this is regulated by
the opening in the diaphragm ; the same size of
opening is used in all of the same power, whether
it is an eyepiece with a large or small diameter.

Defects. —As has been stated many eyepieces
are carelessly constructed and possess defects
which interfere with obtaining a distinct image.
These defects do not show easily in low power
objectives, but can readily be seen with high

119

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.
This may be the difficulty, and before passing
judgment the lenses should be carefully cleaned.

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 parti-
cles 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 distinguished 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 so
closely that it drops slowly into its place from the
compression of air in the tube when objective is
attached.

Eyepieces are now generally made parfocal,
that is the equivalent foci are made to correspond,

120

so that a change in eyepiece does not materially
effect a change in focus and no further adjust-
ment than a slight turn of the micrometer screw
is required.

HOW TO WORK.

To Attach Eyepiece. — As the exterior sur-
faces of the eye lens and field lens are exposed,
they are apt to become dusty and should always
be carefully cleaned before use. 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 oivn 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. To fix in position however, it may be hast-
ened 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 objective may be
forced against the object and thus destroy it, or
the focus may be disturbed.

To Attach Objective.— Take a low power
objective first and after it has been removed from
the box, see that its front lens is clean ; elevate
the tube by means of the coarse adjustment so

122

that the nose-piece shall be at least two inches
from the stage and place the . upper end of screw
of objective in the thread of the nose-piece, hold
the lower end between the fore and middle fingers
(left hand) palm upwards, so that it is in line with
the tube and gently pressing upward, revolve the
objective with the thumb and forefinger of the
other hand (right hand) by the large milled edge
at its upper end, until shoulder sets against
shoulder. To properly attach an objective is not
always simple and cannot be done too carefully.
One danger lies in the fact that the objective may
be dropped onto the object and thus injure or
destroyaone or the other or both and another that
the thread is started wrong by holding the objec-
tive sideways and the threads may thus be
destroyed.

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
objective and object and saving of time, com-
mend them in all cases where two or more objec-
tives 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 c.omes as nearly as
possible in the center of the opening of the stage.

123

The slide should pass easily under the clips-
which however, is not the case when the edge of
the clips are too bluntly rounded and when the
springs 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 instrument is in
an upright position.

With the low power objectives, which are used
on coarse and usually large objects, it will be
found after properly focusing, that a portion of
the object will show itself in the field and by mov-
ing 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 some-
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 and which is
easily recognized by the mounting medium or
small particles of dust on the cover glass, move
the slide about on the stage by grasping one end
with the thumb and forefinger, when it can usually
be recognized by the shadowy outlines of the
object as it flits across the field. The difficulty of

124

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 :

First. — By using a low power eyepiece.

Second. — By using a low power objective as a
finder.

The object is thus found and after moving to
the center of the field the objective is removed
and the high power attached, or in case a revolv-
ing nose-piece is used, swing the high power objec-
tive in position, care being taken not to touch the
slide, and focus in the manner to be described. ' A
low power eyepiece in this connection is also very
useful. 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
attachable 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 absoluely convinced that the w^hole
field of the object has been covered, as for instance
in the search for bacilli.

To Illuminate the Object. — This is an ex-
tremely 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. As it is clearly inconvenient or im-
possible to hold the microscope toward the source
of light, they are provided to reflect the light
upward to the object when this is transparent.
The plane mirror reflects the light in the initial
intensity of its source and is used with low power
objectives. The concave mirror concentrates 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 considera-
tion.

The sources of light are either daylight, or
artificial light from a lamp. In the former the
light from a northern sky is preferable and in the
latter a flat-wick oil lamp, or a Wellsbach gas-
flame. 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, as can
be easily determined by experiment.

When using daylight, place the microscope as
near as possible directly before a window and
when a lamp is employed have it on the table
within easy reach.

126

Light is either transmitted or reflected. When
the former, it is used on 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 transmitted light
is used unless otherwise stated.

The concave mirror converges the light and
therefore 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
l>e longer with the 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 sometimes impossible in
critical work to obtain the best result. For this
and other advantages an additional illuminating
apparatus, called a condenser, is now commonly
used.

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
direct, 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

127

equal intensity, which with daylight is sometimes
difficult, 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 the beginning. There
will be little difficulty in obtaining proper illumi-
nation at the outset, if one will bear in mind the
three necessary conditions when looking at the
back of the objective :

Central illumination,

Even illumination over the entire field,

Mellow illumination.

Defects in illumination which may not be ap-
parent will show when the eyepiece is again
attached, and defective lighting will be indicated,

When dark points or shadows appear in the

field,
When a shadow is thrown at one side of

the object,
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 latter by reducing the amount of light,
(which it may be said in passing, is seldom neces-
sary with medium or high power objectives), by
the use of a diaphragm.

128

When first setting up the instrument the largest
opening of the diaphragm should be used and if
the light is found too intense, it may be reduced
by bringing into position a smaller stop, or in case
of the iris diaphragm, by reducing the size
of opening. The iris diaphragm is infinitely
superior to the revolving or cap diaphragm as
these have fixed openings, whereas with the former
any intermediate gradation may be obtained.

It is now generally conceded that observations
may be made with the microscope to any extent
without any detrimental results to the eyes, pro-
vided however, that the conditions of light are just
right. It is a good rule to follow, to use the least
illumination which will show the structure being
studied and it may also be safely accepted that if
the eye tires or feels uncomfortable, that 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
structure which cannot be seen with central illu-
mination.

How to Focus. — A safe method to follow and
one which is generally in vogue by all careful
manipulators, is to focus itpwards — that is, bring
the objective closer to the object than its working

129

distance and, applying" the eye to the tube, elevate
the objective by the coarse adjustment until it is
in focus. In the lower power objectives, in which
working" distance is considerable, this may appear
unnecessary, but is nevertheless strongly recom-
mended, principally to retain the systematic
method. In the medium and high powers it is very
important as the working- distance is small. In
these, the front of the objective is brought down
nearly in contact with the cover glass and the dis-
tance should be judged by holding the head down
to the level of the stage and observing the distance
as the objective is brought downward. The rack
and pinion is infinitely superior to the sliding tube,
as the distance can be traversed with absolute cer-
tainty without any liabilitv to slip. Then apply
the eye to the eyepiece and rack upward, slowly
however, that the point at which the focus is
reached may not be overlooked, which may easily
be the case in transparent and faintly colored
objects. After a better acquaintance with the
working distance of the objective the proper dis-
tance can be judged quite closely.

After the focus has been found the fine adjust-
ment should be brought into action. This should
only be used after the focus has been obtained with
the coarse adjustment, in order to focus through
the different planes or depths of the object. Its
range of movement is necessarily short and at one

130

end of the screw comes to a stop and at the other
goes beyond the limit of the movement and be-
comes inoperative. It should always be kept as
near as possible at the medium point. During
observation of the object, the milled head of the
fine adjustment should always be kept 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 about.

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, by which lines in a certain axis cannot
be seen 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. But, in order to determine
whether a defect exists, of which the possessor is
not aware, observe closely with both eyes, pre-
ferably an object with fine striations, such as
Diatomacae, to learn whether with one eye a

131

better view is obtained than with the other and
use the one giving the best results.

As already stated, make it a habit at the outset
to keep both eyes open.

There is one point over the lens called the eye-
point 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
either reduced, or shadows or colors will appear in
it. In low power eyepieces the eye-point is farther
than the lens ; in high powers quite close — in fact,
in some so close that the eyelashes may rest upon
the lens and may sometimes appear to be in the
field as dark bars. Generally speaking the best
point is where the entire field is seen and its mar-
gin (diaphragm) sharply defined.

What Objects to Use. — Suitable objects to
test the capacity of objectives are also valuable in
leading the student to a skillful use of the instru-
ment and in giving him proper judgment.

Low Powers — Proboscis of blow-fly. This should
be flat and transparent. For 1 inch, f and | inch
objectives the scales from Lepisma sacharina.

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

High Powers — Oil immersion ^ inch and -^ inch
objectives, Amphiplerua pelucida, Surirella gemma

132

in balsam or styrax. Stained Bacteria and Micro-
cocci.

Test Plate. — This will be an excellent acqui-
sition for all those who can meet the pecuniary
outlay. It consists of a series of 20 diatoms ar-
ranged according- 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 to yoVo mcn- A specimen of Eupo-
discus argus begins and ends the series :

Stiriae in y^
inch.

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

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

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

4. Navicula lyra, Ehrbg., 23. to 30.5

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

6. Stauroneis phoenicenteron, Ehrbg., 31. to 36.5

7. Grammatophora marina, Sm., - 36. to 39.

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

9. Pleurosigma acuminatum (Kg.)

Grun., - 41. to 46.5

10. Nitzschia amphioxys, Sm., 43. to 49.

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

12. Grammatophora oceanica, Ehrbg.

G. subtilissima, 60. to 67.

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

14. Nitschia sigmoidea, Sm., 61. to 64.

15. Pleurosigma fasciola, Sm. var., - 55. to 58.

133

16. Surirella gemma, Ehrbg. - 64. to 69.

17. Cymatopleura elliptica, Breb., - 55. to 81.

18. Navicula crassinervis, Breb. Frus-

tulia saxonica, Rabh., -

19. Nitzschia curvula, Sm.,

20. Amphipleura pellucida, Kg.,

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, are used by opticians to judge their
various characteristics and offer a reliable standard
for testing the resolving qualities, such as no other
objects can. The writer particularly recommends
that the test P. angulatum, dry, form a part of
every outfit, not so much as a test f or resolvability
as to use it for study and to acquire quickly a
knowledge how to judge the phenomena of spheri-
cal aberration. It may be used on powers £ inch
and higher and after it has served its purpose may
be put on one side to work over objects which
come under the particular branch of study which
one is following.

The writer wishes to counteract as much as pos-
sible the opinion, which is too prevalent, that the
use of diatoms indicates microscopical play and is
unworthy of consideration in histological and bio-
logical work, but the fact that the optician deems
them necessary for determination of optical quali-
ties ought at least to indicate that they are a valu-

134

able 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 appearance 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
microscopic observations and upon his knowledge
of the class of bodies to which the particular
specimen may belong. Not only are observations
of any kind liable to certain fallacies arising out
of the previous notions which the observer may
entertain 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 cor-
rected, it is obvious, by general advance in scien-
tific 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 philo-
sophers who have gained the highest repute for
practical wisdom ; and it is one which the micro-

135

scopist cannot too soon learn or too constantly
practice. Besides these general warnings, how-
ever, 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."

How to Set Up the Instrument. — Draw the
instrument from the case by grasping the base,
free it from dust with a large camel's hair brush,
1 inch or 1^ inch wide, or by wiping carefully
with a chamois skin, or old linen handkerchief.
Place the instrument with proper relation to light
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
compressing the chest. Bear in mind always to
sit as upright as possible.

Bring the draw-tube to standard length by
drawing the draw-tube to the length for which the
objectives are corrected. Do this by grasping the
milled edge of the draw-tube and give it a spiral
motion while holding the main tube with the other
hand, which will facilitate its easy movement.
The objection being, however, that in any but
cloth-lined sheaths the polished tube will soon be
scratched, 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 the upper en4 of main tube,

136

Where the graduated draw-tube is provided bring
the proper figure, either 216.0 or 160.0 mm., in line
with the upper end of main tube, in accordance
with the tube length to which the objectives are
corrected.

Attach low power eyepiece.

Attach low power objective.

Place object on stage.

Illuminate object.

Focus on object.

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

How to Work. — It is now supposed that the
instrument is ready for work. To start, it is well
for the beginner to provide a few prepared speci-
mens, as these will help him considerably if it is
his intention, as it should be, to prepare them
later himself. If only a portion of the object can
be seen and it is desired to see a larger surface,
the length of tube may be contracted by means
of the draw-tube. In this case the object will be
placed out of focus and another adjustment be-
comes necessary. If a higher power is desired
the draw-tube may be extended. Observe whether
the field is well illuminated and if not bear in mind
what has been said about properly adjusting the

137

mirror. Now use the micrometer screw, remem-
bering to grasp the milled edge between the
thumb and forefinger, and work to the right and
left, to reach different depths, and note carefully
the beautiful structure which is open to view.

Opaque Object. — We will now suppose that
the object is a mineral or plant. Place this upon
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 if it should be desired to use a higher
power this cannot be done on account of the short
working distance. The light may and undoubt-
edly 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° to its surface. If a
lamp is used and is 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 then well to move it toward the front. Using
the concave mirror, adjust it so that the light will
be concentrated 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,

138

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 by the slide P. angu-
latum. 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 position into an oblique one, on the side
opposite to that of the light and readjust the
mirror. Grasp the ends of the mirror-fork be-
tween the thumb and middle finger and move the
mirror by 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 objec-
tive and it must therefore be brought back until
the light appears equally well over all parts of the
field. It must here also be noticed that if the
diaphragm is still attached to the instrument and
does not swing with the mirror, it may also be the
means of cutting off light. The largest opening
should be used or in the case of the cap dia-

139

phragm, remove it. 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 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 diatom is
not in focus. If the objective is a non-adjustable
one, the proper correction may be approximately
reached by means of the draw-tube. If the lines
appear over the plane of the object, it shows over-
correction and the length of tube should then be
decreased, or contrary when the lines show below
or beyond the plane of the object. If the above
directions have been followed, the lines cannot
fail to be seen with a moderately good J- or \ inch
objective ; but if they are not, the trial should be
repeated. Again, be careful to have no obstruc-
tion between the course of rays from the mirror
to the stage ; get good illumination on the object ;
observe well ; 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, longi-

140

tudinal 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 at intervals
of about 10° 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 a 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
spherical 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, by the
unsatisfactory appearance , of the image, that

141

something is amiss, but will probably not know
how to correct the deficiency.

Using- an objective J, •£• or -J- inch and the test
object P. angulatum, select a diatom which is flat
and move to 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 taken as
being correct. If, however, 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-correction and
the remedy is found in the contraction of the tube
length. This should be done progressively in
spaces of about \ inch and after each change, care-
fully 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 slight oblique illu-
mination. If too much is used the nice differences
will be lost.

Chromatic Aberration. — This may be judged
as described under " Chromatic Aberration " in a
a previous chapter.

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 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 will cause
disagreeable reflections and shadows.

In medium and high powers, the front lenses
will be so close to the water, urine, blood, etc., that
capillary attraction will often cause an adherence
to the front surface of the objective ; besides this,
there is such a considerable 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 a sharp focus becomes impos-
sible. By merely dropping a cover glass upon it
all these objections are overcome.

The covers are commercially classified as No. 1,
No. 2 and No. 3, but there is a variation within the

143

limits of different numbers. The variation is
about as follows :

No. 1, Y^Q to -g-^Q inch, or O.i6 to 0.13 mm. thick ;
No. 2, yfoj. to yfo inch, or 0.25 to 0.16 mm. thick ;
No. 3 -L. to Jf- inch, or 0.50 to 0.26 mm. thick.

According to the prices of cover glasses, when
purchased by weight, the No. 1 gives the greatest
number and No. 3 the least. It may for this
reason be thought that the purchase of No. 1 is
most advantageous, 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 thinner of No. 2
and the thicker of No. 1 should be used, as these
come within limits which are adopted by opticians
as standard and to which medium and high power
objectives are corrected. Test objects which are
prepared to test the reserving power of objectives
and consist of diatoms, are generally covered with
these thicknesses.

An excellent means of determining the thick-
ness 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, and cemented to a slide, each cover being
marked with its thickness.

144

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. If the bands are narrow and the compli-
mentary colors of the secondary spectrum, yellow-
green to apple-green on one side and violet on the
other, indicates the best correction. For deter-
mining the thickness of cover glass and best cor-
rection for spherical aberration we refer to the
cover glass gauge designed by the writer.

If the cover glass is first measured with this
before the object is mounted, the tube length is
given for proper correction for each thickness and
can be read off on the drum. The scale provides
for J, ^, -J, and, ^ inch objectives of the long tube
length and for the £ inch of the short tube length.
It also gives the thickness of cover in millimeters
and inches, and thus indicates the true position of
adjustment-collar in adjustable objectives.

Dry, Adjustable Objectives. — Adjustable ob-
jectives, or objectives with collar correction are
those in which there is a provision to vary the dis-
tance between lenses, in order to make proper cor-
rection for spherical aberration with facility. They
involve a high degree of mechanical perfection and
are therefore more expensive than objectives with

145

fixed mountings. They may, however, be recom-
mended to microscopists who have acquired some
experience in handling objectives and even to be-
ginners who will use judgment in their use, as
they certainly give excellent results and quick
means for obtaining the utmost limit of efficiency
in objectives, a fact best appreciated by those
who use them. In the objectives as at present con-
structed by the best makers, a milled collar is pro-
vided, which in turning imparts a rectilinear
motion to an interior tube, carrying the posterior
systems of the objectives, thus varying the dis-
tances between them and the front system, which
remains stationary. The screw collar is graduated
in such a manner that the figures indicate the cor-
rect point for the proper thickness of cover, thus
10 indicating proper correction for a cover of
0.10 mm., 16 for 0.16 mm. and so on. When set at
the highest figure, for thick covers, the lenses are
in the closest position and the adjustment is said
to be closed. When for the thin covers, are farthest
apart and is then open.

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

Close the adjustment before attaching the objec-
tive as its front may otherwise come in contact
with the cover before the focus is reached. For

OF TH-R

146

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 adjust-
ment collar between the thumb and first finger of
the left hand, keeping the fingers of the right hand
upon the micrometer screw. 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 appearance and
when they do, it will probably be above the plane of
the diatom. This is an indication that the objec-
tive 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 cover.
Observe which number corresponds to the index,
and again return the collar to its closed point and
go through the same proceedings 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

Itt

owing to a want 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 corres-
pond with a 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 ob-
tained 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. 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 by either water or homo-
geneous fluid. Distilled water only should be
used and kept in a suitable bottle. Cedar oil is
used for the homogeneous fluid and is thickened

148

for convenience in use. A small bottle is gener-
ally supplied with each oil immersion objective.
Grent care should be used in keeping it free from
dust, as it often happens that an objective fails to
give satisfaction, due to a small particle of dust
which may float in the fluid before the front lens.
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 mag-
nifier, that there may be no air bubbles present.
These as well as dust may seriously interfere with
obtaining satisfactory results. If bubbles are
present the oil should be removed and a fresh
quantity applied. 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 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 and observe this
by lowering the head. Focus as with dry objec-
tives. The use of immersion fluid in itself involves
a certain amount of inconvenience, but the obser-
vance 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

149

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. — From
the fact that in oil immersion objectives the fluid
has the same refractive index as the cover and
front of the lens, it is not sensitive to variations in
thickness of cover, although many of the most
expert manipulators prefer adjustable mountings
in order to obtain the highest results, if any dis-
turbing element should be present.

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
micrometer screw, place the lamp at the right side
of the 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 there-
fore very difficult to find, a lower power objective
may be used as a finder ; bring No. 1, or Trice-
ratium favus into the center of the field and after
the objective has been removed, attach the immer-
sion objective, which we assume to be a y^, in the
manner prescribed. Get the best possible illumi-
nation with the mirror at the central point and
move the test plate from diatom to diatom until it

150

reaches No. 11, P. angulatum, but observe closely
the structure of each one as it comes into the field.
Next, see whether the objective is corrected. If
the lines and outlines, or middle rib, do not appear
to be in one plane, adjust the collar in adjustable
and the tube length in non-adjustable objectives
until they are and 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° to the left side and readjust-
ing the mirror, observe the effect. It is very
probable that the lines will show and if so, con-
tinue the advance ; if they do not, give 10° or 20°
more obliquity and after the structure comes out,
again go forward. A point may thus be reached,
where with the greatest obliquity which can be given
and with the best possible illumination, the objec-
tive seems to have come to the limit of its perfor-
mance. 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 cor-
rect for oblique light ; perhaps the eyepiece does
not give sufficient magnifying power to distinguish
the stricz. 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 specimens must judge

whether the power is sufficient ; it should be added
that for any over No. 14 and under No. 18 a 1 inch
eyepiece should be used and for those above No.
18 a power of f inch will probably 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 dif-
ferent adjustment is required for obliqtie 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 illlumination ; vary the distance
of the mirror to the object, or if this cannot be
done, vary the distance of the lamp to the instru-
ment 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 concen-
trated, so that the circle of light will not be larger
than the mirror, which can be determined by plac-
ing the hand or a piece of paper back of it. Adjust
when necessary by moving the lamp or bull's-eye.
Keep it a little below the line of the top of the
stage, so that the beam from the bull's-eye will not

152

strike it on its upper surface and as little as pos-
sible on its lower 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, bull's-eye, or lamp, or a touch to the cor-
rection collar or micrometer screw, is sometimes
followed by astonishing 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 manipula-
tion 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 suc-
cess is often attained and then creates confidence.

153

It is, however, only recommended for this purpose
and not for general use.

Stained Bacteria and Micrococci also make
excellent objects for immersion objectives. The
mode of illumination is the same as with dry
objectives.

ILLUMINATION WITH CONDENSER.

Up to this point the matter of illumination has
been treated in its most simple form with mirror
only, as most instruments are of the simple type,
but we must now consider the substage condenser,
which is a most valuable adjunct to the micro-
scope. While skillful treatment of the microscope
will go far toward obtaining good results, it will in
many investigations not suffice. Furthermore the
condenser will give results with ease which' without
it involve effort and skill.

Purpose of the Condenser. — The purpose of
the condenser is not only as its name implies, to
condense light, but is more especially to illuminate
the object with a cone of light having an angular
aperture equal to that of the objective used and
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 substage
condensers is very unique and interesting and
shows, how from having been the subject of no
end of condemnation, which for many years it

155

received, it is now generally accepted as a neces-
sary adjunct to a complete outfit,- in fact should
always accompany an oil immersion objective.
From single lenses, compound, non-achromatic
and achromatic, the use of eyepieces and objec-
tives as condensers with any number of devices
for regulating the light, the generally accepted

Fig. 39.

Optical part of condenser 1.20 Aperture.

Fig. 40.
Optical part of condenser 1.42 Aperture.

forms 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.
39) and the other with, an aperture of 1.42 of three
lenses (Fig. 40). A third is made achromatic with
an aperture of 1.0 which, however, is considerably
more expensive.

156

The most simple form, largely used for instru-
ments for laboratory and everyday work is one
which has attached to it below an iris diaphragm
for regulating the amount and angle of light and
to which is attached a swinging arm to receive

Fig. 41.

15?

blue glass for moderating" light, or stops for dark
ground or oblique illumination. A screw motion
gives a serviceable means of adjustment.

The most complete form is that shown in Fig. 41,
which has adjustments for obtaining every modifi-
cation and character of illumination, with rack and
pinion for vertical adjustment and swinging out of
the way the condenser and iris diaphragm if it is
not desired to use these.

A condenser while useful may be abused so as to
do more harm than good and we deem it proper
to give some instructions in its use, which we trust
may be of service.

Use only plane mirror with the condenser.

Centering the Condenser. — Every condenser
should have a centering arrangement, so as to
bring its axis coincident with that of the objective.
In the simple form of microscopes in which the
character of work is not critical, the condenser is
sufficiently centered for all ordinary requirements,
but even here it should be possible to center if
conditions should demand it.

In the more complete apparatus a pin hole cap
to aid in centering the condenser and fitting over
the upper part of the mounting should be supplied,
and after focusing upon the opening in the cap
with the low power objective, bring into the center
of field with the centering screws. Closer adjust-

158

ment can be made after the high power objective
is attached.

In the simple form, pin hole cap is usually not
provided. To verify correct centering- two easy
methods may be followed.

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

2. Use a f or f- inch objective and bring to its focal
point ; remove eyepiece and look through the tube,
keeping the eye in center, when the small opening of
diaphragm will be seen sharply defined.

This opening in diaphragm may be enlarged to
the full aperture of objective so that the margin
of opening in the diaphragm will just coincide
with that of the back lens of objective.

Centering the Illumination. — This is fully as
important as centering the condenser, tor it is of
little avail to have a centered condenser when the
projected beam of light is not centered. This
depends upon the source of light and position of
mirror and when it is remembered that the mirror
may be so adjusted as to give all gradations of
oblique illumination from the central to the limit
of aperture, the necessity for accurate adjustment
becomes apparent.

Attach f or f inch objective ; open diaphragm to
full extent and focus upon the minute image of

159

flame ; adjust mirror so that the image will be in
the center of 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 proportionately large to the
angular aperture and owing to the variation in
thickness of slides, its focus is variable, falling in
some slides above and in others below its effective
position. In the simple mountings the condenser
is generally so placed that its upper surface is just
below the surface of the stage and for the low and
medium powers this will be generally found suf-
ficiently close, in fact at this point it may often be
found that the illumination is too intense and may
then be reduced either by reducing the aperture
of the diaphragm, or by removing the condenser
farther from the object.

In all objectives with a numerical aperture less
than 1.0 the condenser may be used dry, without oil.
In immersion objectives the top of condenser
should be brought in fluid contact with the slide.
The same phenomena take place by refraction of
the light from condenser through the slide, as
from an object through the cover glass to the
objective. To make contact, place a drop of oil

160

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 f objective ; focus upon
the object, after the slide has been brought in fluid
contact with condenser and tJien adjust condenser
until image of flame will fall in the same plane with
the object,

Relation of Aperture of Condenser to Ob-
jective.— As has been stated the condenser may
be the means of doing more harm than good,
depending mainly upon the angle of illumination
used. Too much illumination decidedly injures
definition by obliterating detail. In the study of
Bacteria and Micrococci, with which the objectives
used are of wide aperture, it is sought to have
them stand out boldly in a bright field, which is
accomplished by bringing the diaphragm to its
full aperture.

In all dry objectives the aperture of the condenser
should be less than that of the objective.

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 aperture is

101

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 rela-
tive aperture of the condenser to that of the objec-
tive may be easily seen, as the outlines of the
diaphragm are sharply defined. If the opening
in diaphragm appears to have the same opening
as the back of objective, the condenser has the
same angular aperture. By experience the fol-
lowing conditions have been found to give most
satisfactory results :

In oil immersion objectives on bacteria use the
full aperture of condenser.

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

In histological and other dense objects use an
aperture equal to about one-half the opening of back
lens in objective.

Oblique Light With Condenser.— Oblique

light may be obtained by setting the mirror 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 however, having such provision, it
is obtained by reducing the opening in diaphragm
so that it shall correspond in size to the back

162

lens of objective and then moving this opening
laterally across the bottom of condenser to such a
point as will bring out the structure of the object,
or to the limit of the aperture of the objective.
When beyond the limit of aperture the field will
appear to become dark. The amount of illumina-
tion may be modified, but in a general way it may
be said that the best results with oblique illumina-
tion are obtained by reducing the amount of
illumination to its minimum practicable amount.
In objects with striated structure, the illuminat-
ing 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.

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 requisition, 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 most simple form is that shown in Fig. 42.

Fig. 42.

With this the microscope must be considerably
inclined and the camera lucida attached to the eye-
piece. The emergent pencil is reflected by the

164

thin film of glass into the eye and apparently
projected through the glass upon the paper.

Another simple form is the Wo lias ton camera
lucida (Fig. 43), which is attached to the eyepiece
in the same manner as the foregoing, with the

^ £3-

Fig. 43.

instrument also inclined. This consists of a quad-
rangular prism, which reflects the emergent pencil
through a small opening into the eye. The eye is
placed over the edge of the prism in such a man-
ner that the rays forming the image enter only a
portion of the pupil, while its other portion views
the paper. In using this however, the eye must
be kept quite steadily in the proper position.

While there are some other forms, they are not
in general use. The very best is one designed by
Prof. Abbe, and which goes under the name of the
Abbe camera lucida (Fig. 44). It is fixed to the
tube of the microscope, permitting the mounting
which contains the revolving prism to be swung

165

back, so that the eye may be applied to the eye-
piece. It is composed of two rectangular prisms,
cemented together at their diagonal surfaces, but
having between them a perforated film of silver.

Fig. 44.

At one side of the prism is a large mirror which
reflects the image of the paper and pencil onto the
silvered surface of the cemented prism and this
in turn to the marginal portion of the pupil,
while the axial portion of the eye views the image
through the perforation of the silvered surface,
thus giving both views coincidentally.

A number of precautions are to be taken to
obtain good results. First of all it must be
remembered that if spectacles are necessary in

166

reading, these will be required in order to see the
pencil point clearly.

The pencil point should be well sharpened in
order to closely follow the minute detail.

The drawing board should be at right angles to
the axial portion of the projected image, otherwise
there will be an elongated picture.

The relative illumination of field and paper
varies with magnifying power and distance of eye-
piece from paper, but should be made as nearly
alike as possible. In low powers the light from
the objective is stronger than that from the paper
and should be modified, in the simple and Wollas-
ton forms by using the plane mirror or by cover-
ing it with white tissue paper, and in the Abbe
form by the use of dark tinted glass, which is pro-
vided with the apparatus. In the higher powers
the paper is brighter in which case it should be
shaded by a screen placed between it and the source
of light, or a substage condenser should be used
to give increased illumination.

Special instructions with reference to the use of
the simple and Wollaston forms are as follows :

Focus the objective upon the object ; then in-
cline the microscope, preferably to the horizontal
position ; raise the microscope by underlaying
with a block or books. The size of drawing may
be varied by change of eyepiece or objective, but
only to a limited extent ; or by varying the dis~

107

tance between eyepiece and table, it being- appar-
ent that as the distance becomes greater the
magnification will proportionately increase. The
usual standard for distance is 10 inches, although
this may be varied to suit requirements. Attach
the camera lucida ; place the paper upon the
table and fasten with drawing tacks ; apply the
eye to camera lucida ; refocus carefully with the
fine adjustment and endeavor to obtain equally
sharp view of image and pencil point. In these
two forms it is quite difficult to see the pencil
point clearly and some care will be required in
modifying the light or shading the paper so that
the image and pencil point may be sufficiently dis-
tinct at the same time to follow the outlines.

In the Abbe camera lucida the optical results are
considerably better, since a direct view of the
image is obtained and the equalization of illumina-
tion is easily accomplished.

While with it, the microscope is intended to be
used in an upright position, the reflecting mirror
is close to the eye so that, if the image is projected
vertically, a portion of the field is cut off by the
stage or base of the microscope and if the mirror
is so inclined as to bring the full field upon the
paper, it will not be round, but elongated or ellipti-
cal and thus also elongate the image. This can
only be corrected by correspondingly raising one
end of the drawing board so that it shall be at

168

right angles to the axis of projected cone, or until
the field appears round. A convenient method of
accomplishing this, with the further advantage of
being able to elevate the drawing board and thus
produce a variation in the size of drawing, is the
drawing table made by Bausch & Lomb Optical Co.
Care must also be used in attaching the camera
lucida to the tube, so that the opening in the
prism shall be in the optical axis. This can be
accomplished by the centering screws and observ-
ing whether any of the field is cut off. If the field
appears smaller with the prism than by directly
looking into the eyepiece, the prism is too close, or
too distant from the eye lens and should be prop-
erly adjusted.

No particular skill is required in drawing as it
is simply a question of copying, but the lines
should be light so that any irregularities may be
corrected after the work with the camera lucida is
completed. To determine the standard distance
of 10 inches, measure from the optical axis to the
axis of mirror and from this point to drawing
paper.

If it is desired to determine the amount of
enlargement of the image on paper, this can be
done by replacing the object by a stage micrometer
and drawing its spaces over the object. If the
divisions or spaces are 0.01 mm. and. it were found
that 10 spaces covered the object and the actual

169

measurement was 80.0 mm. the enlargement
would be 30 times 100 divided by 10, or 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 stage micrometer, a standard is
established for each and further measurements
will not be required. If however, variations from
this standard distance are made, the actual magni-
fication should always be determined.

To Determine Magnifying Power. — While
the magnifying power may be known from tables
accompanying the microscope, these are only
approximate, as there is more or less variation in
eyepieces and objectives and furthermore the
microscope may be used under different conditions.
There are three requisites.

A camera lucid a.

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.

If a low power objective is used, place the stage
micrometer with divisions of 0.1 mm. or 0.01 inch
upon the stage and focus. After attaching 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

170

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
250. If 18.6 on the rule are contained within
three spaces on the paper, the magnification would
be 63 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 millimeters by figuring 25.4 mm. equal
to 1 inch. It may be done by several methods,
two of which are generally followed.

The first of these give 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
instrument set up exactly as for drawing. On the
drawing paper the outlines of object, or that por-
tion which is to be measured are marked, without

in the least disturbing 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 adjustment and, it may
be added, there should be very little variation in
thickness between the two slides. Move the micro-
meter 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 0.001 inch are the values of the
micrometer spaces and the object covers one space,
its size will be 0.001 inch, or covering 7 spaces will
be 0.007 inch.

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 result so long as
objective and micrometer are used under exactly
the same conditions.

The second method is with the eyepiece micro-
meter which in its simple form is a micrometer
mounted on a plate which is placed in the focus
of the eye lens. It is usually graduated to 0.10 mm.,
seldom exceeding 0.05 mm., as closer lines cannot
well be distinguished with the magnification
obtained by the eye lens. The plate is dropped
onto the diaphragm, where it is in focus, or enters
slots which are provided in the tube of the eye-
piece. A better form however, is the eyepiece
micrometer (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.

Fig. 45.

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
micrometer we have on the other only the image
of the object. The valuation 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 micrometer
divided into the same spaces as the eyepiece
micrometer is necessary.

Focus the eye lens on the eyepiece micrometer
and the objective on the stage micrometer, being
careful to bring the first line of the former coinci-

173

dent with a line of the latter, using care to see that
they are parallel. As the lines of the stage micro-
meter will appear to have a certain amount of
thickness, make the first line of the eyepiece
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 it is 8 divis-
ions. 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 careful to leave all the con-
ditions unchanged. Note how many divisions are
required to cover the object and divide by the
figure first obtained with the stage micrometer.
Thus, if there are 40 spaces which we know are
0.1 mm. divisions the real size of the object will be
0.5 mm., or forty tenths divided by eight.

If measurements are made under exactly the
same conditions of tube length, with same objec-
tives, it will be unnecessary to repeat the opera-
tion with eyepiece and stage micrometer, as the
proper ratio may be marked on a card, as it
remains constant.

The most efficient apparatus however, for obtain-
ing accurate results is with the filar or screw micro-
meter (Fig. 46). This consists of a metal case to
the upper surface of which is fitted an adjustable
Ramsden eyepiece. Within is a frame carrying

174

one or several delicate spider lines, which are
moved across the space to be measured by an

Fig. 46.

accurate screw of either 0.5 mm. or -fa inch pitch,
which has at its end a graduated disk divided in
100 or finer spaces, thus giving a definite value for

Field of Large Filar Micrometer
showing cross hairs and recording comb.

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

TO SELECT A MICROSCOPE.

When a person has concluded to obtain a micro-
scope, a suitable selection is a matter of consider-
able importance to him. The varieties are in-
numerable, prices without end, all sorts of claims
made for them.

The variety of special lines of investigation in-
volves 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 paramount 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 firm, the writer is convinced
that it will be honestly and disinterestedly given.

176

When means will permit, the outfit for im-
mediate 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. American or Continental ? — First of
all, choice will have to be made between the two
types and while one's sense of the aesthetic may be
a factor it is mainly the practical utility which
must govern the decision. Whether 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 dur-
ing" manipulation in the upright or inclined position.
In general the low stand is preferred as it permits
of resting the arms upon the table while mc>y;ng
the object and the comfort of looking through the
tube whether the instrument be upright or in-
clined.

Tube Length. — In the matter of tube length
the optical results are the same in both, so that a

177

conclusion must be reached in so far as it effects
the height of the instrument.

Base. — The base is an important feature and
while not 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 of Arm. — This,
without question is an advantage and while it is
an inexpensive addition it will add considerably to
the comfort of working and should invariably be
present, if pecuniary means do not absolutely
prohibit it.

Coarse Adjustment.— Almost all instruments
for reliable work are provided with both fine and
coarse adjustments. The choice of the latter lies
between the sliding tube or rack and pinion. The
former has only the advantage of economy and is
a decided disadvantage in the hands of students in .
injuring objectives and preparations. Further
than this, it is almost impossible for the maker to
center the nosepiece with 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 advantages of the rack

178

and pinion seem to be generally appreciated in this
country and there are few instruments 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 fre-
quently, 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 circum-
stances, the body has the obnoxious habit of sud-
denly slipping further than the microscopist in-
tends, 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 microscopical work can be
done with an instrument whose body slides through
a friction collar. That arrangement 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 through-
out 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 fitted in its bearings, so tha.t

179

there will not be the least side motion and this
should be tested under the magnifying 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
adjustment. It should work absolutely smooth
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 observa-
tion. The micrometer screw should be back of the
pinion, not at the front of the ttibe 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 concerned there is no difference.
Brass however, offers a maker a better opportunity
tor displaying his mechanical skill and while it is
no doubt true, that many highly finished instru-
ments are of poor workmanship in their working
parts, it is also a fact that a well made instrument
is always nicely finished.

180

Size and Weight. — The size of instrument is
worthy of consideration. If an instrument is to
remain stationary in a practitioner's office or labo-
ratory, 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.

Working 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 manipulating a slide, there should still be suf-
ficient space for the convenient attachment of sub-
stage accessories. In this respect the American
instruments, whether of the American or Conti-
nental type, are superior as they are built for the
better accommodation of accessories.

Stage. — A variety of stages is offered on in-
struments of similar construction. The plain, flat
stage while preferred by some, offers no advan-
tages over the ordinary round one, unless specially
made for examining specimens on larger slides than
the standard 3 by 1 inch. Those stages, covered
on top 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 thickness of

181

metal and be so bent as to allow specimens to be
brought under them without resistance and keep
them properly in place, without too much pressure
and consequent friction.

A Glass Stage and Slide Carrier may be con-
sidered a good investment, as it admits of the
convenient manipulation of the slide, without the
grating feeling which usually accompanies the
direct movement of the slide on the stage.

The Mechanical Stage, while an absolute
necessity in petrographical and other work where
a systematic search, as for bacilli or in blood count-
ing, over the entire surface of object is required, it
will also be found a most useful accessory. The
obstacle of considerable cost which prevailed until
the present time, 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 built on a stage.

Attachable mechanical stage, which can be at-
tached to the Continental stands having plain
stage. This has advantages, since it may be re-
moved, leaving the stage plate free but, of course,
cannot be revolved. Either form involves the
most delicate work and while the parts are neces-
sarily small should be built with a view to strength
and durability. They should work with the

182

utmost precision and smoothness and with abso-
lutely no lost motion.

Revolving Stage. — This is also a great con-
venience in all work, while being a necessity in
some directions, and when provided with centering
screws may be used to some extent as a mechani-
cal 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.

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 substage attachment.

Substage Condenser. — If means will permit,
purchase this, as it is in all work most convenient
and in some directions, like bacteriology, abso-
lutely 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

is-,

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 objec-
tive and eyepiece to it and to the work. As a
variety of powers is obtained by a suitable combi-
nation of eyepieces and objectives and while power
in itself can be obtained by increasing the power
of eyepiece, this is not advantageous as we have
shown.

Eyepiece. — Select the Huyghenian eyepiece
and of no higher power than f inch. In cata-
logues many outfits are made up of one eyepiece
and two objectives, but this is only for economy ;
it is always advisable to select two eyepieces, per-
ferably the 2 inch and 1 inch and insist that they
be parfocal, as this will be found extremely con-
venient and will not disturb the optical standard
length. If for any work \ inch or higher powers
are desired, the solid eyepiece may be recom-
mended. With the apochromatic objectives use
the compensating eyepiece only. Every eyepiece
should be marked with its equivalent focus.

Objectives. — For all ordinary student and pro-
fessional work, not involving bacteriological inves-
tigations, the | inch 0.24 N. A., and \ inch 0.62
N. A. for long tube ; and f inch 0.25 N. A., and 1
inch 0.82 N. A. or \ inch 0.85 N. A., for short tube
have generally been accepted as best suited.

184

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

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

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 well to bear in
mind or study the advantage, which is obtained by
objectives of larger angular aperture. These
advantages are absolute and unquestionable, but
whether commensurate with the additional pecu-
niary 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
exceptional, 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 object of inducement, without
any reference to quality. Be distrustful of all
such objectives and if contemplating their pur-
chase, always reserve the right of having them
examined by an expert. Have a distrust especially
of all " nameless " objectives. It is safe to assume
that if the maker cannot attach his name he is
doubtful of their quality.

185

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 unre-
liable maker. This mode of offering objectives
was in vogue many years ago when the principles
of optics and facilities for making were limited and
when a higher price was asked for those which
might be termed a happy combination. There is
no excuse however, at the present day, for any-
thing of this kind, because every conscientious
optician has his standard for every objective.

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 in,-
strument as to the object itself and if the optical
parts are inferior, it will require but a short ex-
perience to become convinced of it — usually as
soon as a comparison can be made with reliable
work. The investment in one of these objectives
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
as have been spoken of are 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 sonsiderably

186

greater than purchasing 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 accessories are considered
useful. Some of them are absolutely necessary in
some special lines of work, in which case however,
the student is generally conversant with the re-
quirements 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 pro-
hibited by necessity, should accompany each outfit.

Abbe substage condenser, preferably the com-
plete substage attachment giving all adjustments.

Double, triple or quadruple nosepieces according
to the number of objectives accompanying the
microscope.

Abbe camera lucida.

Revolving or attachable mechanical stage.

Eyepiece and stage micrometer.

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

Pocket magnifier, preferably Aplanatic or Hast-
ings triplet.

Cover glass gauge.

Flat-wick oil lamp.

Dissecting stand or dissecting microscope.

187

Besides these there are other requirements such
as slides, covers, mounting media, forceps, etc., the
necessity of which, however, can be better deter-
mined 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.

Photomicrographic camera.

Live box or compressors.

Eye shade.

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 likelihood 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 condition as when first received. Accidental
injury may of course occur to them, but if a syste-
matic manner of working is followed and a special
receptacle for each part is provided, this may
usually be avoided.

To Take Care of a Stand. — Keep free from
dust is one of the* first rules to be observed. This
may be done in a manner formerly prescribed. If
dust settles on any part of the instrument remove

189

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.
Keep the working and sliding parts absolutely
free from dust, as this grinds and will thus soon
cause play.

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 a spiral motion.

To lubricate any of the parts, use a slight quan-
tity of soft tallow or good clock oil, or pararfine oil.

If the pinion ivorks 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.

In using a screw -driver, grind its two large sur-
faces so that they are parallel and not wedge-
shape and so it will exactly fit in the slot of the
screw-head.

In 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.

When repairs or alterations are necessary, always
have these make by the manufacturers, who can,

190

from a system of duplicated parts, not only do it
cheapest, but best.

To Take Care of Objectives and Eyepieces.—

The utmost cleanliness must be observed with
objectives and eyepieces. When indistinct, dark
specks show in the field, the cause may usually be
looked for in the field lens of the eyepiece, although
sometimes in the eye lens also. The dust may be
removed by a camel's hair brush, but when this is
not sufficient use a well washed piece of linen, such
as an old handkerchief. From its fine texture
chamois skin is desirable, but as it is fatty it should
never be used until after it has been well washed.

The same method applies to cleaning objectives.
Clean an immerson objective immediately after it has
been used, first by removing the fluid with a moist
linen and then by iising a dry piece, or by means
of Japanese lens paper. Never separate the systems
of objectives, even if they can be unscrewed by the
fingers ; it is always dangerous as they are liable
to become decentered.

Keep the objectives especially in a place where
they are not subject to extreme and sudden changes
of temperature, as the unequal expansion and con-
traction of glass and metal may cause the cement
between the lenses to crack. Also keep them from
direct sunlight.

191

Screw them into the nosepiece and unscrew, by
grasping the milled edge.

Avoid any violent contact of the front lens with
the cover glass. Usually the latter suffers, but it
is as liable to injure the former.

Above all, the owner should make it a mle that
no one except himself handles the microscope and
accessories. One person may be expert in the
manipulation of one instrument and still find it
difficult to work with another. The fine adjust-
ment particularly causes the greatest difficulty, as
in some instruments it corresponds with the move-
ment of the micrometer screw, while in others it is
contrary and thus the objective as well as the
object is endangered.

INDEX.

ABBE CAMERA LUCIDA, .T^^iii ._.... 164

Abbe Condenser, '154, 182

Aberration, Chromatic, 24, 25, 94, 142

Aberration, How Corrected, 25, 26

Aberration, Spherical, 22, 23, 96, 140

Accessories, 186

Achromatic Lens, 26

Adjustments, 47

Adjustable Objectives, 98

Adjustment, Closed, 98

Adjustment, Open, • 99

American Type Microscope, 176

Amplification, no

Angular Aperture, 81

Apertometer, 91

Aperture, 83, 87

Aperture of Condenser, 160

Aplanatic Triplet, 30, 38

Apochromatic Objectives, 112

Arm of Microscope, 45

Attachable Mechanical Stage, 57, 181

Axis of Lens, 19

Axis, Optical, 49

BASE OF MICROSCOPE, 45, 67, 177

Binocular Microscope, 61

Blood Counting, 58

Body of Microscope, 45, 61

Bruecke Lens, 31

Burning Point, 21

CAMERA LUCIDA, 163

Camera Lucida, Abbe, 164

Camera Lucida, Wollaston's, 164

Cap Diaphragm, 70

Cedar: Oil, 77

Centering Illumination, 158

11)4

Centering Screws, 48, 55

Chromatic Aberration, 24, 25, 94, 142

Clips, 48

Closed Adjustment, 98

Coarse Adjustment, 47, 62, 177

Coddington Lens, 29

Collar, 47

Collective Lens, 74

Colors, Primary, 24

Compensating Ocular, ..-..- 112, 117

Compound Microscope, 15, 42

Concave Lenses, 18, 19

Condenser, Abbe, 154, 182

Condenser, Achromatic, 155

Condenser, Aperture of, 160

Condenser, Centering of, 157

Condenser, Focusing of, 159

Condenser, Purpose of, 154

Continental Eyepiece 115

Continental Microscope, 50, 176

Convex Lenses, 18, 19

Corrected Lens, 26

Cover Glass, 49, 83, 96, 142

Cover Glass, Corrections for, 96-103

Cover Glass Gauge, ; 101

Cover Glass, Thickness of, 96

DIAMETERS 39

Diaphragm, 24, 49, 70, 114

Diatoms, How to Resolve, 150, 151, 152

Diatom Test Plate, 132

Diatoms, When Resolved, 139

Dispersive Quality, 24

Dissecting Microscopes 28

Dissecting Microscope, How to Use, 35

Dome Diaphragm, 70

Double Nose-piece 60

19,-)

Doublet So

Drawing Board, 166, 168

Draw Tube, 46, 66

Dry Objectives, 77, 85, 86, 144

ENGLISH EYEPIECE, 115

Excelsior Dissecting Microscope, 35

Eye, Which to Use, 130

Eye Lens, 74

Eyepiece, 46, 72, 112, 113, 182, 183

Eyepiece, Care of 180

Eyepiece, Diaphragm in 119

Eyepiece Micrometer, 171

Eyepiece, Parfocal, 119

Eyepiece, Rating of 117

Eyepiece, to Attach, 121

FIELD, FLATNESS OF 104, 118

Field Lens 74

Field of Objectives, uo, 118

Filar Micrometer, 173

Fine Adjustment, 47, 65, 179

Flatness of Field 104, 118

Flint Glass, 27

Focusing, 128

Focal Distance, 19

Focal Point, 19, 21

Focus, Real, 20

Focus, to Determine, 20

Focus, Virtual 20

GLASS COVERS, 49, 83, 96, 142

Glass, Crown, 27

Glass, Flint, 27

Glass, Jena, 74

Glass, Reading, 31

Glass Stage 56

Graduated Head 65

Graduated Stage, 55

196

HASTINGS TRIPLET, 30

High Power Objectives, 79

Homogeneous Immersion Fluid, 78

Homogeneous Immersion Objectives, 85, 86, 147

ILLUMINATION, 154, 158, 161, 124

Image, Magnified, 42

Immersion Fluid, 77

Immersion Objectives, 77, 147

Index, 65

Iris Diaphragm. , 70

JENA GLASS, 74

Joint for Inclination, 68, 177

KELLNER EYEPIECE, 116

LENS, ACHROMATIC, 26

Lens, Bruecke, 31

Lens, Coddington, 29

Lens, Collective, 74

Lens, Eye, 74

Lens, Field, 74

Lens, Hastings, 30, 38

Lens Stands and Holders, 32

Lens, Triplet, 30, 38

Lenses, 15

Lenses, Convex, 18, 19

Lenses, Concave, ) 8, 19

Lenses, Converging, 18

Lenses, Diverging, 18

Lenses, Meniscus, 18, 19

Lenses, Optical Properties of, 15

Lenses, Stopped Down, 24

Light, Refraction of, 16, 17

Linear Magnification, 39

Linen Tester 33

Long Tube Microscopes, 50

Low Power Objectives, 79

MAGNIFICATION, Linear, In Areas, In Diameters, 39

19?

Magnified Image 42

Magnifier, Tripod, 32

Magnifiers, 28

Magnifiers, How to use, 35

Magnifying Power, 21, 38, 109

Magnifying Power, to Determine, 40, 169

Mechanical Stage, 56, 181

Medium Power Objectives, 79, 138

Meniscus Lenses, 18, 19

Micrometer, Eyepiece 171, 116

Micrometer, Filar, 173

Micrometer Screw, 65

Micrometer, Stage, 170

Microscope, American Type, 176

Microscope, Binocular, 61

Microscope, Care of, 188

Microscope, Dissecting, How to use, 35

Microscope, How to Select, 175

Microscope, How to Work with, 136

Microscope, Material of, 179

Microscope, Parts of, 45

Microscope, Purpose of, 15

Microscope, Simple and Compound, 15, 28, 42

Microscope, Size and Weight, 180

Microscope Stand, 43

Microscope, To Set Up, 135

Microscopes, 28

Microscopes, Classification of, 49

Microscopes, Continental Model, 50, 176

Microscopes, Jackson Model, 49

Microscopes, Ross, 49

Milled Edge, 47

Milled Head, 47

Mirror, 48, 69

Mirror Bar, 48, 69

Mirror, Focal Point of, 69

19S

Moellers Test Plate, 132

Monocular Microscope, 61

Movable Object Carrier, 56

NEGATIVE EYEPIECE, u4

Nose-piece, ... 46, 59

Numerical Aperture, 87

Numerical Aperture, How to Measure, 89

OBJECT, 49

Object Carrier, Movable, 56

Object, Opaque, 137

Object, Resolution of, 92

Object, to Find, 122

Object, to Illuminate, 124

Object, to Measure, 170

Object, When Centered, 49

Objects, How to Draw, 163

Objects, Test, 132

Objects, Test, Which to Use, . . 131

Objectives 46, 72, 77, 85, 86, 138, 147, 182, 183, 184

Objectives, Adjustable, 98, 144

Objectives, Angle of, Si

Objectives, Apochromatic, 112

Objectives, Care of, , 190

Objectives, Classification of, 79

Objectives, Powers of, 79

Objectives, Rating of, 78

Obje .tives, Systems of, 79

Objectives, Testing of, 95, 96, 149

Objectives, to Attach, 121

Objectives, Tube Length of, 78

Oblique Illumination, 161

Ocular, 46, 72, 112, 114, 182, 183

' Oculars, Care of, 190

Oculars, Rating of, 117

Oil Immersion Objectives, 77, 78, 85, 86, 147

Opaque Object 137

199

Opened Adjustment, 97

Optical Axis, 49

Orthoscopic Eyepiece, 116

Over Correction, 95

PARFOCAL EYEPIECES, 119

Penetration, 103

Periscopic Eyepiece, 116

Pillar of Microscope, 45

Pinion , 63

Plankton Work, 58

Pleurosigma Angulatum, 109, 131, 132, 133, 138, 141, 146, 150

Positive Eyepiece, 114

Power, Magnifying, 21, 38, 109

Primary Colors, 24

Principal Axis, 19

Principal Focus, 19

Projection Eyepiece, 117

RACK AND PINION, 47, 63, 177, 178

Radius of Lens, 19

Ramsden Eyepiece, 1 16

Reading Glass, 31

Real Focus, 20

Real Image, 22

Refraction, 16, 17

Resolving Power, 91

Revolving Diaphragm, 70

Revolving Nosepiece, 60

Revolving Stage, 55

SCREW MICROMETER, 173, 65

Screw, Universal, 46

Searcher Eyepiece, 115

Secondary Colors, 94

Short Tube Microscopes, 50

Simple Microscopes, 15, 28

Slide, Object, 49

Sliding Tube Adjustment, 47, 177

200

Slips, Glass, 49

Slow Motion, 65

Society Screw, 46

Solid Eyepiece, 115

Spectrum, 24

Spectrum, Secondary, 94, 112

Spherical Aberration, 22, 96, 140

Spring Clips, 48

Stage 47, 53, 180

Stage, Glass, 56, 181

Stage, Micrometer, 170

Stage, Revolving, 55, 182

Stage, Space Below, , 1 80

Stand, 43, 176

Striae, 119

Substage, 48, 182

Substage Bar, 48

Substage Condenser, 154, 182

TESTING OBJECTIVES, 95, 96, 149

Test Objects, 132

Test Objects, Which to Use 132

Test Plate, 132

Triple Nose-piece, . . . 60

Triplet, Hastings, .... 30, 38

Triplet Lens, 30, 38

Tripod Magnifier, 32

Triplet System, ._ 80

Tube Length, 50, 78, 176

UNDER-CORRECTION, 95

Universal Screw, 46

VIRTUAL FOCUS, 20

Virtual Image, 22

WOLLASTON CAMERA LUCIDA, 164

Water Immersion Objective, 85, 86

Working Distance, 106

Working Distance, to Measure, 107

The BAUSCH & LOME

OPTICAL COMPANY

Issue the following- Catalogues:

MICROSCOPES, MICROTOMES AND
LABORATORY SUPPLIES.

^t

PHOTOGRAPHIC LENSES AND
SHUTTERS.

^t

EYEGLASSES AND LENSES, ETC.

at

Offices:

ROCHESTER, N. Y.,
NEW YORK CITY, CHICAGO, ILL.,

130 Fulton Street. 408 Masonic Temple.

Bauscb $ Comb

Optical Company,

manufacturers of

Microscopes, Microtomes,

Photomicrographic Cameras, Microscopic

Accessories, Centrifuges.

^
Sterilizers, Incubators, Autoclavs, Etc.

j*

Photographic Lenses,

Photographic Shutters, Etc., Etc.,

<*

Offices:

ROCHESTER, N. Y.,
NEW YORK CITY, CHICAGO, ILL.,

J30 Fulton Street. 408 Masonic Temple.

UNIVEESITY OF CALIFOENIA LIBEAEY,
BEEKELEY

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

Books not returned on time are subject to a fine of

st?

OCT 2 0 i^b
UNIV.* CALIF, BERK.

.

50m-8,'26