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THE
MICROSCOPE
CONRAD
BECK
QH205
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This book was presented by
Theodore G. Rochow
QH205
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NC STATE UNIVERSITY D.H HILL LlBRAR
S001 39483 S
Collins & Gray
Scientific FJooks & Periodicals
■iQA Museum Street,
] London, W.C.I. hol '>l''n
THIS BOOK IS DUE ON THE DATE
INDICATED BELOW AND IS SUB-
JECT TO AN OVERDUE FINE AS
POSTED AT THE CIRCULATION
DESK.
\0 2)UK^C2-
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THE MICROSCOPE
THE MICROSCOPE
A SIMPLE HANDBOOK
BY
CONRAD BECK
FIRST EDITION
TWO SHILLINGS AND SIXPENCE NET
LONDON
R. & J. BECK, LTD.
68 GORNHILL, E.G.
1921
PREFACE
This book is intended as a guide to the use of the microscope.
The correct use of the instrument follows directly from a know-
ledge of the functions of the different parts. It has therefore
been found best to develop the method of manipulation in the
course of the descriptions of the component portions. Such
particulars as are given of the principles of its optical construc-
tion are of the simplest character, and their comprehension
requires no optical knowledge. Certain theoretical matters
are stated in this book, but are not explained. Some of the
descriptions may appear to be extremely elementary to experi-
enced microscopists, but it is hoped that even they may find useful
matter which is not available in the ordinary text-books.
The author hopes to publish a further volume at a later date,
which will deal with the optical theory and the use of the
microscope in greater detail.
CONTENTS
CHAPTER I
PAGE
A SIMPLE DESCRIPTION OF THE MICROSCOPE . . .11
CHAPTER II
ILLUMINATING APPARATUS AND SOURCES OP ILLUMINATION . 26
CHAPTER III
APPARATUS FOR HOLDING SPECIMENS .... 50
CHAPTER IV
SUNDRY APPARATUS ... .... 67
CHAPTER V
OBJECT GLASSES AND EYEPIECES 76
CHAPTER VI
THE MICROSCOPE STAND 92
CHAPTER VI r
THE MICROSCOPE AS A RECREATION 125
INDEX .......... 143
DESCRIPTION OF FIG. 1
A. Base to support instrument.
B. Pillar to support instrument.
C. Joint for inclining instrument.
D. Stage for reception of object to be examined.
E. Hole for attaching mechanical stage.
F. Mirror for illuminating transparent objects.
G. Substage for carrying illuminating apparatus.
H. Substage focussing adjiistment.
J. Substage condenser for regulating the illumination,
K. Iris diaphragm for varying the light.
L. Limb for holding the body and stage.
M. Body for carrying the observing lenses.
N. Drawtube for lengthening body.
Nl. Drawtube stop to prevent reflections from tube entering eyepiece.
O. Eyepiece, combination of lenses nearest observer's eye.
P. Fine adjustment for delicate focussing of body.
Q. Coarse adjustment for rapid focussing of body.
R. Object glass, combination of lenses nearest the object.
S. Nosepiece with universal screw for carrying object glasses.
T. Eyepoint position where all emergent light passes through a small
area and where observer's eye should be placed.
U. Position where the primary image formed by the object glass is
produced.
V. Position where final virtual image formed by eyepiece is produced.
8
Fig. 1. — Diagram of a Microscope.
THE MICROSCOPE
CHAPTER I
A SIMPLE DESCRIPTION OF THE MICROSCOPE
The microscope is an apparatus for producing an enlarged
image of a small object. In its complete form it is an elaborate
instrument, but to understand its constrviction it may be looked
upon as a complex form of magnifying lens with the addition of
means for making delicate adjustments both for moving the lens
and the object and for obtaining special forms of illumination.
It consists primarily of three parts — the body, which carries the
observing lenses, the stand or framework, and the illumination
apparatus.
The body (M) carries an object glass (R), which is attached to The body.
the object end by a standard size screw thread, and an eyepiece
(0), which slips loosely into the tube at the eye end in a standard
size fitting. It has a telescopic tube, called a draw- tube (N), for
varying the distance between the object glass and the eyepiece,
and a diaphragm (Nl) to prevent reflections from the inner sur-
faces of the tubes from entering the eye.
The body and its lenses combined form the magnifying
apparatus.
The object to be examined is placed on the stage (D) of the
microscope. The object glass if used by itself acts in the same
manner as a lantern lens. It throws an enlarged picture of the
object to a position (U) at the upper end of the body, just as a
lantern lens throws an enlarged picture of a small lantern slide
upon a white screen, but instead of its being thrown upon a
white screen it is thrown into space. This image is examined
with a magnifying lens called the eyepiece (0), by which it is
further magnified. If the primary image were projected upon a
lantern screen and one were to cut a hole in the screen and stand
behind it with a magnifying lens focussed upon the plane of the
screen, one would have the same kind of instrument as a micro-
scope on a large and inconvenient scale.
11
12
THE MICROSCOPE
of object
glass.
The object The object glass (E) in the earliest instruments was a single
^^*^' double convex lens (Fig. 2) ; it gave an enlarged but very im-
perfect picture of small objects, the outlines were ^^777777^
surrounded by coloured fringes, and the details '^ii.r^^^f
were fuzzy and indistinct. Such lenses were made Fia. 2.
several hundred years ago, but in the early part
of the nineteenth century is was discovered that the defects of
a single lens could be overcome by using several lenses in
combination, made of different kinds of glass and of suitable
shapes and sizes.
Focal length Modern object glasses are made of different powers to give
different magnifications in the primary image, and, in general,
the more an object glass magnifies, the larger the number of lenses
that are required to produce a perfect image- For instance,
Fig. 3 shows the optical construction of the 2/3-inch (16-mm.),
^ ^ l/6-inch(4-mm.),
and 1/12 - inch
(2-mm.) object
glasses.
The name
2/3, 1/6, or 1/12
inch, as applied
to an object
glass, represents
its focal length.
It indicates its
magnifying
^ power. If an
ordinary single
Fig. 3. — / = focal length ; w = working distance, \qy\s, of 2-inch
focal length is
used as a hand magnifying glass, it has to be placed about
2 inches from an object to give a clear image, and the
2/3, 1/6, and 1/12 inch require to be placed at about these
respective distances from the object when in use — thus the
higher the magnifying power of a lens, the closer it must be
to the object.
Object glasses are not single lenses, but are composed of
several, and consequently the focal distance is measured from a
point about half-way between the front and back surfaces of the
component lenses. The distance between the foremost lens
and the object is, therefore, always considerably less than the
true focal distance. This is called the working distance to signify
the space between the end of the microscope and the object
when it is so adjusted that a clear picture is obtained, or when
it is, as it is called, " in focus." A list of the working distances
of different object glasses is given on page 82.
An examination of the diagram (Fig. 1) on page 9 illustrates
Working
distance.
Magnifying
power.
A SIMPLE DESCRIPTION OF THE MICROSCOPE 13
the formation of the images. An enlarged pictm-e of an object
placed upon the stage (D) is formed in the neighbourhood of
the eyepiece at U, and the eyepiece again magnifies this image,
projecting the light into the eye as if it came from an object
situated at V.
The eye, when placed in a small area (T) through which all Eyepiece.
light passes, and which is known as the eyepoint, sees the final
picture of the object as if it were a real object placed at V,
10 inches from the eye.
It is assumed for convenience of measurement that this picture virtual
is actually 10 inches away, though it may be formed at a some- ^'"*°®'
what different position according to the adjustment or condition
of the observer's eye. Whether the virtual image is actually
at 6, 10, or 20 inches is of no importance. It makes no.
difference to the size of the picture, because when the virtual
image is formed farther away it becomes proportionally larger.
In Fig. 4, if E is the eye and 0 0' 0'' are objects of different
sizes, they produce the same size pictures in the eye if placed
at such distances that they
subtend the same angle.
The magnifying power of
the microscope will depend
upon the size of this final
image formed at V (Fig. 1)
compared with the size of the
object being examined. In this
connection it should be under-
stood that if a microscope is said to magnify 100 diameters,
it means that the picture that is seen is 100 times as long and
100 times as wide as the object would appear if it were
taken from the stage (D, Fig. 1) and placed in the position V,
10 inches from the eye.
In order to express how much larger an object appears
when seen through the microscope than when seen by the naked
eye, a standard distance must be taken, because an object appears
to the naked eye to be of different sizes at different distances.
A sixpence is almost invisible at a distance of 100 yards, but it
is a large object at 8 inches. Therefore, some standard
must be taken for comparison purposes, and 10 inches has
been universally adopted. The magnifying power of a micro-
scope always denotes the relative size of the picture com-
pared with that of the original object when placed 10 inches
from the eye.
If a microscope has a magnifying power of 100, such magnifi- ^^^J^^i^*^
cation may be produced by different methods. The object obtaining
glass may magnify the object twenty times in the primary image, ^^s^^^e
and the eyepiece increasing the primary image five times will
give a total of a hundred. This magnification may also be pro-
y
power.
14 THE MICROSCOPE
duced by a lower power object glass which magnifies the object
ten times, and a higher power eyepiece which magnifies it
again by ten. The same result is obtained as far as magnifying
power is concerned, but a different result as regards the quality
of the image.
Another method of varying the magnifying power is by increas-
ing the distance between the object glass and the eyepiece. To
enable this to be done the microscope is supplied with a sliding
drawtube (N), which allows the tube length to be varied from
140 to 200 mm. The reason for this increase in magnification
is well illustrated by reference to the lantern, in which case the
lantern lens gives a larger picture when it projects it upon a
screen that is at a greater distance. In the same way the micro-
scope object glass produces a more highly magnified primary
image if by sUght adjustment in the focussing of the instrument
the picture is formed at a greater distance, and the drawtube
of the microscope is extended so as to examine the picture
formed at this greater distance.
Field of The " field of view " is a term applied to the size of the object
^^^' that can be seen at one time by means of the microscope. To
assist in increasing the size of field an eyepiece is made of two
lenses instead of a single one. The lower field lens is situated
below the position U (Fig. 1), where the primary image is
produced, and increases the field of view while the upper lens
does the magnifying.
Suppose that the apparent field of view is a circle of about
8 inches diameter at the position V, where the final image
seen through the microscope appears to be. It is evident that
with a microscope magnifying 100 diameters, the size of the largest
object that can be observed at one time is only 1/100 the size
of this field, or about 1/12 inch, so that for this reason alone
it is important that a microscope should possess a means of
varying the magnifying power. It is sometimes desirable to
examine a large area of an object with a small magnifying power,
at others a small area with a large magnifying power. A table
of the fields of view given by different lenses appears on page 82.
The question arises as to whether it is preferable to vary this
magnifying power by means of changing the eyepiece, by means
of changing the object glass, or by means of lengthening the
drawtube.
This is influenced by an optical consideration of great
importance.
Aperture. In the early days, before it was understood how to correct
the errors of a single lens, microscopes were constructed in
which the object glass was a single lens, the defects of which
were reduced by putting a very small aperture — almost a pin-
hole— in front or behind this lens. This meant that only an
extremely fine cone of light from each point of the object could
Small Aperture Large Aperture
Lens. Lens.
Fig. 5. — a = angular aperture.
A SIMPLE DESCRIPTION OF THE MICROSCOPE 15
enter the instrument (see Fig. 5). It was soon found tliat
when this was the case, although great magnifying power could
be obtained, fine detail could not be seen, but merely a repre-
sentation on a larger scale of the coarse structure which could
readily be seen with a lower magnifying power.
In order that an advantage should be obtained from the use
of higher magnifying power, it was necessary to admit into the
microscope a correspondingly larger cone of light from each point
of the object, as unless this
were done, no advantage could
be obtained in the observation
of fine details. Such a plan
had the further advantage that
it collected a larger amount of
light and rendered the object
more brilliant. The size of the
cone of light admitted into the
microscope from each point of
the object is called the aperture
{a, Fig. 5). It is expressed
either by the angle of the cone
of light entering the micro-
scope or by a figure called the numerical aperture, or N.A.
The aperture is of such paramount importance, that the limit
of what can be seen with the microscope does not depend upon
what magnifying power can be obtained, but upon what size
cone of light can be collected from the object by means of the
object glass ; and lenses can be made with a much higher magnify-
ing power, but they cannot be made with a larger aperture, than
those in use at the present time.
The aperture, therefore, has a direct bearing upon the best
method of increasing magnifying power, because, if an object
glass can only admit a certain aperture of light, the use of an
eyepiece does not alter this property, and therefore to increase
the magnifying power by high eyepieces is of no service, when
carried beyond that power which is sufficient to enable the detail
that can be shown by the aperture of a particular object glass
to be seen.
The best method of increasing the magnifying power is,
therefore, by changing the object glass. Most object glasses
have sufficient aperture to allow of the use of an eyepiece of as
high a power as 15, but, in general, magnification of more than
10 by means of the eyepiece should only be used in special cases,
and the object glass should be changed rather than the eyepiece.
The same reason makes it undesirable to depend for increased
magnifying power upon extending the drawtube of the micro-
scope, and the more so in this case because the object glass can
only be constructed to work at its best with a particular length
Limit of
vision
dependent
on aperture.
Best method
of increasing
magnifying
power.
standard
length of
body.
16
THE MICROSCOPE
of body. To obtain the most perfect results a tube length of
160 mm. should be used — the drawtube of the microscope is
graduated, and can be set at this figure. If a revolving nosepiece
is in use, this lengthens the body 15 mm., and the drawtube should
be set at 145 mm. instead of 160 mm. ; with a Sloan object glass
changer measuring 10 mm. it should be set at 150 mm.
Thickness of The thickness of the cover glass used over the object has no
cover glass. g£fgg^ ^^}j q^j^ immersiou lens and but slight influence with the
low powers, but is a matter of importance with a high-power
dry lens. A 1/6-inch object glass can only be optically correct
for one thickness of cover glass, and it is most important to
always use those known as No. 1 thickness. The object
glasses, unless otherwise ordered, are always made for a thickness
of '007 inch ('18 mm.), which is the average thickness of No. 1
cover glass. Thicker cover glasses should only be used for
objects to be examined with low powers.
The delineation of fine structure depends upon the aperture
of the object glass being sufficiently large to produce an image
of this fine structure, but combined with this it must possess a
sufficient degree of magnification to enable this image to be clearly
seen. We may know that the finest lines of an etching or steel
engraving exist in a print, but it may be necessary to magnify
the image in order to make them visible as single lines to the
eye. If the print is magnified further, the fine lines appear
thicker, but no further fine lines are there to be seen. Thus
lines which are invisible require a certain degree of magnification
to see them clearly, but extra magnification beyond this point
is useless. So with a microscope object glass, it must possess
a large enough aperture to produce the detail in the image, and
the magnifying power need not be more than enough to enable
the eye to see it clearly.
Each object glass has a particular aperture, sufficient to
form an image of all the detail that can be seen with the magnify-
ing power given by it in conjunction with a moderate eyepiece.
The following table gives the apertures of standard object glasses :
Apertures
suitable for
different
powers.
Table of
apertures
and powers.
Focus.
Angular
Aperture.
Numerical
Aperture.
Initial
Magni-
fying
Power.
Magnj
fying Power with
Eyepiece.
42 mm.
25 mm.
17 mm.
1^ in. = 40 mm. .
19''
•16
3
20
34
50
1^ in. = 32 mm. .
17°
•15
4
25
45
65
2/3 in. =16 mm. .
32°
•28
10
62
110
155
1/3 in. = 8 mm.
60°
•5
18^5
115
200
285
1/6 in. = 4 mm. .
116°
•85
40
285
490
690
1/8 in. oil immer-
sion = 3 mm.
•95
60
427
735
1,015
1/12 in. oil immer-
sion = 2 mm.
1^3
90
530
900
1,275
A SIMPLE DESCRIPTION OF THE MICROSCOPE 17
The 1/6-incli is receiving from the object, cones of light Anf>ie in air
of 116°, as shown in Fig. 6. It could not be made to collect wiKaUn
a very much larger angle of light because it cannot be used in s^^ss.
absolute contact with the object. Sufficient space must be
provided for a thin glass cover and a small distance for focussing
adjustment. It will be noticed in Fig. 6 that the cone of light,
which is 116° as it enters the
object glass, is only 68° when it ^i^
passes through the object. It rQbif'
is spread out by refraction as it ^^^
enters the air between the cover /^^^^^u- {Ut^l^
glass and the lens. If the air ^^^^^^^'' ^^^•/^
space between the cover glass • z::.^::,^^.:.:C:)^^S■:^uC^^■Z::::^ r,, ,
and the lens could be filled up I-^^vvM^'^v^i'-vSI^
with glass, this spreading out of '• -"■■'-'■^;^j ••■•••"- :y^^.^- ■■■;-:_
the cone would not occur, and " UG"*'
the cone of light would remain Fig. 6.
68° when it entered the lens.
As far as the power of depicting detail is concerned it would be
equal to a 116° cone in air. It is the same body of light and has
just the same properties in this respect. If therefore the space
between the object and the lens is glass throughout, a larger
angled cone than 68° can be collected by the object glass, and a
greater power of depicting detail, what is known as resolution,
can be reached, and a further power of seeing fine structure
obtained.
Cedar- wood oil is a liquid which has the optical properties of immersion
glass, and if a drop of this oil is placed between the front of gfaS.
the object glass and the cover glass, the whole distance between
the object and the lens is equivalent to glass. A much larger
effective aperture can thus be obtained with corresponding
increase in resolution.
Thus object glasses of higher power than 1/6 inch (4 mm.)
are generally what are called immersion object glasses. They are
so constructed that a drop of cedar-wood oil must be placed
on the front lens so that it connects it to the object being
examined.
The method of describing the aperture by the term numerical Numerical
aperture (N.A.) instead of by the actual angle of the cone is to ^^^^ ^^^'
enable the resolving power of a microscope to be correctly stated.
A 1/12-inch oil-immersion object glass is generally made to admit
an angle in glass of 117°, which corresponds to an angle of more
than 180° in air. It is almost the same actual angle as the 1/6 -inch
admits from air, but the numerical aperture (N.A.) which gives
its true resolving power is 1*3 N.A., while that of the 1/6-inch is
only -85 N.A.
Dry lenses such as the 1/6-inch cannot be used with cedar-
wood oil as immersion lenses, and immersion lenses cannot
18
THE MICROSCOPE
Immersion
fluids.
Flatness of
field.
Depth of
focus.
Coarse
focusing
adjustment.
be used without the cedar-wood oil because the lenses must
be specially constructed for the conditions under which they
are used.
No immersion fluid but cedar-wood oil, or a fluid sold for the
purpose with exactly the same optical properties, must be used.
Water, paraffin, or several other substitutes which are some-
times inadvertently employed, entirely destroy the fine quality
of the image formed by an oil-immersion lens.
In every optical instrument the centre of the field gives the
finest definition, and the object being examined should be placed
near the centre. An absolutely flat field is incompatible with
the finest definition in the centre, and although in certain types
of telescopes and photographic lenses the importance of a flat
field is so great that a compromise is made, no deterioration of
the central image can be aUowed in the microscope.
The penetration or depth of an object glass or the number of
difierent layers of an object that can be seen sharply at the
same time with a microscope is very small. With lenses of a
high aperture, and therefore in general of a high magnifying
power, the penetration decreases at a very rapid rate, and the
power of seeing different planes sharply must depend on adjusting
the instrument. It has been said that the depth of focus of a
high-power microscope is really the fine focussing adjustment.
The fine adjustment in the hands of a skilled observer is in constant
motion, focussing first to one plane and then to another ; by this
means a perception of depth is obtained which could never be
given by an object glass fixed at one focus.
■ The penetration of the microscope may be increased by insert-
ing a stop with a small aperture immediately behind the object
glass, but such a method reduces the aperture and consequently
the detail that can be seen. It is seldom adopted except for
photographing certain objects where the image from the upper
or lower portion of the object obscures the layer being
photographed, or for photographing objects with compara-
tively coarse structure. An iris diaphragm is made that will
screw into the body of the microscope between it and the object
glass for this purpose.
There is only one position in relation to the lenses where an
object can be placed to give a perfectly clear picture. This
position is generally called the focus, and the microscope is said
to be '* in focus" when it is so adjusted that the object is in this
position. It is more convenient to effect this adjustment
by moving the body which carries the lenses rather than by moving
the object. The coarse focussing adjustment is actuated by
a helical rack and pinion which moves the body along a slide
towards or away from the object. Turning the milled head so
that its upper edge moves towards the observer, raises the body ;
away from the observer, lowers it. It is a sufficiently delicate
A SIMPLE DESCKIPTION OF THE MICROSCOPE 19
motion for focussing with object glasses of lower power than
1/6-inch (4 mm.).
The fine focussing adjustment does exactly the same as the Fine
coarse adjustment, but the movement is far more delicate : ^oc'issing
', • , . T -I • T1 '11, adjustment.
It IS actuated by a micrometer screw and a lever moving the whole
body along a second slide. A complete turn of the screw moves
the body about a quarter of a millimetre. Turning the fine
adjustment milled heads moves the body in the same direction
as those of the coarse adjustment. In the " Standard " micro-
scope the left-hand milled head is twice as delicate a motion as
that on the right-hand side. The fine adjustment is required
for the focussing of high powers and for examining the different
layers of an object.
In moving the body of the microscope up and down to obtain The best
the correct focus, care is required to prevent the front of the Scu^s(5,°.^
object glass being forced into contact with the object by racking
it too far down. It is easy to break a valuable specimen by this
means ; and although for its protection the metal mount of
the object glass projects slightly in front of the front lens, it is
delicate in construction, and can be damaged by being brought
into contact with the specimen.
Experienced microscopists can focus a lens downwards and
stop at the position where the object is sharply seen, but it is
unsafe. The correct method is to set the body of the microscope
so that the front of the object glass almost but not quite touches
the object, and then to rack backwards, turning the milled heads
so that the upper portion turns towards the observer, and raise
the body until the correct focus is found. With high-power
object glasses, especially oil-immersion lenses, this method is
not so easy because the distance of the correct focus may be
below the point at which the body has been set in the first instance.
If, however, the slow motion is used to make the final adjustment,
damage is not likely to occur, as it lowers the body very gradually,
and the latter is only pressed down upon the object by a spring.
When using an oil-immersion lens ajdrop of cedar -wood oil should
be placed on the object glass, and the body of the microscope
racked down until the drop of oil touches the cover ; the final
focussing can then be done with the fine adjustment.
Some objects are so transparent that it is quite easy to pass
by the focus and miss the correct position. In these cases dust on
the cover glass maybe focussed first, and the fine adjustment lowered
by an amount representing the thickness of the cover glass. If
the slide be moved backwards and forwards on the stage during
the process of focussing, the movement will be seen directly the
correct position is nearly reached.
It may appear absurd to mention that if a slide happens to
have been placed on the stage upside down a high-power object
glass will not focus through the thick glass slip, but the writer
20
THE MICROSCOPE
ReTolving
nosepiece.
Sloan object
changer.
has more than once made such a mistake and wondered why he
could not focus his specimen.
The nosepiece (S) of a microscope is the lower end of the
body (M) provided with a screw for attaching object glasses. A
revolving nosepiece is an appliance which
screws into the nosepiece and which
carries a revolving plate into which two
or three object glasses can be fixed,
known as double or triple nosepieces
respectively. By rotating the revolving
plate each object glass can be rapidly
Fig. 7. — ^No. 3301, Dust- brought into use, being held in the
tight Triple Nosepiece. correct position by a spring clip. The
best form is made so that no dust can drop into the back of the
object glasses and they can be safely left attached to the micro-
scope. The extra length of the body caused by the length of a
nosepiece is 15 mm., and the drawtube should be closed by that
amount or set at 145 mm. instead of 160 mm.
An object glass changer is an apparatus for rapidly changing
the object glasses by another method. Each object glass is
screwed into a fitting which slips into an adapter that is fixed to
the nosepiece of the microscope, and as each fitting is provided
with two adjustable abutment screws the object glasses can be
individually adjusted so that they exactly register as regards
the position of the field of view. Changing an object glass by
this means is nearly as rapid and more accurate than that of a
revolving nosepiece, and is far more convenient when the object
glasses are to be used on
different instruments or where
more than three are used.
It consists of an adapter
which has on one side a
sloping projection (A), and
on the other a clamp screw
(B) which actuates a bevelled
nut (C). The adapter is
clamped to the nosepiece of
the microscope by a screwed
ring (D), which is provided
with slots, into which a half-
penny will fit for tightening
it 'up.
Loose fittings (Fig. 9) are
supplied, one of which is screwed on to each object glass.
Each fitting has a bevelled gap which fits loosely over the
bevelled nut (C) of the adapter and swings round into position
when a turn of the milled head (B) forces the fitting against the
sloping projection (A) and holds it firmly in position. Each
'•«w..* ^
Fig. 8.— No. 3280, Sloan Object
Glass Changer.
A SIMPLE DESCRIPTION OF THE MICROSCOPE 21
adapter has screwed studs with clamping screws, which form the
stops in both directions when the object glass is in the correct
position. These can be adjusted by means of a spanner supplied
for the purpose, so that each lens can be centred with an accuracy
that is never possible with a revolving
nosepiece, because ' the error of each
individual object glass cannot be com-
pensated with the latter.
The construction of this apparatus
is so simple and rigid, having no slides
to wear loose, that it remains in ad-
justment permanently.
The total extra length of the micro-
scope body caused by its use is 10 mm., -pia. 9 No 3281 Fit-
and the drawtube should be set at ting of Sloan Object
150 mm. to obtain the standard tube Glass Changer,
length.
A box is supplied to carry object glasses with fittings screwed
on ready for use, held against dust-tight pads.
The illumination of an object seen with a microscope is ofiiiumina-
almost as much importance as the quality of the lenses. It is*^°°*
interesting to find that the methods worked out by those who
were enthusiastic in the use of the microscope as an enjoyment,
and to a great extent as an amusement, have been one by one
adopted by the more serious scientific worker who has sometimes
been ready to consider the time spent on the pure manipulation
of the instrument to be of little value. The proper use of the
substage condenser to regulate the light in viewing transparent
objects is now acknowledged to be of first importance for correct
observation. Dark-ground illumination, which has been considered
by some to be only useful to show in an attractive manner what
could be seen equally well by direct light, has proved to be of
paramount importance for the study of living bacteria and colloid
particles. The methods devised for illuminating opaque objects
have formed the basis for the observation of metallurgical
specimens, and the much- criticised study of the markings of
diatoms and insects' scales has proved to be of the greatest value
in enabling the images seen by the microscope to be correctly
interpreted. A bad lens can never be made to give a perfect
image, but a good lens will only give the best image when the
illumination is satisfactory. Most objects seen with the naked
eye only require that a sufficiently powerful light should fall upon
them. They reflect back the light that they receive, or the
greater portion of it, in all directions. It is not of importance
where the light which illuminates them comes from, although
occasionally, when the light falls upon them from one side only,
such deep shadows may be formed that it is difficult to recogm'se.
the true appearance.
22
THE MICROSCOPE
Visioa of
natural
objects.
The mirror.
Direction of
the light.
The same holds true of opaque objects examined with the
microscope, but the greater mimber of microscope specimens are
either transparent or semi-transparent, and must be viewed by
sending a beam of light through them from behind. This beam
then passes through the microscope into the eye. Natural objects
are seldom viewed in this manner, but in order to examine the
water mark of paper or a photographic transparency, they must
be held between a strong light and the eye, and the ability to see
the pattern of the water mark or the view in the transparency
depends on certain portions of the light being blocked out which
would otherwise enter the eye. In the black portion the whole
light is stopped, in others only a portion is absorbed, and thus a
complete range of tone in the picture may be obtained. This is
the method by which semi-transparent objects are seen with the
microscope.
The conditions are not the same as ordinary vision, and the
direction and character of the beam of light used to illuminate
them are a matter of great importance.
The mirror of the microscope (F, Fig. 1) is used to direct a
beam of light from some source of illumination through the
object into the microscope. The mirror swings in gimbals and
can be moved in all directions. It has
on one side a flat, silvered surface
which gives a plane reflection, and on
the other a concave surface which
concentrates a more powerful beam
upon a small area of the object.
The direction of the light should be
such that it shines directly along the
line that passes through the centre of
the microscope — the line that is known
as the optic axis. If the light comes
from the side it passes obliquely
through the object, and even if it does
not give an erroneous appearance it
prevents a clear image being formed.
To illustrate the effect on the object,
one should examine the appearance of
a fairly thick piece of wood which has
fine holes drilled in it : light from one
side would not pass through and would
not reach the eye. The holes will not
Oblique light. Direct light, be visible unless the light is passing
Fig. 10. through them centrally. The effect of
an oblique beam of light as it passes
through the microscope lenses is shown in Fig. 10.
The left-hand diagram in Fig. 10 shows the object glass
transmitting oblique light only. The light which actually forms
A SIMPLE DESCRIPTION OF THE MICROSCOPE 23
the image is a fine bundle thrown to the edge of the object glass,
so that the object glass acts as if it had only a pinhole aperture
at one side, and is consequently no better for depicting detail than
the early pinhole lenses which were made before the modern
achromatic microscope was discovered.
This diagram also shows how the direction of the light can be
immediately recognised by focussing the microscope. The only
light producing the primary picture is shown in the left-hand
diagram of Fig. 10. It is on the right or the left of the axis,
according to whether it is above or below the true focus. There-
fore, by putting the object in and out of focus with the focussing
adjustment, the direction of the light can be observed. When
the light which illuminates the object is oblique instead of being
truly central, the object will not only become indistinct on either
side of the focus, but will appear to move from side to side ; whereas
if the light is truly central, the object will become less distinct
on either side of the focus, but will not alter its position.
The mirror can always be adjusted until the object remains
stationary as the microscope is being focussed, and the centring
of the light is thus assured.
Below the stage of the microscope an iris diaphragm (K, Fig. 1)
is fitted, and if this is shut down to a small aperture the light will
not pass through the microscope at all if the light is very far away
from the axis, though this is not in itself sufficient to make the
final adjustment.
The nature of the illumination may be varied according to Nature of
whether it is parallel, divergent, or convergent. If the flat side
of the mirror be used and the ,
source of light is at a consider- -;;;;;;; !!\ ;''. ';|.'(''
able distance, a beam of nearly .i,'';:!; ft '•,:1;;;:.','.'
paraUel light is obtained (a, v';;';;: ;Z ;'¥-,','
Fig. 11). If the source of iUu- %^km ^m^m Sfe^v..v.,
mination is very close, a diver- ^|j^=-= ^|^i ^|^-i-_-----:-v.-::
gent beam is obtained (c, Fig. 11). * ^ ▼ ^
If the concave mirror is used, it a be
will be a slightly convergent Fig. 11. — Mirror reflecting parallel,
beam (6, Fig. 11). By means of convergent, or divergent light.
a substage condenser (J, Fig. 1)
with an iris diaphragm below it— described later— the light
can be rendered still more convergent and can be regulated
with accuracy. As the light which enters the condenser at its
margin emerges as the outer portion of the cone, the effect of
reducing the aperture of the diaphragm of the condenser is not
only to reduce the amount of illumination, but to alter its
character by reducing the size of the cone of emergent light.
Thus with a very small aperture an almost parallel beam of light
can be obtained, and by opening the iris diaphragm a more and
more highly convergent cone of light may be used (see p. 27).
24
THE MICROSCOPE
niumina- As regards the best kind of illumination for transparent
transparent objects, the light may be a nearly parallel beam from the flat
objects. mirror, or a slightly divergent beam from the flat mirror used
with a lamp near the mirror. The light from a lamp may be
rendered nearly parallel by placing a bull's-eye condenser close
to the lamp. To find the correct position for the bull's-eye to
give parallel light, an image of the flame or filament of the lamp
should be observed on a distant wall and the bull's-eye moved till
the lamp or filament is in sharp focus on the wall. The light is
then approximately parallel, and the microscope should be so
placed that the mirror is in the beam of light about 8 or 10 inches
away from the lamp.
The light may be made slightly convergent if the bull's-eye
be arranged to give parallel light, but the concave instead of the
flat mirror be made use of.
The light may be condensed by a substage condenser, which
not only increases the brilliancy of
the illumination, but also gives a
strongly convergent beam of light
which may be modified to any extent
by the use of the iris diaphragm and
stops, as described more fully under
the description of substage condensers.
The question as to whether the
best results will be obtained by
parallel, divergent, or convergent il-
lumination, depends to a great extent
on the nature of the object. When
light shines through certain kinds of
objects it is distributed or scattered
in all directions. A cut-glass lamp-
shade breaks up the light that falls
upon it and scatters it all round; the same thing occurs to
a lesser extent in the case of a botanical or histological section
of tissue when each cell or irregularity acts like a facet of a cut-
glass lamp-shade. In this case, whatever the nature of the
iUumination, there is a sufficiently scattered light to fill the
aperture of the object glass, and the general structure of the tissue
will be accurately depicted. This, however, does not apply to
all kinds of objects. Some do not scatter light, and the question
as to whether the aperture of the object glass is filled with light
depends on the nature of the illuminating beam. If an object
which does not scatter light is illuminated as shown in Fig. 12 {a),
the object glass might just as well have nothing but a pinhole
aperture ; and to make use of the aperture a convergent cone of
light must be thrown upon the object, as shown in Fig. 12 (6).
oft^Tbj ^^ ?^^^^ objects spread the light slightly by diflraction,
object. though in the case of a single dark object on a white field, the
a
Fig. 12.
A SIMPLE DESCRIPTION OF THE MICROSCOPE 25
amount of such spreading is relatively small and need not be
considered. If the object is a regular periodic structure, like a
series of dots and lines, the spreading due to this cause may be
very considerable, and such an object may not require so large
a beam to use the full angle of the object glass.
This is very noticeable in the case of the fine periodic structure
of diatoms, where the structure may often be shown when the
illuminating cone of light is considerably less than that required
to fill the whole of the aperture of the microscope. In such cases
it will be observed, if the eyepiece of the microscope is removed,
that the central direct beam illuminates the central portion of
the back lens of the object glass, but the rest of the lens may be
illuminated almost as strongly by the large amount of diffracted
light scattered by the periodic structure of the diatom.
For the use of the microscope with any but the lowest
magnifying powers, a substage condenser should be used in order
that the nature of the illumination may be completely varied at
will.
CHAPTER II
Substage
condensers.
ILLUMINATING APPARATUS AND SOURCES
OF ILLUMINATION
A MOST important part of the microscope has now to be con-
sidered, namely, the substage condenser, which is essential with
all higher powers to converge a beam of light upon the object
in order to illuminate it brilliantly and to vary the character
of the illumination.
There are three difierent kinds of substage condensers. The
simple so-called Abbe condenser consists of two lenses with an
iris diaphragm close behind the back lens and a tray below for
the insertion of patch-stops or colour filters, as shown in Fig. 13.
It was in use under various names long before the time of Abbe,
who, however, popularised it in a particular form of mounting.
It does not focus the rays correctly to one spot (see Fig. 14),
— 0
Fia. 13.— No. 3286, Abbe Condenser.
Fig. 14.
the oblique rays coming to a nearer focus than the more direct,
and does not form a definite image of the source of illumination
due to the uncorrected lenses of which it is constructed. It
has an aperture of 1 N.A. — that is to say, it will give 180° in air,
the maximum aperture obtainable with a dry condenser. A large
beam of light from the mirror thrown upon the back lens is
concentrated upon a small area (0). This area is illuminated by
an imperfect image of the source of light. The condenser fits into
the substage of the microscope by which it can be moved up and
down, or "focussed," or can be moved laterally, or "centred," until
the illuminated area (0) coincides with the object being examined.
The object is by this means brilliantly iUuminated. A powerful
illumination is often required to overcome the loss of light due
26
ILLUMINATING APPARATUS
27
to the large magnification obtained with high-power lenses ; but
this is not the only advantage gained by the use of a condenser,
as illumination might be increased by other means — for instance,
by bringing a source of light closer to the object. A substage
condenser receiving an approximately parallel bundle of light
from the mirror of the microscope converts it into a wide angle
cone of light. When this light is centred and focussed, the object
is illuminated by light falling upon it in all directions.
The achromatic condenser (Fig. 15) has the same aperture as Achromatic
the Abbe condenser 1 N.A., but it is corrected almost as care- ^^^ ^^^'^'
fully as a microscope object
glass, so that the rays come to
exact points, and a very perfect
image of the source of illumina-
tion is formed in the plane of
the object, much reduced in size.
It is provided also with an iris
diaphragm and a tray for patch-
stops and filters. Fig. 16 shows
a beam of light (A, B, C, D,
E, F) passing through this con-
denser to the central point of
the object at 0. The rays
A, F, which are at the margin
of the beam of light as it enters, emerge as the most oblique
rays falling upon the object 0, and the rays C, D, which enter
near the centre of the condenser, emerge nearly parallel. ^ Thus, if
the iris diaphragm which is placed below the condenser is gradu-
ally closed, it excludes more and more of the oblique rays. Fig. 16
shows a large solid cone of light of great angle converged upon the
object, the iris diaphragm being fully open. Fig. 17 shows a small-
angled cone transmitted by the same condenser, the iris diaphragm
being partially closed. Fig. 18 shows the same condenser in
which the iris diaphragm is fully open and
0 C an opaque patch or stop is placed below
"^ the condenser so that the object is being
Fig. 15.-
No. 3288, Achromatic
Condenser.
m
^\\
s\-
nN^
WW
m
m
m
A B C D E
Fio. 16.
Fig. 17.
Fig. 18.
28
THE MICROSCOPE
Immersion
achromatic
condenser.
Use of a
substage
condenser.
To focus a
substage
condenser.
Fig. 19.— No. 3291, Dry and
Immersion Condenser.
illuminated by a hollow cone. Stops or patch-stops with apertures
of different shapes, or in different positions below the condenser,
used in combination with the iris diaphragm, regulate the illumina-
tion so that light in any direction may be passed through the
object.
The dry and immersion achromatic condenser is of even higher
quality than the achromatic condenser,being equal in its corrections
to a microscope object glass, and has an aperture of 1*3 N.A., so
that it can, if used in immer-
sion contact with the under-
surface of the slide, fill the whole
aperture of an oil-immersion lens
with light.
For transparent objects with
object glasses as low in power as
a IJ-inch (32-mm.) a condenser
is not required. The aperture of
such a low- power lens is small,
and the angle obtained by the
use of a concave mirror is usually
sufl&cient to make the best use of
this lens.
The same applies to some extent to the 2/3-inch (16-mm.)
object glass, but as this lens is so frequently used as a finder for
a high-power, it is not always convenient to rapidly remove the
condenser, and it is customary to use the condenser, but to put
it somewhat out of focus in order to fill the whole field with an
even illumination. A condenser should always be used with
the 1/3-inch (8-mm.), 1/6-inch (4-mm.), 1/8-inch (3-mm.), or
1/12-inch (2-mm.) object glasses.
A substage condenser is only corrected for light which is
parallel or slightly divergent ; therefore the flat mirror should be
used. The concave mirror giving convergent light is quite un-
suitable for use with a condenser.
Daylight as a source of light is not recommended with a
condenser, as the finest detail cannot be shown bv its means.
For ordinary microscopic examination of not too critical a
nature, daylight is satisfactory, but even then the more delicate
details may escape notice.
Assuming that the source of light is a paraffin lamp with a
flat wick, or other small source of illumination, the condenser
must first be focussed and centred.
In order to focus the condenser, sufficient light must be thrown
through the object to render it visible, and the object glass must
be focussed upon the object. The iris diaphragm of the condenser
should then be shut down to about one- quarter its size, and it
should be focussed up and down until an image of the flame of
the lamp is seen sharply in focus at the same time as that of the
ILLUMINATING APPAEATUS 29
object ; the mirror will require to be adjusted in order to direct
the light through the condenser. If the lamp be turned round
so that the edge of the flame is opposite the mirror, it makes
an easier object to focus, and its image will appear as a slit
across the field of view. When focussed the lamp may be turned
round, with the flat side of the flame facing the mirror, thus
illuminating the whole field. It is found in practice that the best
resolution is obtained when the source of light is almost, but not
quite, in focus.
The accuracy of centring of the simple Abbe condenser is not To centre
important. The image that it gives is not accurate, and it is col'denS
generally sufiicient to move the mirror slightly till the image of
the object does not move from side to side, while the body
of the microscope is being focussed up and down in a similar
manner to that described on page 22 for setting the mirror
when used alone. Microscopes fitted with this condenser are
frequently not provided with a centring adjustment to the
substage.
When using the achromatic or immersion condenser centring
is a matter of importance. The iris diaphragm is placed in a
position at such a distance below the condenser lenses that if
the condenser be moved downwards the source of light will be
put out of focus and an image of the small aperture in the iris
diaphragm can be sharply focussed. When doing this the iris
diaphragm should be closed to its smallest aperture, so that
the image is of a sufficiently small size to be seen in the field of
view. The condenser may then be moved by the centring
screws until the image of the diaphragm is in the centre of the
field of view. If the image of the small aperture is so far out of
centre that it is not in the field, the aperture can be enlarged
until its edge begins to appear. When the condenser has been
thus centred it should be moved upwards till the image of the
lamp is again in focus on the object, and the mirror readjusted
if the light is not in the centre of the field.
The achromatic condenser is now in the best position for use
with a 1/3-inch (8-mm.), 1/6-inch (4-mm.), or 1/12-inch (2-mm.)
object glass, though slightly better resolution is obtained if the
source of light is a little out of focus. If it is required to use a
2/3-inch (16-mm.) object glass, the condenser can be focussed
down to give an evenly illuminated field, being brought back
into focus when a higher power is used.
The substage condenser having been focussed and centred. Effect of
the eyepiece of the microscope should be removed, and the cEi^^iris*^
effect of opening and closing the iris diaphragm be observed by diaphragm of
looking down the tube of the microscope. This, effect is best
observed when an object glass with a fairly large aperture such
as 1/6-inch (4-mm.) is used. When the iris diaphragm is fully open
the back lens of the object glass will be completely filled with
30
THE MICEOSCOPE
Effect of
focussing a
Bubstage
condenser.
light appearing as a uniform circular disc; closing the dia-
phragm will at first not make any change in the appearance,
because the condenser is co verging upon the object a beam of
light of greater angle than can be collected by the object glass,
and it will receive the full illumination until the light from the
condenser becomes smaller in angle than the aperture of the object
glass. If the diaphragm be now closed to its fullest extent, the
back lens of the object glass shows a small spot of brilliant light
in the centre ; and as the aperture of the iris diaphragm is slowly
opened, the spot of light slowly increases in size until the whole
of the back lens of the object glass is completely filled with
uniform light. As soon as this is done the whole of the aperture
of the lens is receiving direct light from the condenser shining
through the object as shown in Fig. 16.
While the microscope is in this condition, the effect on the
appearance of the back lens of the object glass, which is produced
by putting the condenser in and out of focus, should be
observed. The iris diaphragm being open to the full extent, it
will be found that unless the condenser is in correct focus the
whole area of the back lens will not be equally filled with light,
and from the appearance so observed it can be realised why an
uncorrected condenser like the Abbe condenser cannot readily
be made to fill the whole area of the object glass with uniform
light, the reason being that the light of difierent obliquity is
brought to a focus at different positions. It may be that only
one ring of light at the edge of the object glass, or a small area
in the centre, or a combination of both, is being illuminated.
The importance of the character of the illumination has been
ufmninatSig referred to, and the question arises as to what aperture cone of
light it is best to use. A well-corrected substage condenser
centred and in focus gives the means of completely controlling
this, and from what has so far been said it might be supposed
that the full aperture of the object glass should be always illu-
minated ; this is not necessarily the case, further research is
required before definite rules can be laid down to meet all condi-
tions. For the best resolution the diaphragm in the condenser
should never be opened to admit a larger angle of light than that
of the object glass, or " glare " may produce a misty appearance
which will destroy the crispness of the image.
This can be avoided by reducing the size of the diaphragm
until the aperture just becomes visible at the edge of the back
lens of the object glass. To what extent the angle of light
admitted by the condenser should be smaller than the aperture
of the object glass depends upon the nature of the object. Full
resolution will only be obtained if the whole aperture of the
object glass is transmitting light, for reasons previously explained ;
and if the light from the condenser were not filling the aperture
of the object glass, and there were no object on the stage, the
Best
cone
ILLUMINATING APPARATUS 31
aperture condition would not be fulfilled until the iris dia-
phragm was open to the desired amount to fill the back lens of
the object glass; but the object itself always has some, and oftert
a very great, power of scattering light. Even when the condenser
diaphragm is cut down to a minute aperture, such an object as
a diatom or a podura scale scatters so much light that on look- Resolution
ing at the back of the object glass it is almost as bright over its s,fatSred
whole surface as at the spot where the direct light passes through, ^y object.
and the image is formed by the scattered quite as much as by the
direct light.
The markings of the podura scale form a good illustration
of this point. The scales of this small insect appear to have
markings somewhat like small quills. If the aperture of the con-
denser is reduced so as to send direct light through only a com-
paratively small fraction of the object glass, the best image of
these quills is formed somewhat as Fig. 20 (A). If the diaphragm
be opened beyond a certain amount the clearness of this image
is reduced. If a diaphragm is inserted at the back of the object
glass to cut off all the scattered light, and only let
the direct light from the condenser through, the image
will be absolutely fuzzy and indistinct, somewhat as
Fig. 20 (B), shomng that in the delineation of this
object the light scattered by the object is doing the
work of resolution, and that it is not done so well if
the full cone of light is passed through the object by the
condenser. The same applies to a lesser extent to
some diatom structure, though in this case the finest
structure is always best shown by a large cone of light from
the condenser.
The tubercle bacillus embedded in tissue is extremely difficult
to see unless a large cone of light is used, and in the early days of
its discovery its existence in the tissues themselves was doubted
by some of those who were not in the habit of using a condenser
to its best advantage. Some delicate semi-transparent objects
are quite invisible when a large cone of light is used, but can
be seen with a pinhole aperture in the condenser. It should be
understood, however, that appearances created by the use
of very small apertures may be incorrect representations of the
correct structure. In the case of the podura scale and the
structure of diatoms, the nature of the actual structure is not
definitely known, and it is not certain that the images obtained
of these apparently well-marked structures are even approximately
correct. Observers have frequently claimed the discovery of
delicate envelopes around bacilli and micrococci which a skilful
microscopist can at once refer to the result of incorrect illumina-
tion. It is generally best to use the largest aperture in the con-
denser that does not produce indistinctness, for the image is
more likely to be a correct representation ; and it is well to try
32
THE MICROSCOPE
Variation
of light
intensity
when iris
diaphragm
of condenser
is altered.
Double
wedge light
moderator.
I
different apertures when it is required to observe very fine struc-
ture.
There is a difficulty in making use of different apertures because
the light varies so greatly by the change in the aperture of the
condenser, that the variation in intensity becomes a serious incon-
venience. The observer should never be tempted to overdo the
brilliancy of the illumination ; several pieces of ground glass should
be at hand to place between the lamp and the mirror to modify
the light as the condenser is opened up.
The most satisfactory appliance for critical work is an adjust-
able pair of neutral tint glass wedges mounted on a stand which
can be placed between the light and the mirror of the microscope
to vary the intensity of the
light at will. The apparatus
consists of a frame which
carries two neutral glass
wedges which slide in fit-
tings and are connected
together so that they move
over one another in oppo-
site directions by sliding
a knob. The total thick-
ness of the neutral tint
glass is varied and the
brilliancy of the illumina-
tion increased or decreased
within very considerable
limits. The frame is at-
tached by means of a clamp
to a rod fixed to a strong
stand. Its height from the
table may be varied from
2 inches to 8 inches, and
two slides are provided in
front of the wedges for the
reception of colour filters or
ground glass.
As the illumination is increased by opening the diaphragm
of the condenser, it can be reduced by thickening the neutral
tint layer of glass by sliding the two wedges over each other.
This apparatus can be provided with wedges of different intensities
according to the strength of the illuminant with which it is to
be used.
The use of a small bundle of oblique light directed upon the
object at a particular angle has been studied in connection with
the delineation of line structure. It is accomplished by cutting
a small hole in an opaque sheet of card, metal, or celluloid, and
placing this aperture in different positions under the condenser,
Fig. 21.— No. 3328, Double Wedge
Moderator.
ILLUMINATING APPARATUS 33
so that one or more selected beams of oblique light may be used to
illuminate the object. An experiment with finely ruled parallel
lines shows that if a small oblique beam of light is used to illu-
minate them at right angles to their length, finer lines can
be distinguished than with direct light ; but it is doubtful
whether this method is of any advantage for ordinary objects,
and it is liable to give rise to quite erroneous impressions of
structure.
Dark-ground illumination is obtained by throwing light upon
the object in such a manner that the object is illuminated, but
that none of the light enters the microscope except that reflected
by the object itself.
The illuminator must be capable of throwing light upon the
object at a greater angle than can be received by the object glass in
use. The illuminator must have a larger aperture than the
object glass. Dark-ground illumination with a substage con- park-ground
denser in which the object is illuminated by light that is so oblique '^'th^*^^^"
that it cannot enter the object glass is accomplished by placing substage
a glass with a central black patch below the condenser and by
opening the iris diaphragm to its full extent, as shown in Fig. 18,
page 27. With the Abbe form of condenser this method is useful
for low powers — IJ-inch (32-mm.) and 2/3-inch (16-mm.) — but is
not sufficiently corrected to cut off the central light with the
accuracy required for a high power. It will perform fairly well with
a 1/3-inch (8 mm.) achromatic, which has an aperture of '5 N.A.,
but not for lenses with a larger aperture. With the achromatic
or immersion condenser, dark-ground illumination can with care
be used with a 1/6-inch (4-mm.) object glass, but it is better to use
a special liigh-power illuminator described later for the 1/6-inch
(4-mm.), 1/8-inch (3-mm.), and the 1/12-inch (2-mm.) object glass.
This is partly because the high-power illuminator is of shorter focal
length and gives more brilliant illumination, and partly because
the stop of a substage condenser is
some distance below the lenses and
allows some light to spread round the
stop employed. It does not produce
so black a background.
For dark-ground illumination with ^^^^""^f^^;^ | pat'htp'ot
a condenser, an adjustable stop in-
vented by Mr. Traviss (Fig. 22), on
the principle of a reversed iris dia-
phragm, is a very convenient appliance. Fig. 22. — No. 3284, Tra-
By moving the handle the size of the viss Patch-stop,
central patch is enlarged or diminished.
The high-power dark-ground illuminator is a reflecting device High-power
or o J! "11 ' n dark-ground
by means of which a very small image or trie source or luurmna- iuomiuator.
tion is focussed upon the object, and this image is formed by rays
of light which fall upon the object at a very oblique angle. Fig. 23
3
34
THE MICROSCOPE
Fig. 23.— No. 3295, High-power Dark-
ground Illuminator.
shows the optical portion of this illuminator. The light from
the mirror being thrown upon the under- surface of the glass
reflector, the light is reflected by two curved surfaces so that
a ring of light is focussed to a point upon the object at a very
oblique angle, as shown
by the white portion of
the diagram. The whole
of this light is so oblique
that it will all be totally
reflected inside the glass
and will not emerge from
the illuminator unless
the latter is brought into
immersion contact with
the under-surface of the
slide by placing a drop
of cedar-wood oil be-
tween the top of
the illuminator and the
slide. It must be used
in immersion contact
with the slide in the
same way that an oil-immersion object glass is used in immersion
contact with the cover glass. With this iUuminator any dry
lens or an immersion lens with an aperture of less than 1 N.A.
can be used, and no direct light, but only that reflected by the
Use of oil- object, enters the microscope (see Fig. 24). A special oil-
objUcr^^"^ immersion 1/8-inch (3-mm.) focus, with an aperture of '95 N.A.,
glasses with jg made for work with this illuminator ; or an immersion lens
groun . ^^^^ ^ larger aperture can be used if it be stopped down by
means of a small diaphragm placed behind the back lens of the
- object glass.
In the latter case the object glass must be stopped down to
a considerably smaller aperture than 1 N.A., because the stop
cannot be placed in the best position, which is between the lenses
themselves, and with a stop behind the back lens a certain amount
of direct light is not properly excluded by a stop of the theoretical
size, because it is not in the correct position.
There is a peculiarity in dark-ground illumination. The
object must be exactly at the crossing point of the beams of light —
that is, in its focus — or it will not be illuminated at aU (see Fig.
24), whereas with an ordinary condenser, even if the object is
not in the exact focus, it will still be illuminated, though, perhaps,
not so brifliantly. The non-focussing dark-ground illuminator
has no adjustment ; as the front portion of the illuminator must
be in immersion contact with the under-surface of the slide, it
cannot be moved up and down, and therefore the slides used
with this illuminator must be 1 mm. thick. SUdes of this thick-
ILLUMINATING APPARATUS
35
Fig.
24.— No. 3294, Focussing Dark-
ground Illuminator.
ness can be selected for examining living specimens, but mounted
objects can seldom be examined.
To overcome this difficulty, R. & J. Beck, Ltd., have designed Focussing
the dark-ground iUuminator (Figs. 24 and 25) with a focussing lurj^Stor."^
adjustment. The upper
lens (C) remains in im-
mersion contact with the
slide, but the reflector
(D) can be moved up
and down, which raises
and lowers the illumi-
nated point, enabling
any slide of from J mm.
to IJ mm. thickness to
be used.
A convenient means
of setting the focus for
a particular slide is
arranged for in the
mount of the illumi-
nator. Fig. 25 shows
the focussing illuminator
mounted for use on the Standard London Microscope. Turning
the lever (C), which projects from the lower portion of the mount,
moves the focussing lens (D, Fig. 24) ; in doing this it also
moves the pin (A, Fig. 25) up and down, and alters its distance
from a flange (B, Fig. 25) on the mount.
If the lever (C) is moved till this pin (A, Fig. 25) is at its farthest
distance away from the ring (B, Fig. 25), the slip that is to be used
may be placed between the ring (B) and the pin (A) ; and if the
lever (C) be again moved till the pin (A) just clamps the slip,
the illuminator will be approximately set to the correct focus
for this thickness of sHp. The
final adjustment may then be made
when the object is in position.
The high-power dark-ground
illuminator has been found specially
valuable for the examination of
living bacteria, rhizopods, and other
transparent and unstained speci-
mens that are difficult to see with
direct light. Such objects, however,
due to their transparency, reflect
only a small portion of the light
that falls upon them, and a strong illumination is necessary. To
accomplish this the high-power illuminator is made to produce
a very minute image of the source of light, so that all the light
may be concentrated on a very small area, almost a point.
Fig. 25.— No. 3294, Focussing
Dark-ground Illuminator.
36 THE MICROSCOPE
To centre It is therefore necessary that a centring adjustment^should
dark'-^ound ^^ provided, SO that the illuminated point may be exactly in
illuminator, the field of view. If the substage of the microscope is not pro-
vided with a centring adjustment, the form of dark- ground
illuminator mount which has centring screws should be used.
To centre the illuminator the following is a satisfactory method
of procedure :
Remove the eyepiece and object glass from the microscope
and swing out the substage. Place the eye six or eight inches
above the tube of the microscope, and move the eye until the
lower end of the microscope tube, where the object glass screws in,
is central with the upper edge of the drawtube. Then, without
moving the eye, move the mirror until the light appears in the
centre of these apertures. Swing in the substage with the dark-
ground illuminator without moving the microscope or the mirror,
place a drop of cedar-wood oil upon the upper surface of the
illuminator, put the object to be examined on the stage, and move
the substage up till the illuminator is in immersion contact with
the sHp. This having been done, put a low-power object glass,
say 2/3-inch, into the microscope, use a low-power eyepiece, and
focus the slide. There will be sufficient dirt or particles on the
slide to show the small illuminated point, which will probably
not be in the centre of the field. By means of the centring
screws of the substage or the illuminator mount, this illuminated
patch may be brought into the centre of the field, and the 2/3-inch
object glass may then be replaced by the object glass which it is
desired to use. A further slight adjustment may be made
for the new object glass if necessary, after which the centring
screws should not be touched, but slight alterations should be
made by altering the position of the mirror. Until the centring
of the illuminator has been completed, the position of the lamp,
the microscope, or the mirror, should not be altered.
To focus Focussing, as previously mentioned, is almost impossible
d?rk'-ground '^^^^ *^® nou-focussiug illumiuator, although a very small move-
iiiuminator. ment of the substage is possible without breaking the film of oil
between the illuminator and the slip.
With the focussing model it is best to set the focus approxi-
mately by the pin on the mount, as described, but a small
movement of the adjusting lever while the object is being
observed is particularly useful in obtaining the best result.
If the illuminator is out of focus, a dark circular patch will
appear in the centre of the field surrounded by a bright ring ;
when focussed, the central dark patch disappears.
Objects mounted dry cannot be examined by this illuminator.
They must be in some fluid or medium, as no light will reach the
object if there is any layer of air between it and the illuminator.
It is important to see that the slips and cover glasses are
thorpughly clean and that there are no air bubbles in the oil or
ILLUMINATING APPARATUS 37
the fluid containing the object, as reflections from dirt or bubbles
may cause a glare that destroys the black background against
which the illuminated objects stand out.
A strong source of light for use with this illuminator is essential, intensity
It is referred to under the heading of " Illuminants." If the°^"=^^-
light is of only moderate intensity, a bull's-eye condenser should
be used. It should be placed at such a distance that an image
of the lamp is formed in the centre and on the surface of the
mirror. The correct position is best ascertained by holding a
white card on the mirror while the bull's-eye is adjusted between
the lamp and the mirror.
When a colour filter is used, a stronger light than would other-
wise be required should be available. The light, however, must
not be too strong, for although a weak light will not illuminate
such transparent structures as bacteria sufficiently to render
them visible, too strong a light shows up certain diffraction
images and destroys definition.
It is, therefore, necessary to reduce the illumination to just
such an extent that these diffraction effects are not aggressively
apparent.
A very brilliant source of illumination, such as an electric
" Pointolite " lamp, gives more light than is required, except for
use with colour filters ; but used in combination with the adjustable
neutral tint wedges (p. 32), gives every intensity of illumination
that is required for this and all other classes of microscopic work.
Those who have not used this form of illumination cannot Resolution
realise the large amount of extra structure that can be recognised ^^^^^^ ^^^J^^^
by its means in certain classes of objects. The spines or illumination,
pseudopodia of Coscinodiscus were discovered by dark-ground
illumination. Hidden structure in bacteria has been revealed,
and the markings of diatoms are shown with greater brilliancy
by this illuminator than by any other means. The resolution of
diatoms is much easier with dark-ground illumination because it
gives far greater contrast. If an object is not opaque and only
appears slightly darker than the background, it is but faintly
seen when viewed by transmitted light ; but if it has even a small
power of reflecting light it can be made to show brilliantly upon
a black ground provided the illuminating source is sufficiently
powerful.
Reflection of light takes place from any object that has a
different refractive^ index or density from that of the material in
which it is situated. A very small difference of density only is
required to give reflection. An interesting experiment to illus-
trate this consists of taking two plates of glass with a layer of
water between them, and flowing in from one side a few drops of
a highly refracting fluid, which gradually mix with the water
and raise its density above that of glass. As the fluid mixes,
the density is gradually raised from point to point, and at the
38
THE MICEOSCOPE
Bull's-eye
condenser.
Fig. 2b.— No. 3215,
Bull's-eye Con-
denser on Stand.
Increase in
illumination.
position where it actually readies exactly that of the glass an
extremely fine line shows on looking at the surface obliquely,
indicating that only at that exact point where the density of the
fluid and the glass are absolutely the same is the grey effect pro-
duced by reflection destroyed and turned into black.
A bull's-eye lens on a stand used in conjunction with a lamp,
or attached to the lamp itself, is required for the illumination of
opaque objects. It is also useful for in-
creasing the illumination with a dark-
ground illuminator or for obtaining a
moderately convergent beam of light in
combination with the mirror when a sub-
stage condenser is not used. It may be
used in connection with a substage condenser
for increasing the size of the image of the
source of light. This is of service when a
very small luminous source of light, such as
the electric " Pointolite," is employed.
On an optical bench a condenser will
transfer the light source to a position nearer
the microscope. Except for these purposes,
it should not be employed with a good
substage condenser. It does not give
additional light, and it generally ruins the performance of a
perfectly made condenser due to its own lack of correction.
Its action in increasing the illumination of opaque objects can
be explained by Fig. 27. A bull's-eye lens placed between the
source of light (S) and the object (0) produces a small image of
the source of light at (0). The upper diagram in Fig. 27 shows
the lens (L) forming a somewhat reduced picture of the source of
light. The lower diagram shows the distances so arranged that
it is forming a much reduced image. The size of the image will
depend on the relative distances from the light source to the
lens and the image to the lens. In the upper diagram L 0 is
half L S, and the image is half the size of the luminous source ;
in the lower diagram L 0 is one- quarter L S, and the image is
one-quarter the size of the luminous source. The lens does not
only collect a much larger amount of light than would other-
wise reach the
object at 0, but
it also compresses
it into a very
small size image,
especially in the
case of the lower
diagram, and
therefore it pro-
duces a very con- Fig. 27.
ILLUMINATING APPARATUS
39
centrated and brilliant small patch of light which is made use
of for the illumination of opaque objects viewed with low
powers.
Fig. 28 shows the Beck electric lamp used with a bull's- Condenser
eye condenser in this manner for condensing a powerful beam °° ^^^'
of light upon the
top of an object on
the stage of the
microscope (see also
p. 46).
A method of
displaying the struc-
ture of opaque ob-
jects is sometimes
adopted in which
two lamps and two
bull's-eye condensers
are used, one of
which has a blue
and the other a red
glass to colour the
light. If these are
used at different
angles, difficult
structure may be
more easily inter-
preted by observing
the different colours
of the shadows.
A suitable illumination of opaque objects is required for illumination
botanical, entomological, and general work, and is of paramount objects vSth
importance for metallurgy. When low-power object glasses are ^"(^g'^Jer.
used there is sufficient working distance (see Fig. 3) between
the front of the object glass and the object to throw light in from
one side by means of a bull's-eye condenser either attached to a
lamp, as shown in Fig. 28, or on a separate stand (Fig. 26).
"When a bull's-eye is used on a separate stand there is another
method of using it which is useful for even high powers. If the
bull's-eye condenser is placed
with its flat surface upwards,
nearly parallel with the direc-
tion of the light, as in Fig. 29,
the light enters the curved
surface and is condensed ; when
it meets the flat surface it is
reflected back in such a way
that a very powerful narrow ribbon of light is emitted ; this
band is so narrow that it can be made to illuminate the object
Fig. 28.— No. 3332.
Fjg. 29.
40
THE MICROSCOPE
Parabolic
reflector.
Sorby
reflector.
Glass
reflector.
Vertical
illiiminator.
Fig. 30.— No. 3360,
Parabolic Reflector.
even when a moderately high power, such as a 1/6-inch (4-nim.)
is used, because the band of light is sufficiently narrow to be
directed through the small working distance between the object
glass and the object. This method is particularly useful for the
examination of alloys of metals or substances with fine laminae,
as the heavy shadows shown by such oblique illumination
indicate the character of the structure.
Another method of illuminating opaque objects is by means
of a silvered parabolic mirror, which can be attached to either a
IJ-inch (40-mm. or 32-mm.) or a 2/3-inch
(16-mm. or 14-mm.) object glass. The front
lens of the object glass being removed, the
tubular portion of the reflector can be slid
on to the cylindrical part of the object glass,
which is of a standard size. This may be
lightly clamped in position by the milled
head and the front lens replaced. The
object glass having been screwed into the
microscope and focussed, the reflector, which
has an adjustment up and down, should be placed so that
its lower edge almost touches the object. The light should
then be directed by a bull's-eye condenser in a horizontal
direction parallel with the stage, so that it illuminates the
whole of the reflector (Fig. 30). The reflector condenses it to
a focus on the object, and a slight movement of the reflector
up or down or a slight turn will give the best result. This
produces a very brilliant illumination, and as the light falls upon
the object from a large number of directions, the shadows
produced are not, as a rule, misleading in interpreting structure.
Mr. Sorby devised an addition to this reflector, which
can be used with the IJ-inch (32-mm. and 40-mm.) object glass,
which consists of a small, flat, silvered mirror which swings in and
out of the optic axis, and when it is in position it covers half
the object glass. It reflects a beam of
light directly downwards upon the object,
which illuminates it in such a manner that
no shadows are produced.
A modification of this apparatus is also
used for metallurgy in which a thin transparent
plate of glass is placed at 45° between a low-
power object glass and the object (see Fig. 31).
This has the advantage that it does not reduce
the aperture of the object glass. It can be
used with either a IJ-inch (40-mm. or 32-mm.) or a 2/3-inch
(16-mm.) object glass.
For the illumination of opaque objects viewed with high
powers, a system was first invented by Mr. Richard Beck in which
a thin glass disc was placed behind the object glass, and a beam
Fig. 31. —No.
3362, Thin Glass
Reflector.
ILLUMINATING APPARATUS
41
No. 3363.
of light reflected througli the object glass itself upon the
object.
This illuminator is generally known as a vertical illuminator, use of
It is screwed into the nosepiece of the microscope, and the object Ju^^nator.
glass screwed into the illuminator mount. The body of the
microscope is then racked into the
position where the object glass is ap-
proximately in focus. The illuminator,
which can be turned round, should be
rotated until the two apertures in the
mount are pointing one to each side,
while the milled head, which carries
the thin glass reflector, is at the front
of the instrument. A lamp is now set up on one side, pre-
ferably the left-hand, at the height of the apertures in the
illuminator, so that light will shine through the two apertures
upon a card held on the right-hand side. A bull's-eye con-
denser may now be placed in front of the lamp, and the beam
of light concentrated by this means. The milled head, which
carries the thin glass reflector, should be turned round until
the light is reflected downwards through the object glass
upon the object. The milled head has engraved on it a line
parallel with the reflector, and this enables the reflector to be set
to approximately the correct angle (45°) before commencing work.
The microscope can now be accurately focussed, and a slight
alteration of the position of the lamp or the reflector will throw
the light in one direction or the other.
For critical work the lamp-flame or source of illumination
should appear, if the object be flat, as a small sharp image on its
surface. This can be effected by having the lamp, if used alone,
about 6 inches away from the microscope, or, if a bull's-eye
condenser is used, by adjusting the distance of the bull's-eye
from the lamp.
A disadvantage of this form of illumi-
nation is that the surfaces of the lenses of
an object glass are convex and that a certain
amount of light is reflected back into the
eye by these surfaces, tending to produce a
glare ; but this can frequently be overcome
by small adjustments in the position of the
lamp or reflector so that such reflections
are directed on the sides of the interior of
the microscope.
Another form of this illuminator is made in which the trans- Prism
parent glass reflector is replaced by a small prism which occupies illuminator.
half the aperture of the object glass. This method reflects more
light, but reduces the aperture and resolution of the object glass.
The prism illuminator gives more light when used with low powers
Fig. 33.— No. 3364,
Prism Uluminator.
42
THE MICROSCOPE
Colour
screens.
than the Beck illuminator, but most prefer the thin glass form
for high powers, and the Sorby reflector for low-power work.
As a convenient and universal illuminator for all powers where
the highest resolution is not required, the prism illuminator,
especially with an electric light bulb attached to it, is popular
for metallurgical work. Fig. Ill (p. 120) shows this illuminator
provided with a small focussing lens, a receptacle for colour
filters, and a 16-candle-power electric light to suit either the
100- or 200-volt current in a metal casing.
Colour screens are of use for several important purposes.
They are either coloured glasses or coloured gelatine mounted
between two glasses. They give greater contrast where objects
being examined are stained or are naturally coloured, and
give truer rendering of
natural colours where
artificial light is em-
ployed.
Every substage con-
denser should be sup-
plied with a green glass,
but a set of different
coloured screens is
very useful for increas-
ing colour contrasts,
both for visual and
photomicrographic work.
If a specimen of
bacteria is stained
KODAK
Fig. 34.
faintly with red, the
use of a green screen
will make them appear almost black and much more distinct.
If a specimen is stained —
a red
a red
filter should be used.
55
55
55
55
55
55
55
5 5
Blue,
Green,
Red, a green ,,
Yellow, a blue
Brown, a blue
Purple, a green
Violet, a yellow
If the screen is too dark, and the light and shade contrast
too great in consequence, fine detail in the structure may be
somewhat clogged or obscured, but faintly stained or coloured
specimens are rendered much more visible by the use of the
correct colour filter.
Another marked advantage in the use of colour filters of green
or blue is obtained by the greater power they give to an object
glass of resolving fine structure.
ILLUMINATING APPARATUS 43
When discussing the aperture of an object glass, the resolution
was stated to be dependent on its aperture, but it is also depen-
dent on the colour of the light. White light, when split into its
component parts by, for instance, a rainbow or a prism, con-
sists of certain pure spectrum colours — red, yellow, orange, green,
blue, and violet. The resolution obtained with white light is that
due to the orange-coloured portion of its component parts, because
this coloured light is more powerful than any of the others that
go to make it up. If a green light be used, the resolution of a
microscope can be increased about 15 per cent., and if a purple
be used, about 25 per cent.
A purple light, or even a very dark blue, is unpleasant to the
eyes of most observers, but a bluish-green light is very restful,
and is the best colour to use as regards
microscope resolution.
Apochromatic object glasses are so
perfectly corrected for colour that the
brilliancy of their images, quite apart
from resolution, will not be improved
by the use of colour screens ; but
achromatic object glasses are always
slightly, and under some conditions
considerably, improved in their per-
formance by the use of a screen
which transmits a pure colour.
The green glass supplied with
the substage condensers trans-
mits a moderately pure colour,
but is not so good as the special
Wratten &; Wainwright gelatine
filters.
A glass trough about 3/4 inch
thick, filled with a nearly saturated solution of acetate of colour
copper, makes a fairly pure blue-green screen. It is rather more ^^°^^^'
transparent than a gelatine filter.
Various fluid screens have been used, but they are so much less
convenient than the Wratten gelatine screens that they are not
so frequently employed.
The best form of illuminant for the microscope depends upon sources of
many circumstances. The author is of opinion that daylight is illumination.
the worst form for accurate observation, but that when it is used
a screen or card with a hole in it about 2J inches diameter should
be placed in front of the mirror of the microscope about 8 or
10 inches away. This ensures a moderately parallel beam of light
falling upon the mirror. A paraffin lamp with a flat flame is
probably the most convenient light for general purposes,
but it is not powerful enough for the use of colour screens or for
high-power dark-ground illumination. The electric light of
Fig. 35.— No. 3366, Monochro-
matic Light Trough.
44
THE MICROSCOPE
Relative
intensities
of different
sources of
illamination.
the ordinary type is unsatisfactory unless used with a ground
glass or tissue paper in front of it. A form of 1/2- watt bulb
called the " Grid " is a good light, as the filaments, when looked
at from the correct side, appear as a fairly large ribbon of almost
homogeneous light. The " Pointolite " electric arc is extremely
good for the highest power work. The incandescent gas mantle
lamp is a useful illuminant, and a modification of this, heated
by a methylated spirit lamp, is an excellent light for those who
have not gas or electricity.
The relative intensities of a similar size small area of different
illuminants are approximately according to the following table
taken from Mr. A. P. Trotter'^ book on Illuminating Engineering :
Candle
Daylight (blue sky)
Paraffin lamp
Incandescent gas mantle
Carbon electric filament
Metal electric filament
1/2-watt electric biilb .
" Pointolite " electric bulb
Arc lamp ,
Direct light from the sun
2i
2
4 to 9
50
300
1,000
5,250
12,000
80,000 to 110,000
. 800,000
From the above table it is evident why ordinary daylight is
not sufficiently powerful for high- power
microscope work. The sun, even in a clear
climate, requires the use of a heliostat, and
the arc lamp requires a special equipment.
It is of great advantage to use a very in-
tense light modified with the neutral tint
wedge moderator described on page 32.
The brightness of the light can then be
perfectly regulated to meet requirements.
This advantage of a very powerful illu-
minant has been referred to in connection
with substage condensers and dark-ground
illuminators, but care should be exercised
in its use. When direct light
is being used through a con-
denser, it is damaging to the
eyes if too strong a light is em-
ployed. A strong illuminant is
necessary for high- power dark-
ground or opaque illumination,
but it must be modified when
direct light is thrown through
the object. Some colour screens
require a strong light, but imme-
diately they are removed the
Just enough light to show the
Fig. 36.
-No. 3335, Paraffin
Lamp.
hght should be cut down.
ILLUMINATING APPARATUS
45
object readily should be used, and no more. If this precaution
is taken, microscopists need have no fear of injuring their eyes,
however long they work. The light should be more powerful
than is required for general purposes, it should be powerful
enough for dark-ground illumination and to allow of the use of .
colour filters.
Fig. 36 shows a good form of parafhn lamp for microscopic Paraffin
work. It has a single flat wick 5/8 inch wide. The burner ^^™p*
has a revolving motion and may be used with its edge facing the
mirror to give a strong illumination, and with the flat surface
facing the mirror for a softer light. It has a means of raising
and lowering it from the table to enable it to be used for illumina-
ting opaque objects with a bull's-eye condenser or parabolic
reflector, or for setting it to the correct height for using the
vertical illuminator described on page 41.
The reservoir and burner are carried on a support which
passes through the centre of the reservoir so that the weight is
well balanced over the centre of the stand. The lamp glass is
simply a 3 X 1-inch microscope slip carried in a thin metal
chimney. The burner is insulated from the reservoir by a fibre
ring, which is always cool enough to touch for turning the burner
round. The metal chimney can be removed and the burner
hinged back for trimming the wick. The reservoir has a large
screw stopper for filling. A bull's-eye condenser on a separate
stand may be used in combination with this lamp for illuminating
opaque objects or
for high - power
dark- ground illu-
mination, al-
though this lamp
is not recom-
mended for the
latter purpose.
The ordinary
electric incan-
descent lamp pro-
vided with a
frosted or ground
glass bulb is a
handy lamp for
ordinary observa-
tion, but is not
sufficiently bril-
liant for many purposes. It is supplied on an adjustable table
stand.
The best equipment is the " Pointolite," or 1/2-watt " Grid"
lamp, with a neutral glass double wedge, a set of colour screens,
and a bull's-eye. It does everything that is required for every
Electric
lamp.
Fig. 37. — No. 3336, Electric Lamp on Stand.
46
THE MICROSCOPE
Electric
lamp.
Fig. 38.— No. 3332, " Pointolite " Lamp
class of illumination ; and the Beck electric lamp is a convenient
form which takes either kind of electric bulb.
The lamp has adjustment so that the beam of light can be
placed at any height
between 3 and 9 inches
above the level of the
table.
The stand (A) is in
the form of a heavy
ring with a section cut
ofi so that it can be
placed close to the
microscope.
A vertical rod (B) is
fixed into this ring. On
this rod a bracket (C)
slides up and down
and can be clamped in
position at any point
by a milled head (E).
This bracket can also
be inclined at any angle and clamped by another milled head (F).
These clamps are independent of each other, and either can
be used without disturbing the other adjustment.
On the bracket (C) is fixed another vertical rod (G), which, by
means of the arm (H), carries the electric light bulb. This can
be moved up and down the vertical rod G and fixed by a
clamp screw (J) in such a
position that the incan-
descent point of the
" Pointolite " or the most
luminous portion of the
filament of an electric
bulb can be placed in the
optic axis of the bull's-eye
condenser (K).
The arm (H) has at-
tached to it a thin metal
cylindrical tube with a
circular aperture which
forms a shield to cut off
stray light from the room,
and when the electric bulb
has been adjusted to the Fig. 39. — No. 3332, "Pointolite" Lamp.
optic axis of the condenser,
this tube can be moved up and down till the aperture in the
metal casing is also opposite the condenser (K).
The condenser (K) is carried in a mount which has two slides
ILLUMINATING APPAKATUS
47
for colour screens or ground glass. It is supported on a rod (L)
which moves backwards and forwards parallel with the optic
axis for obtaining either parallel or convergent light, and can be
clamped in any position.
If the condenser is not required it can be swung to one side ;
or if it is required to use colour screens alone, the lens of the con-
denser can be removed from its mount.
The illustrations show the lamp (Fig. 38) for use with the mirror
of the microscope for transparent or dark- ground illumination
by means of a dark-ground condenser, or for metallurgical or
photomicrographic work. Fig. 39 shows it tilted for use without
a mirror, or Fig. 40
shows it arranged for
the illumination of
opaque objects from
above.
The lamp is pro-
vided with a ground
glass and a signal-
green glass ; it is com-
pleted by the addi-
tion of the Wratten
& Wainwright's colour
filters and the neutral
glass moderator. It is
provided with 12 feet
of cable and an at-
tachment for fitting it
to a lamp fitting of
an ordinary house
supply. For use
with the " Pointolite "
lamp, which is an in-
candescent disc about
the size of a small
peppercorn, a direct Current of any voltage from 100 to 250
volts is equally satisfactory, a variable resistance being supplied
to adapt it to any current between these limits. The candle-
power is 100, but as it is all concentrated in the one point it is
at least twenty times as powerful as the filament lamp focussed
with the condenser.
If a 100-candle-power 1/2- watt lamp or 40- or 60-candle-
power metal filament lamp is used, it is suitable for either
direct or alternating currents, and for a voltage from 100 to
200 volts, although a lamp suitable for the voltage must be
selected.
No special wiring is required, any ordinary house current
being sufficient.
Fig. 40.— No. 3332, "Pointolite" Lamp.
48
THE MICROSCOPE
Incandes-
cent gas
launp.
Incandes-
cent spirit
lamp.
An incandescent gas mantle, either of the ordinary or inverted
type, makes a good light. Its only disadvantage is that it
cannot be conveniently used
with its image exactly in focus,
because the fine mesh of the
mantle does not then give a
continuous surface. This light
is sufficiently powerful for high-
power dark- ground illumination
if dark colour screens are not
used.
For those who have not gas
or electric light, but who require
a more powerful light than a
paraffin lamp, an extremely use-
ful lamp, which is quite simple
to use and gives excellent re-
sults, consists of an incandescent
mantle heated by a methylated
spirit flame. The reservoir
having been filled with spirit^
the method of lighting the lamp
is as follows. The cap of the
reservoir must be screwed off, and the bellows attached by screwing
in the nipple at the end of the tube. The bellows must be
j^
Fig. 41.— No. 3337.
Fig. 42.— No. 3338.
squeezed till the burner is hot. The U-shaped metal piece
covered with asbestos should now be soaked in spirit and placed
ILLUMINATING APPARATUS 49
on the supporting tube below the burner, as shown in Fig. 42,
and ignited. This will heat the burner which is inside the
hanging incandescent mantle. When the asbestos-covered
U-piece has almost burnt out, the bellows should be gently
squeezed two or three times, which will drive the spirit from the
reservoir to the burner, where it will become volatilised and
burn with a steady flame. The bellows may be gently squeezed
every five minutes if the light appears to be failing. The handle
below the burner regulates the air supply, and should be adjusted
till the best illumination is obtained.
The electric arc lamp is useful for photomicrography or
projection, but is troublesome for general use.
CHAPTER III
APPARATUS FOR HOLDING SPECIMENS FOR
EXAMINATION
stage clips.
Vertical
position of
microscope.
Sliding
ledge.
Fig. 43.— stage Clips.
All microscopes are provided with some means of attaching
slips of glass or similar appliances to the stage, and with a means
of moving them about in order to bring difierent portions of the
slide into the optic axis of the instrument.
If the microscope is placed in a vertical position, the stage
then forms a horizontal table, and the slide or slip may be allowed
simply to rest on the surface, but it is difi&cult to move it about
with a regular and even motion unless it is held in some way.
The simplest holding device is a pair of springs called " stage
cHps" (Fig. 43), which fit into two
holes in the stage and press the slide
down upon its surface. They give
sufficient friction to enable the slide
to be pushed with the fingers with an
even and steady movement.
The microscope should, however, not be used with its body in
a vertical position unless it is necessary. It causes the observer
to bend down in an unnatural manner, which is fatiguing, and is
said to interfere with the proper circulation of the blood, and it
allows the fluid on the surface of the eye to collect in the line
of sight, interfering with perfect vision. If the microscope is
inclined to a suitable angle, so that the observer can use it
comfortably, most objects can be as readily examined as is the
case when the instrument is in a vertical position. Even those
objects which are mounted in fluid can be used in this manner,
as they are almost invariably enclosed between a glass slip and
a cover glass. When used in this position stage clips or some other
holding device are essenital.
A sliding ledge (Fig. 44), which fits on to the edge of a square
stage and can be slid up and down, is the most convenient simple
apparatus for holding a 3 X 1 slip, as a very even motion vertically
can be obtained by pushing the ledge up and down, and laterally
by pushing the slip to and fro along the edge of the ledge. There
are two springs on the ledge which press the slip on to the stage
which can, however, be turned aside if not required. A slide
can be searched in this way, as the object can be raised by an
50
APPARATUS FOR HOLDING SPECIMENS
51
^ te &
Mechanical
stage.
Fig. 44.— No. 3307, Sliding Ledge.
amount equal to the field of view of the microscope, and the
specimens pushed all the way along. It can then be raised a
similar amount and
pushed back, and
so on till the
whole area has
been searched. It
is not so conveni-
ent as a mechani-
cal stage , but
makes an inex-
pensive substitute.
A mechanical
stage is an appa-
ratus which holds
a slide or object-
holder, and bv
me ans of two
racks actuated by
milled heads moves
it in a dehcate manner in either direction. One milled head
travels the object laterally, the other longitudinally. This
appliance is almost essential for the delicate movement of the
object when exacting work is being performed, and it has
other important uses. It enables the whole of a specimen to
be systematically examined over its entire surface step by
step, in a manner that is impossible by hand. It is provided
with scales and verniers, so that any position in a specimen can
be recorded. The readings of the scale may be written upon a
label on the slide, and the specimen found at any future time by
setting the stage to the same reading (see page 72).
Fig. 94, page 99, shows a form of mechanical stage that is
very popular. It can be attached to a microscope and removed
at will, and it does not interfere with the adjustments of the
substage apparatus or alter the level of the stage. It consists
of a frame which holds the ends of a 3 x 1-inch sHp, and moves
it on the flat stage of the microscope, with which the slip is always
in contact. A spring presses the slip down on to the stage and
may be turned aside when not required. It has a lateral travel
of 2 J inches (65 mm.) and a vertical travel of 1 inch (25 mm.)
Fig. 45 shows a concentric rotating stage, with a mechanical Rotating
stage built into its surface. In this case the slide moves longitudin- ^gg^"'*^^^
ally along the base-plate of the mechanical stage, but the whole
base-plate moves laterally to and fro. The mechanical portion of
the stage can be racked completely off the circular stage, leaving
a plain stage- plate for the examination of large objects ; but in
this case a small readjustment of substage apparatus is required
to compensate for the thickness of the travelling base-plate which
52
THE MICROSCOPE
has been removed. This form has adjustable slide- holders, so
that slides of any length between 2 and 4 inches can be held.
Fig. 45. — No. 3306, Rotating Mechanical Stage.
Centring
rotating
stage.
Glass slip.
2J inches
The ordinary mechanical stage is made to take only the 3-inch
standard length slide. It has a lateral travel of
(55 mm.) and a vertical travel of 1 inch (25 mm.).
Fig. 98, page 105, shows another form of mechanical stage, in
which the actuating milled heads are at the side instead of
being vertically over the stage. It can also be removed from the
large square stage. It has a lateral travel of 3 inches (75 mm.)
and a vertical travel of \\ inches (30 mm.).
Fig. 97, page 103, shows a plain rotating stage with two stage
clips and two centring screws. The centring screws are primarily
intended for moving the axis of rotation so as to adjust it to the
exact optic axis, but they may also be used as a means of adjusting
the position of the object to a small extent in both directions.
This stage has only a travel of about 1/6 inch in either direction,
and cannot be used for searching a specimen or for registering
positions on the slide. It forms a means of finally adjusting a
specimen that has been roughly adjusted by the fingers.
A revolving stage is necessary for petrological work. It is
very useful in observing opaque objects illuminated with oblique
light, as the behaviour of the shadows, where the stage is rotated,
assists in the interpretation of the
structure. It is of great service in
adjusting an object into the correct
position for drawing, measuring, or
photomicrography.
Holding an object for examina-
tion under the microscope calls for various appliances, according
to the nature of the object. The most universal method consists
Fig. 46.— No. 3400, Glass Slip.
APPAKATUS FOR HOLDING SPECIMENS
53
of placing the object between a glass slip and a tbin cover
glass. Such glass slips are made 3 inches long and 1 inch
wide, and it is only for a few special pur-
poses that slips of any other size are used.
The thickness of such slips varies from J to
1 J mm. Most forms of illuminating apparatus
can be adjusted to focus through slips of
such thickness, but apparatus which cannot be
focussed is constructed for slips of a thickness
of 1 mm., which must be specially selected.
Cover glass is a specially thin form of glass
prepared for use with the microscope. It is
made in squares or circles of 5/8 to 7/8 inch
diameter, or can be cut to any particular size
required. It is made in three thicknesses :
No. 1. Average thickness . "006 in. '15 mm.
2. „ „ . -008 „ -2 „
o. ,, ,, . *Ui ,, 'JiO ,,
Cover glass.
Fig. 47. — Thin
Glass.
The thickness varies about 20 per cent, in different individual Measuring
pieces, and absolute uniformity of thickness can only be obtained "''^^^ ^^^^*
by selection. A screw micrometer is the most useful form of
appliance for measuring cover glasses.
Cover glass can also be measured by the microscope itself.
The fine adjustment milled head of a microscope is provided
with a series of divisions, and the amount that the body tube of
the microscope is moved by the revolution of the milled head for
one division is given on page 96. A high-power dry object
glass should be used, and the cover glass to be measured placed
under the microscope, resting on a glass slip so that one edge
of the cover glass appears near the centre of the field of
view. The microscope should now be carefully focussed on
to specks of dust on the upper surface of the cover glass
and the position of the fine adjustment milled head ob-
served. The milled head provided with the divisions should
then be turned till the dust on the slip is in focus and the
number of divisions that the milled head has moved to make
the alteration noted. This number multiplied by the value
of one division gives the thickness of the cover glass. It is
necessary to focus particles of dust which are situated on
the slip to one side of the cover glass, and not seen through
it, as the optical path seen through glass is not the same as that
in air.
If it is desired to ascertain the thickness of a cover glass of Measuring
a mounted specimen where the edge of the cover glass cannot ^^^e^jP^sg^
be observed, the microscope may be focussed to the dust on the ot mounted
surface of the cover glass and then to the object itself, but the^^^^^'"
result so obtained will be too small, and must have one-half as
54
THE MICROSCOPE
Cleansing
cover
''lasses.
Tliickness
of cover
glass.
Glass slip
with ledge.
Blood films.
much added to it. If the motion of the adjustment is ten divisions,
the true thickness is 15.
It is essential that cover glasses before use should be thoroughly-
cleansed, and all specks, hairs, and fibres be removed. In most
cases a little soapy water will remove all dirt and grease, after
which they should be rinsed in clean water and dried with a clean
linen duster or chamois leather. Some microscopists use two
flat boards covered with chamois leather, between which the
cover glasses are rubbed, reversing the glass during the process
to make sure that it does not adhere to one board, thus cleaning
only one side.
With low-power object glasses — IJ inch (32 mm.), 2/3 inch
(16 mm.), 1/3 inch (8 mm.) — the thickness of cover glass used is of
little importance ; but for high powers — 1/6 inch (4 mm.) or higher
power dry lenses — it is most important to always use the thinnest
covers (No. 1), because with high-power object glasses which are
not immersion lenses a variation in the thickness of the cover
glass affects the correction of the object glass. An object
glass can only give the most perfect image when used with a cover
glass of a particular thickness, and they are always adjusted for
the No. 1 cover glass (see page 81). The 1/6-inch (4-mm.) object
glass is very sensitive in this respect, and one apochromatic
lens of this power is provided with a correction collar to adjust
for cover glass of different thicknesses. As microscopic cover
glass is sold by weight, the cost of the No. 1 glass is not materially
more than the No. 2 or 3, because a larger number go to the
ounce.
If a specimen in the nature of a leaf, a fibre, or powder, is to
be examined under a high power, it is best to place such a specimen
on a glass sUp and place a cover glass over it to flatten it out and
hold it in position, preferably in a drop of water.
In this case a slip with a ledge against which the cover
glass may rest is a convenience.
If the specimen is to be ex-
amined in fluid, a drop should
be placed on the slip and a cover
glass put down over it at an
angle in such a manner that the
cover glass touches one side of
the drop first, and is then allowed
to gradually fall so as
to prevent air bubbles
being enclosed (Fig. 49).
Blood films, or speci-
mens of bacteria which
are to be examined
and then destroyed, may be dried by heating over a spirit
lamp upon the slip or cover glass. If they are to be examined
Fig. 48.— No. 3406, Slip with
Ledge.
Fig. 49.
APPARATUS FOR HOLDING SPECIMENS
55
witli a dry object glass, they should be dried upon the cover
glass and placed film downwards upon the slip. If an immersion
object glass is used they may be dried on to the slip and the
use of a cover glass dispensed with, for the whole space between
the object and the lens is filled with what corresponds to glass.
The thickness of the cover glass, therefore, makes no difference
optically, but unless the object is thoroughly dried a cover glass
may be required to prevent the object from floating off into the
immersion fluid. By putting a drop of Canada balsam between
the cover glass and the slip, and firmly pressing them together, a
permanent mount may be prepared.
When objects in a drop of fresh or salt water are placed between Examination
a cover glass and a slip, the superfluous fluid around the cover fluj^^^*^** ^
glass should be removed with blotting or filter paper, and capillary
attraction wiU hold the cover glass in position when the slide is
placed at an angle.
If a specimen is to be examined for a long period, a piece of
cotton may be placed between the cover glass and the slip, one
end of which dips into a bottle or capsule of water at a higher level
than the slip, and the other in a similar bottle at a lower level.
By this means the slide will be kept moist and objects can be kept
alive for a considerable period.
Small organisms, such as infusoria, bacteria, or protozoa, Slip with
have sufficient room in the thin layer of water between the cover
glass and the slip to live and move
freely, but larger objects, such as
rotifers, entomostraca, etc., require
more room. For use with such
objects, slips are made with cavi-
ties, and are known as slips with
hollows. They are used in the
Fig. 50.— No. 3405.
Fig. 51.— Cells.
same way as ordinary slips, the water which
fifls the cavity holding the cover glass in
position by capillary attraction.
CeUs or rings of vulcanite metal or glass Ceiis.
may be cemented to 3 X 1-inch slips with
Hollis glue, forming deeper cavities for the
reception of large specimens (see page 58).
When such objects are in fluid, the removal
of the superfluous water is sufficient to make
the cover glass~adhere to the cell.
If insects are to be examined dry,
the cover glass may be made to
adhere to the ceUs by placing a
smear of grease or vaseline around
the upper edge.
For the examination of aquatic weeds, algse, and animalcula Trough.
with low powers, a trough is a useful apparatus. Fig. 53 shows a
m
a.
Fig.
52.— Slip with Cell
and Cover.
56
THE MICROSCOPE
Adjustable
trough.
Live box.
Fia. 53.— No. 3413, Slip with
Trough.
convenient form mounted on a 3 X 1 slip ; it has usually a space
for water about 2 mm. tliick. It is made with an upper glass
either of thin microscope cover
glass, about "25 mm. thick, or a
thicker glass about 1 mm. thick.
Fig. 54 shows a glass trough
cemented together. This has
dimensions of IJ xlj- xj inches,
and is made of glass about
1 mm. thick. It is only suitable,
owing to the thickness of the
glass, for use with low powers.
A very useful form of trough, known as Beck's glass trough,
is made of a 3 X 1 glass plate, into which are fixed two screv/s
and milled nuts, each holding a clamping plate. A half-circle
of indiarubber made from an elastic band is laid on the 3x1
slip, and a glass cover plate of any re-
quired thickness is placed on the top.
The whole is clamped together by the
milled nuts. As all the parts take to
pieces, it can be readily cleaned, and
cover glasses or separating bands of any
thickness can be used. Separating bands
of the very thinnest material, such as
dental rubber, or even paper, can be
used, so that the layer of material being
examined is exceedingly thin. This is of
great convenience when it is desirable to
examine the specimens by dark-ground
illumination or with high powers. It is a
very convenient appliance also for the
examination of aquatic specimens. These
can be first arranged in position on the lower 3 X 1-inch slip
within the area surrounded by the rubber band, the cover
may then be placed in position and sufiicient fluid dropped in.
If a small circular cover glass be cemented in the centre of the
3 X 1-inch lower glass, a small
drop of fluid can be confined to
the centre of the field for ex-
amination. It can be used with
substage condensers or dark-
ground illuminators.
A live box consists of a plate
3x1 inches, with an aperture
in the centre of wliich is fixed a
short brass tube carrying at the
top a glass plate. Over this tube sUdes a cap, in the top of which
a cover glass is held by a screwed cell. The object to be examined
Fig. 54.— No. 3415,
Trough.
Fig.
55
No. 341G, Bsck's
Glass Trough.
APPARATUS FOR HOLDING SPECIMENS
57
i^l
1
VAY///m///.','ZM
Wy/'jjwj/fjj/f^
is held between the two glasses. This appliance is useful for
examining living insects or for flattening out thin, uneven objects,
such as a piece of a leaf or
fabric. It is chiefly used with low
powers, as substage illuminating
apparatus cannot be readily used.
A form of live box known as
the Rousselet live box is useful for
high powers. The principle is that fig. 56.— No. 3420, Live Box.
of an ordinary live box, but the
fixed lower glass plate is on the level of the stage, and a
substage condenser or high-power illuminator can be used with
this live box. When a very small object is to be examined, a
still smaller cover glass may be cemented with Canada balsam
to the centre of the lower glass plate, and the object is thus
confined to the centre of the field. It is 2| X If inches, and is
not suitable for use on a mechanical stage.
The Beck compressor is a 3 X 1-inch plate of glass at one end Compressor,
of which a circular pillar is fij?:ed. This pillar carries an arm
which holds a thin cover glass 3/4 X If inches. The arm is
raised or lowered by a screw
at the top of the pillar, which
mechanically varies the space for
holding the specimen. The arm
carrying the thin glass can be
swung to one side for placing the
specimen in position and then
lowered to the required amount.
For many purposes this compressor is more convenient than a
live box, for by means of the delicate screw motion a living object
may be held stationary without being crushed. Also, the slip
being made of glass, it can be kept clean, and the thin glass which
is attached to the arm by spring clips can be readily removed
for cleaning, or replaced if broken. It can be used with substage
condensers and dark- ground illuminators.
A convenient method of holding small solid objects for stage
observation under the microscope is by means of a pair of stage ^o^^^^p^
forceps, which are attached to a 3 X 1-inch ebonite plate. The
plate is either held by the mechanical stage or, if the microscope
is not fitted with the latter,
by means of the spring stage
clips. On the plate is a
metal fitting holding a rod,
which has at one end a small
pair of spring forceps opened
by pressing the two pins
together, while at the other end is a cork into which specimens
may be pinned. The forceps can be unscrewed from the rod,
Fig. 57.— No. 3421, Beck's
Compressor.
3S.
Fig. 58.— No. 3422, Stage Forceps.
58
THE MICEOSCOPE
Mounting
specimens.
Turntable.
Haema-
cytometer.
which can then be reversed in its fitting so that either the forceps
or the cork can be brought into the centre of the field ; they can
also be rotated so that all parts of the object they hold can be
examined. This apparatus is useful for the examination of small
insects, botanical specimens, fragments of rock, tissues, and other
small solid bodies.
A case can be supplied containing apparatus for holding
objects, which includes 3 X 1-inch slips, a slip with ledge, a sUp
with hollow, a trough on slip, a Beck glass trough, a live box,
a Beck compressor, stage forceps, and a supply of thin glass.
The Rousselet live box is not included, as it is not of the standard
3 X 1-inch size.
Mounting permanent specimens for the microscope is a subject
that is beyond the scope of this book. The microscopist should
be pro^dded with a bottle of Canada balsam dissolved in benzol
or xylol, which is a transparent cement, and a bottle of HoUis glue,
which is a brown shellac cement. Many objects can be mounted
by means of these two cements. Small shells, botanical and
entomological specimens, diatoms, and other small objects may
be attached to a 3 X 1-inch slip with gum or Canada balsam inside
a cell of paper, vulcanite, or glass of a thickness sufficient to
protect them, and with a cover glass cemented to the cell. A
narrow ring of cement of the diameter of the cover glass, dried
upon the slip, is often sufficiently thick to protect small objects
when a cover glass is cemented to the surface of this ring.
A turntable (Fig. 59) is an appliance for making rings of
cement on a 3 X 1-inch glass slip and for placing a protecting ring
of cement round a circular cover glass or
cell after it has been cemented on. The
slip is held on to the circular revolving
table by spring clips, and by holding a
camel's-hair brush, which has been dipped
into cement, against the slip as this table
spins round, a layer of cement is left
in a neat circular ring.
Many objects can be placed on a slip and a drop of Canada
balsam dropped upon them, a cover glass being then placed over
the drop before it has set. The specimen is thus permanently
preserved. This is all that is required with such specimens as
dried blood films or stained bacteria.
It is essential, however, that such specimens should not be
moist, as water will not mix with Canada balsam, and some objects
require to be first soaked in absolute alcohol or turpentine to
remove the water or air.
The hsemacytometer is an appara,tus for counting the blood
corpuscles, and consists of a counting chamber, two mixing pipettes,
and suitable optically plane cover glasses. The blood is first
diluted with a solution known as " Toisson's" solution, for either
Fig. 59.— No. 3386,
Turntable.
APPARATUS FOR HOLDING SPECIMENS 59
the red or white, or with a solution of acetic acid when a count
of white cells only is being made. In counting red corpuscles
a dilution of 1-200 is generally used, but in certain cases 1-100
may be employed. The blood is drawn into the pipette up to the
mark 0*5 in the case of 1-200 dilutions, and up to the mark 1 for
1-100 dilutions. The pipette is then immediately placed in the
Fig. 60.— No. 3325a, Pipettes for Red and White Corpuscles.
diluting fluid, which is drawn up to the mark 101 above the bulb.
Both ends of the pipette are then closed with the fingers, and the
pipette shaken to ensure an even mixing, the glass bead in the
bulb facilitating this. For white corpuscles, a dilution of 1-10 is
employed and the other pipette is used. For filling the counting
chamber, a drop of the mixture is blown out of the pipette, after
allowing several drops to go waste, into the centre of the counting
chamber. The cover glass is then placed over the cell. The drop
of blood must not be allowed to overflow the platform into the
groove which surrounds it, and the cover glass must be in perfect
contact with the object slide, and all must be scrupulously clean.
A B C
C
Fig. 61.
DeptK
•lOin.in.
1
400 sqm.m.
THOKA
HAWKSLEY
Fig. 62. — No. 3325a, Thoma-Hawksley Counting Chamber.
The counting chamber consists of a plate of glass with an
annular groove ground upon it. The circular portion inside the
groove is ground and polished to a distance of '1 mm. below the
level of the plate of glass.
In the Thoma haemacytometer this portion is ruled with a
diamond into squares l/400th of a square mm. each in area. It
Tiirck.
Thoma.
Elzholz.
Centre Ruling of Thoma.
Biirker.
Nebauer,
j^^-^^-.
h=^H**"=^
ir -
J
1
1 1
I 1
5 = — , -—=4=
- :
;Eii:-ii:
T
1
1
Tr F~^^
-====^=^.
Fiichs and Rosenthal.
Fig. 63.— Rulings.
Breuer.
60
APPARATUS FOR HOLDING SPECIMENS 61
will therefore be seen that the amount of liquid resting upon
each square has a cubic capacity of l/4,000th of a cubic mm. The
hquid which has been placed in the counting chamber is allowed
to settle, and the corpuscles will therefore be in contact with the
bottom of the cell. It will be found that it is a simple matter
to count the corpuscles contained in each square. The usual
method is to count, say, 100 squares, and it must be noted that
m dealing with those actually on the lines, only those on two sides
of the square should be counted, and this rule should be applied
throughout.
The number of corpuscles in 1 cubic mm. "of undiluted blood
is then obtained by multiplying together the rate of dilution, the
number of corpuscles counted, the volume of each square (l/4,000th
of a cubic mm.), and dividing by the number of squares counted.
The above is the general method of counting the red corpuscles ;
but in the case of the white corpuscles, as there are a very much
smaller number of these, the method generally employed is to
count the total number of the whole ri3ed area of the counting
chamber, which is 1 sq. mm.
^^ There are other forms of counting chambers, such as the
Biirker, Fiichs -Rosenthal, Breuer, and Zapperts ; the method of
employment in all these is the same, but the ruHng and also the
counting are different in each case.
The use of a mechanical stage greatly assists the counting.
A simpler form of haemacytometer can be used which depends
for its action on Mr. Rheinberg's beautiful process of making
graticules and glass scales. A glass
plate is photographed with squares
in the pattern of a chess-board, so
that alternate squares are tinted, al-
though they are transparent. This
plate is dropped into an eyepiece be- Fig. 64.
tween the lenses, and by means of a
stage micrometer the drawtube can be varied until a definite
number of squares are equal to '1 of a millimetre in the mi-
crometer. The chess-board glass plates are supplied with squares
either 1/4, 1/2, 1, or 2 mm. in size. They are made to cover the
whole field of view, or as a small block of squares in the centre
of the field. The latter are to be preferred for blood counts.
The only other requirement is a 3 X 1-inch sHp with a metal
ring cemented to it which is '1 mm. thick, into which the blood
is placed covered with an ordinary cover glass. Suppose a 1/6-inch
object glass is being used, a 1-mm. chess-board plate dropped into
the eyepiece can be made by drawing out the drawtube to the
required position according to the eyepiece and object glass
employed, of such an apparent size that nine squares, three each
way, correspond to '1 mm., and the count of nine squares will
give the number in a cubic tenth of a millimetre. If a 1/2-nmi.
62
THE MICROSCOPE
Culture
plates.
Warm stage
chess-board plate be used, then thirty-six squares, six eacb way,
correspond to a cubic millimetre. The most convenient size to
select will depend upon the class of object to be counted and the
object glass that is used.
Due to the alternate squares being tinted, a count can be made
with much less eye-strain than with the ordinary haemacytometer,
and this method is preferred by some apart from the question of
the cost of the apparatus.
The preparation of culture plates and the methods of cultiva-
tion will be found in text-books on bacteriology. They are large
square plates covered on one surface with nutrient gelatine, upon
which isolated colonies of bacteria are growing. They should
be examined with a low-power IJ-inch (32-mm.) object glass,
the mechanical stage having been removed from the surface of
the stage for the purpose. The required colonies having been
recognised, a morsel of the gelatine can be removed with a
platinum needle, while the colony is in the field of the microscope,
and can be smeared on a cover glass. A drop of distilled water
having been added, it can be spread out on the cover glass and
examined in a living state with the high- power dark- ground
illuminator or dried in a spirit flame, stained and mounted on
a 3 X 1-inch slip with a drop of Canada balsam.
A warm stage is an apparatus for applying warmth to a speci-
men under continuous observation. A simple form consists
of an oblong copper plate 3x1 inches,
from one side of which projects a long
narrow strip and which has an aperture
1/2 inch diameter in the centre of the
3 X 1-inch portion. It is placed on the
stage of the microscope and held like an
ordinary 3x1 glass slip in such a posi-
tion that the long strip projects in front
of the microscope. A spirit lamp is placed
under the far end of the projecting strip
and adjusted so that its flame impinges
on the strip, or is slightly to one side,
until the portion of the copper plate
which is near the 1/2-inch aperture is at
blood heat. The correct temperature is
readily ascertained if a small piece of a
mixture of cacao butter and wax is placed
on the copper near the aperture. The
mixture is made in such proportions that
it melts at blood heat, and when the piece melts on the copper
the correct temperature has been reached.
The drop of fluid to be examined is placed on a large cover
glass and a smaller cover glass is placed over it, and the two laid
upon the copper plate. To prevent evaporation the upper
Fig. t)o.— No. 3384,
Warm Stage.
APPARATUS FOR HOLDING SPECIMENS G3
cover glass should be smeared round its edge with olive oil
or vaseline.
A centrifuge is a small hand machine for revolving test tubes centrifuge.
of fluid at a very rapid speed, so that the heavy portions of
sediment may be rapidly separated from the fluid. Two glass
test tubes encased in aluminium covers are revolved at a speed
of about 2,500 revolutions per minute by turning a handle.
The examination of urine is greatly facilitated by this method,
and hyaline cysts can be deposited without breaking them or
altering their form. Milk is separated by the centrifuge so as to
give the percentage of fat, and micro-organisms can be readily
concentrated to the bottom of the test tube, from which they may
be extracted with a pipette.
A simple microspectroscope for the examination of blood has Micro-
been designed by Mr. Rheinberg; it consists of the micrometer ^p'^''*''"°^^°p^'
eyepiece, as described on page 68, with a slit in the position
where the divided glass plate is generally placed, and a diffrac-
tion grating placed in the eyepiece. On looking through the eye-
piece, the slit is observed, while to one side a spectrum is formed.
If a low-power object,glassbeused,and the object to be examined
placed on the stage of the microscope, its spectrum will be seen
some little distance to one side of the slit. If a comparison
slide of a fluid be prepared close to the edge of a glass slip,
it can be placed on the stage in contact with the fluid to be
examined on the edge of another slip, and the two spectra can
be seen at the same time one above the other. If colour filters
are to be examined they can also be compared by this
method. It is very useful for the examination of blood, chloro-
phyll, dyes, or other colouring matter.
The preparation of metaUurgical specimens for examination Metai-
under the microscope consists of cutting ofi a smaU piece of the gpeSeas.
metal to be examined with a hacksaw and grinding a small
portion to a flat surface and polishing it. It is then etched with
such solution as will remove certain constituents from the
surface, leaving the rest unaffected. Where a fracture of steel
is to be examined, it is sometimes advantageous to cover it, before
grinding and polishing, with a coating of copper by electro-
plating, as by this means a fractured edge shows up very clearly
against the different colour of the copper.
The piece so polished is then mounted by embedding it in a ^oj"*^^^'^^
lump of wax placed on the slide. The best wax for this purpose ^^th^ax,
is one prepared in such a manner that it will hold its position
for a long period and yet remain plastic under pressure. It is
known as S.I.R.A. wax. The specimen should be attached so
that its surface is paraUel to the slip upon which it is mounted,
and this is done most readily as follows :
Cut two square or circular pieces of wood or vulcanite from
the same piece of material of a thickness greater than the specimen
64
THE i\IICIlOSCOPE
Grinding
and polishing
specimens.
Grinding
and
polishing
machine.
m
and about 1 inch diameter. Lay one of these at each end of a
3x1 glass slip, and lay the metal specimen face downwards
on the glass slip in the space be-
' tween the two pieces of wood.
Take another 3x1 slip with a
lump of wax adhering to the
Y^Q 66 centre, and, holding it with the
wax downwards, press it down
upon the wooden plates until it is in contact with them ; the wax
will adhere to the metal specimen and cement it to the upper
slip. This can now be removed and turned over, and the speci-
men is ready for examination (see Fig. 66).
The grinding and polishing is generally done on a machine
with a horizontal revolving disc with carborundum and emery,
and polished on the same machine with rouge or diamantine. The
following describes
a special machine
made for the
purpose which is
driven with an
electrometer from
the ordinary light-
ing circuit.
It is com-
plete and self-
contained, and
only requires to
be connected with the electric current supply by the usual fittings
to be ready for immediate use.
Fig. 67 gives a general view of the machine, which consists of a
vertical spindle carrying a grinding or polishing disc, driven by
a small electric motor.
The machine consists of a vertical spindle (A) carrying a
grinding or polishing disc (B) driven by a small electric motor (L),
and gives in a compact, convenient form all that is required for
preparing metal specimens
for examination.
The spindle (A) is made
of steel, and is bored out
at the upper end to re-
ceive the disc upon which
the polishing or grinding
material is to be placed.
The lower end is hard-
ened to prevent undue
wear. This spindle is
with pulleys of varying
of a belt from the driving
Fig. 68.— No. 1292.
furnished with
diameters, and
a speed cone (F),
is driven by means
APPAEATUS FOR HOLDING SPECIMENS
65
,1
cone (G), whicli in its turn is driven from the motor. By shifting
the belt on the speed cone, a range of speeds varying from
about 300 to 1,000 revolutions per minute can be obtained.
The disc B is made of brass, and fits, by means of a tapered
fitting, into the spindle A, which allows of its easy removal
and at the same time ensures accuracy in the running.
A lip (E) projects downwards and prevents any grinding or
polishing material reaching the bearing.
The cloth for polishing, or emery paper for grinding, is
secured to the disc by a simple but very effective device. A
groove (K) is made in the edge of the disc, and the paper or
cloth is stretched over the surface of the disc and is held in
position by means of a garter made of a stiff
brass spiral spring, which presses the material
into the groove. In this way the cloth, or paper,
is held in contact with the disc, no matter what
its thickness may be (see Fig. 68).
In order to collect the spent polishing
materials, the disc is surrounded by a catcher
(C), which can be easily removed for cleaning.
In the top of the catcher is fitted a guard
ring (D) which, being wide, forms a rest for
the hand, and by being continued downwards
below the surface of the disc, and nearly touch-
ing the edge, prevents any specimens that are
being polished from falling into the catcher
should they be let slip from the fingers.
This ring is also used for stretching the paper
or other material on the disc in the following
manner :
The catcher (C) being removed, the paper or
material is placed on the disc (B) and the ring
(D) pressed over the paper until the ring (D) is
about half-way down the edge of the disc (B).
garter is stretched over the edge. The ring (D) is now pressed
right down over the disc, and the garter spring is pressed home
into the groove.
If it is desired to remove a piece of paper that has been fitted
to the disc so as not to disturb the folds of the paper, the garter
spring should be removed downwards. The paper should be
replaced in the manner described above. .
Should the disc at any time become so firmly nxed m
the spindle that it cannot be removed by hand, a pair of liftmg
levers are suppHed, which can be placed resting on the edge of
the catcher with one end under the disc ; a steady pressure on
the other end wiU raise the disc from its fitting in the spnidle
A cover is provided to protect the revolving disc from dust
when it is not in use.
Fig. 69. —
Pipettes.
The spring
66
THE MICROSCOPE
li
'^■■■:'i
The motor is supplied with flexible connecting wire and
plug adapter, so that it can be connected with any ordinary
lamp fitting.
The machine can be made to suit any voltage specified, and
for direct or alternating current; in the latter case, the phase,
cycle, etc., must be given.
Pipettes. Pipettes (P'ig. 69) are small glass tubes of various shapes, and
are useful for taking specimens out of fluid and transferring to the
slip or object-holder
for examination. If
the upper end of
the tube be closed
with the finger, the
lower end can be
immersed in a fluid,
and the air within
the tube prevents
the entrance of the
liquid. On removal
of the finger from
the upper end, the
fluid enters the glass
tube, carrying with
it small bodies sus-
pended in it; by
replacing the finger,
the fluid will be re-
tained in the tube,
and thus transferred
to a slip, live box,
or compressor. Two
Instruments, or three ucedles, a pair of fine forceps, a pair of scissors, and a
scalpel are required for the manipulation of unmounted objects
before examination. For the collection of aquatic organisms
from either fresh or salt water, a collecting stick and net are
of great use. The net is made of fine bolting cloth, and is of a
conical shape with a glass bottle secured to its apex (Fig. 71). A
surface net that is towed behind a boat may be made in a similar
manner, and should be provided with a
smaU calico bag attached to its front edge
which may be fiUed with stones to enable
it to be towed along when sunk below the
surface of the water.
Most of the free swimming fauna in
open water are near the surface during the
day, but there is often a great variation in the fauna to be
found at different levels.
Fig.
70. — Dissecting Instruments.
Collecting
net.
Fig. 71.— No. 3460,
Collecting Net.
CHAPTER IV
Fig. 72. —
No. 3279,
Rilled Eye-
piece Plate.
SUNDRY APPARATUS
The drawing of specimens seen under the microscope by free- Drawing
hand suffers from the disadvantage that it is difficult to obtain Jecimena*
accuracy in dimensions and relative proportions. Microscope
drawings are seldom required as works of art, but must be
accurate. The simplest aid to accurate drawing is paper ruled ituied
with lines in squares used in combination with a glass plate ruled ^^i"*""^-
into squares dropped into the eyepiece of the microscope. If
the top lens of the eyepiece be unscrewed, it will
be seen that about half-way down the tube there
is a stop ; a ruled plate (Fig. 72) can be dropped
upon this stop, when it will be found to be in the
focus of the top lens. If the lines are not quite
distinct when the top lens is screwed home, the
latter may be slightly unscrewed till the lines come
sharply into focus.
This method of drawing is popular because the
position of the main outlines and salient features
of an object can be accurately ascertained, and as much of
the detail as is desired filled in freehand. A sketch showing the
points of importance, leaving out much of the extraneous detail,
is sometimes of more scientific value than a photograph, which
shows so much detail that it is difficult to pick out the features
of special interest.
Objects sketched in this manner may be measured by reference Measuring
to a stage micrometer. This is a 3 X 1-inch glass sHp with lines specunena.
ruled on it 1/10 and 1/100 of a millimetre, or 1/100 and 1/1000
of an inch ; and if it be placed on the stage of the microscope
and viewed under the same conditions as the object that has been
drawn by means of the squared paper, it is easy to see how many
1/lOOths of a millimetre or 1/lOOOths of an inch are included in
each square. This can be noted on the paper, and the dimensions
of the object may be obtained by measuring the drawing.
A glass plate 4x1 inches, with divisions etched on its lower
surface, is the most convenient scale for making such measure-
ments on the drawing.
The measurement can also be made without making a drawing,
for once the value of a square in hundredths of a miUimetre or
67
68
THE MICROSCOPE
Ruled
micrometer.
Micrometer
eyepiece.
thousandths of an inch has been ascertained with a particular
object glass and a particular tube length, the measurement can
be made direct in the microscope. For this pur-
pose ruled squares in the eyepiece are not always
convenient. An eyepiece micrometer is a plate of
glass ruled with a finer series of hues (Fig. 73). It
drops into the eyepiece in a similar manner to the
square ruling. An even better method of making
such measurements is by means of the Beck mi-
Eyepiece crometer eyepiece.
Micrometer. This consists of a complete eyepiece with a
magnifying power X 8, and a special vernier milli-
metre scale (A, Fig. 75) placed in its focus which is outside the
lenses.
It is provided with a coUar (B) which fits over the draw-
tube and can be clamped in position by a milled head (C). The
eyepiece itself can be focussed up and down by revolving it in
its fitting till the scale A is in exact focus for the observer's eye.
The scale (Fig. 76) is in millimetres with a vernier reading to
1/lOth of a mm.
On the left is a vertical series of divisions divided in half-
millimetres for rough measurement. For fine measurement the
object to be measured is placed in a horizontal position, and the
length is measured in 1/lOth mm. by use of the slanting line on
the right. The image of an object as shown in the diagram
measures 3*25 mm., because it covers three large divisions
and extends to the oblique line at a point half-way between the
•2 and '3 of tenth -millimetre vernier divisions. '
To obtain the actual size of the obiect itself, this result has
^
Fig. 74. — No. 3275,
Micrometer Eyepiece.
^S^
^
21223 -
^^l
1— -A
Fig. 75. —No. 3275,
Micrometer Eyepiece.
'^:V'
4 3 2 10
Fig. 76.— Scale
of Micrometer
Eyepiece.
merely to be divided by the initial magnifying power of the object
glass. (See table of magnifying power on page 77.)
In cases where great accuracy is required, each object glass
can be verified as to its initial magnifying power by the use of a
SUNDRY APPARATUS 69
stage micrometer. For this purpose, focus the scale of a stage
micrometer carefully ; if 1/lOth of a millimetre now measures
2-5 mm. in the scale with the correct tube length of 160 mm.
and a particular object glass, the magnifying power of that object
glass is 25.
The first image formed by a microscope is produced by the initial
object glass at a position rather above the stop of the eyepiece. Sfwer^^'"^
This initial magnification depends on the focal length of the object
glass, and also the position of this image, which is governed by
the length of tube of the microscope. The approximate initial
magnifying power of each object glass or the enlargement pro-
duced in the first image is engraved on each Beck object glass
for a standard tube length of 160 mm. It can only be approxi-
mate, because different eyepieces have their stops in slightly
different positions, and therefore a small variation in the theoreti-
cal tube lengths is caused by the use of different eyepieces.
The eyepiece magnifies the first image formed by the object glass
by a fixed amount, according to the focal length of the eyepiece,
and does not vary, and the total magnifying power at the 160-
mm. tube length is obtained by multiplying the power of the
object glass by the power of the eyepiece.
The Beck micrometer eyepiece measures the size of the first
image formed by the object glass in millimetres and tenths of
a millimetre. The result obtained when the drawtube has been
set at 160 mm. has only to be divided by the initial magnifying
power of the object glass to give the actual size of the object
being measured.
Small variations may occur in individual lenses, but they
are usually not sufficiently great to be of consequence in ordinary
work.
The camera lucida is an apparatus for making correct drawings. Horizontal
It is made in four models, suitable for three different positions Sda^
of the microscope. '
The Beck horizontal camera lucida (Fig. 77) requires the
tube of the microscope to be in a horizontal position, and the
paper upon which ,
the drawing is to j^^^^^ ^^^^r '
eyepiece of the ^l^^MLg^y "J"
microscope. The ^^^^^^^^^ «
camera lucida is a •
small half - silvered Fig. 77. — ^No. 3368, Horizontal Camera Lucida.
prism held in a
mount which fits on to the drawtube of the microscope in
such a position that one surface is close to the front lens of
the eyepiece. The observer places his eye immediately above
the prism, and the image seen in the microscope is reflected
70
THE MICROSCOPE
Vertical
camera
lucida.
Abbe
camera
lucida.
i
I
<^-
(Ar
(
Upwards into his eye by means of a reflection in the prism from
a half- silvered surface. The eye also sees the paper and pencil
through the half- silvered surface, and can draw the object
seen through the microscope accurately and rapidly, because it
appears to be superimposed on the paper.
If the eyepiece of the microscope is closer to the paper than
about 10 inches (the near point of vision), the pencil will not
appear sharp ; and to obviate the necessity of raising the micro-
scope, a lens is supplied below the prism which enables the pencil
and paper to be clearly seen at a distance of about 6 inches, which
is the usual height of a microscope body. The lens is also a
great assistance even when the paper is 10 inches away. It
fits into a recess in the mount and is held in by a turn-button.
The Beck horizontal camera lucida is superior to the old
WoUaston form, as the eye does not require to be held in an
exact position during the process, and there is no training required
for its use. The^only^care that is required is to see that neither
the illumination of
the object nor the
paper is so brilliant
as to obscure the
one or the other.
The relative illumi-
nation can be easily
regulated by a neu-
tral tint glass placed
either between the
prism and the paper
to reduce the apparent brightness of the paper, or between the
microscope eyepiece and the prism to reduce the apparent bright-
ness of the microscope image, or the illumination of the micro-
scope may be varied by any of the means previously referred
to. A slot is provided in the two positions to receive the neutral
glass.
The Beck vertical camera lucida (Fig. 78) is a prism which
acts in a similar manner except that the microscope must be
placed in a vertical or an inclined position. When the microscope
is in a vertical position, the drawing paper must be placed on a
slanting board at an angle of 30° in front of the microscope.
In other respects the manipulation is the same. When the
instrument is used in an inclined position, the tube of the micro-
scope must be set at an angle of 60° and the paper may be placed
upon the table. The same arrangements are made for the
reception of the lens and neutral glass.
The Abbe camera lucida (Fig. 79) consists of a prism over the
eyepiece and a large mirror placed a few inches to one side
in a horizontal direction. The prism has a completely silvered
surface, with a small aperture in the centre, and is not so easily
Fig.' 78.-
-No. 3369, Beck Vertical Camera
Lucida.
SUNDRY APPARATUS
71
used as any of tlie other forms. With this apparatus the
instrument is placed in a vertical position, and the drawing paper
Fig. 79.— No. 3370, Abbe Camera Lucida.
Fig. 80.-
-No. 3371, Modified Abbe Camera
Lucida.
placed on the table at one side. The mirror must be inclined
at such an angle that the centre of the field of view appears
below the centre of the mirror, or a distortion in the picture will
be caused. This generally limits the size of the drawing to a small
portion of the centre of the field of view, because of the closeness
of the mirror to the side of the microscope. This can be remedied
if the paper on which
the drawing is to be
I made be tilted up so
1 that the distortion is
S'lf ^^ corrected, for the image
I \ can then be thrown to
I V a greater distance to
the side of the instru-
ment. In order to find
the correct angle at
which the paper should
be tilted to avoid distortion, the circular margin of the field of
view as seen upon the paper may be measured in two directions,
sideways and fore and aft, and the angle of the paper altered
till the two measurements are the same. This method can also
be adopted with the Beck vertical camera lucida, when it is
required to set the inclination of the microscope or drawing-board
to the correct angle experimentally.
A camera lucida (Fig. 80) of the
Abbe type is made in which the
bulky mirror is replaced by a small
tilting prism attached close to the
eyepiece, and the prism is half-
silvered. In this case the drawing-
board must always be placed at an Fig.
angle which can be ascertained as ex-
plained above. A lens and neutral
tint glass can be used in the same manner as previously described. Drawing
A table (Fig. 81) which can be set at any desired angle is ^^^®'
supplied which is a convenience where the drawing paper requires
Modified
Abbe
camera
lucida.
81.— No. 3375, Draw-
ing Table.
72
THE MICROSCOPE
Finders.
Vernier.
Polarising
apparatus.
•o
to be at an inclination. It is marked for the correct position
for the use of the Beck vertical camera, or may be set at any
other position.
A large amount of time is saved in examining specimens if
the position of a particular object or of a portion of a slide can
be recorded for future reference. For this reason mechanical
stages are provided with divided scales and verniers. The
readings of these scales are taken when the desired object is in
the centre of the field. These readings can be written on the
label on the slide, and the object in question can always be
found again by setting the stage so that the scales read these
numbers.
For those who are not familiar with the use of a vernier, the
following description may be useful. The scales of the mechanical
stages are all divided in millimetres with a vernier which reads
to 1/10 of a mm. For a rough reading the first line with arrow-
head on the right-hand scale (Fig. 82) may be used
as an index, and the distance which it is beyond one
of the lines estimated, thus the reading of the scale
as shown in the figure would be about 13 J. For a
more accurate reading the other lines on the right-
hand scale, which form what is known as the vernier,
should be examined. The line with arrowhead is
not opposite any division on the long scale, but it
will be found that one of the lines on this scale
is opposite a division — in the case illustrated it is
the fourth line — this shows that the true reading
is not 13 J, but is 134. If it had been the eighth
line that was opposite a division, it would have
been 13-8, and so on.
Polarising apparatus consists of a polarising (Nicol) prism in
a revolving fitting which pushes into the substage of the micro-
scope, a plate of selenite in a detachable tube sliding over the
polarising prism, and an analysing (Nicol) prism in a revolving
mount which screws into the nosepiece of the microscope between
the body of the microscope and the object glass. An analysing
prism in a special eyepiece may be used instead of the analyser
over the object glass if preferred. A polarising apparatus is
essential for the study of rocks, and is always supplied in petrologi-
cal microscopes ; but it is used on an ordinary microscope for the
study of crystals, starch, and many organic substances. A starch
granule can always be recognised by its means, as it shows under
polarised light a black or coloured cross, due to the crystalline
refraction of the material. Sugar and other crystals display
brilliant colours, and such materials as horses' hoofs, wax, or
finger-nails, show the structure in a manner that is not other-
wise seen. An explanation of the reason for the appear-
ances obtained with polarised light involve a full discussion
Fia. 82.—
Vernier.
SUNDRY APPARATUS
73
Eyepiece
with cross
lines.
of the theory of light, which is not within the scope of this
book.
An eyepiece with a movable pointer or indicator (Fig. 83) is Demon-
a useful aid to teaching. It consists of an eyepiece magnifying f "g**!?^
X 10, which has a fine movable index eyepece
which can be made to point to any portion
of an object under consideration, or can
be turned out of the field when not re-
quired. It is invaluable for demonstra-
ting cell-structure, crystals, etc.
An eyepiece with a pair of cross lines
(Fig. 84) is necessary for petrology where
angles are to be measured by means of a
rotating stage, and is useful for other
purposes.
An eyeshade (Fig. 85) which clips Fig. 83.— No. 3263, Eye- Eyeshade.
on to the drawtube of the microscope P^®^® ^^^^ Indicator,
obscures the unemployed eye and saves
much inconvenience and eye-strain with a monocular microscope.
It enables the observer to keep both eyes open without his
attention being diverted.
An erecting eyepiece is a very low-power eyepiece which Erecting
does not invert the image. It drops into the tube ^^^p^^^®*
of the microscope in the ordinary way, and is
made for use with a 2/3-inch object glass. It
gives a magnifying power from 10 to 40 diameters
by extending the drawtube. It gives a very large
field of view and an erect image, so that it at
once converts an ordinary microscope into a thor-
oughly efficient dissecting microscope ; and a slight
alteration in the length of tube gives great varia-
tion in magnifying power.
To take photographs through the microscope which are Photo-
entirely satisfactory for most purposes is simple, and does °^"°°'"^p^^
not require much apparatus or special appliances.
The microscope is first
arranged to give the best
visual image, the particulars
as regards illumination given
in the earlier part of the book
having been carefully followed.
The ordinary eyepiece is re-
placed by the 30-mm. focus
compensating eyepiece, and the
photomicrographic camera is attached to the tube of the micro-
scope. The image must now be carefully re-focussed upon the
ground glass and the plate-holder inserted. The light is cut
off from the microscope by placing a card between the light and
Fig. 84.—
No. 3264,
Cross Lines
of Eye-
piece.
Fig. 85.— No. 3257, Eyeshade.
74
THE MICROSCOPE
Vertical
photo-
micro -
graphic
camera.
the stage of the instrument, and the slide of the plate-holder
is drawn. The exposure may now be made by withdrawing the
card, replacing it and closing the plate-holder.
The use of colour screens (see page 42) is of great service in
photography to increase contrasts, but the student is referred
to books on this subject for detailed information as to photo-
graphing difficult objects.
At the same time, the photography of most microscopic
objects is so simple that
the ordinary observer
need not be deterred by
the complexity of the
instruction given for the
most advanced work.
There are two general
forms of photomicro-
graphic cameras. One is
vertical and is used with
the microscope in a ver-
tical position. The other
requires the microscope
to be placed in a hori-
zontal position, and con-
sists of a metal bar
on raising and lowering
screws which carries a
camera with a variable
extension adjusted by
means of bellows.
The vertical camera
consists of a frame stand-
ing on three strong legs
splayed out to give sta-
bility. It has a slide
Fig. 86. — No. 3342, Vertical Photomicro- on its upper surface into
graphic Camera. ^j^j^h either a ground
glass screen or a double
plate-holder is inserted. Below this frame is a flexible bag
which fits over the upper end of the drawtube of the microscope
and can be attached by a cord. The size of plate used is 4^ X 3J
inches (quarter-plate), and the distance from the upper end of
the standard microscope is such that, with the 30- mm. compen-
sating eyepiece, it gives a circular picture of about 3 inches. It
is rigid, and light, and extremely convenient. When the micro-
scope is adjusted with all the care required to obtain the best
image the camera is placed over it, attached to the tube by means
of the bag, and a touch of the fine adjustment is all that is neces-
sary. In order to focus the image on the ground glass accurately,
SUNDRY APPARATUS
75
Fig. 87. — Focus-
sing Glass.
a focussing glass should be used ; this consists of a high-power Focussing
lens niounted in an adjustable tube, which can be set so that^^^^*
when it is stood upon the ground glass the latter is sharply
focussed. A small portion of the ground glass screen in the centre
is left clear so that the image can be
viewed with the focussing glass without
being partially obscured by the ground glass.
The method of setting the focussing glass
is as follows : Loosen the top cell which
hold s the lens combination by slightly un-
screwing it, then screw the outer one of
the tubes downwards away from the cell,
leaving the screw of the top cell exposed.
Now hold the inner tube and screw the top cell backwards and
forwards until a pencil mark on the lower side of the ground glass
is sharply defined, while the focussing glass is held against the
upper side. Then screw up the outer tube, which will form a
lock nut and fix the top cell in the correct position.
The horizontal pattern of photomicrographic camera is Horizontal
illustrated in Fig. 88. When this is used, the microscope must Stro-
be placed with its tube in a horizontal position. It enables a ^^p^°
... • j_i • r 1 . 1 1 • ^ T camera.
variation in the size oi the picture to be obtained according to
the extension of the bellows. It is' of unusually solid construction.
It has an extension of 30 inches and takes a 4|- X 3J inches
(quarter-plate) size negative. It consists of a heavy steel
hexagonal bar fix:ed to two steel cross bars which are supported
on four levelling screws. Along this bar slide three frames with
connecting bellows, each frame being provided with a clamp
screw. The frame at one end holds the ground glass or double
plate-holder, the frame at the other end carries a flexible bag to
attach to the microscope. It can be adjusted up and down so
that its centre is at any distance from the ground between
5| and TJ inches, and it can be raised to a higher level for use
Fig. 88. — No. 3340, Horizontal Photomicrographic Camera.
with large microscopes by putting four feet under the levelling
screws.
There are but few purposes for which a larger photomicroscope
than 4J- X 3J inches isjequired, and for this size 30 inches is
ample extension.
CHAPTER V
OBJECT GLASSES AND EYEPIECES
The object glasses and eyepieces are of such paramount impor-
tance in the performance of a microscope that their use and
selection is a matter which should receive the careful consideration
of the microscopist. Each object glass is a complicated combina-
tion of lenses and metal parts. In some as many as ten, and in
none less than four, lenses, mounted in their cells at specified
distances apart, form the complete whole. The adjustment
and setting of these demands the utmost skill and care in
manufacture ; an error of 1/10000 of an inch may damage the
quality of a high-power lens.
Scratches Scratches upon the. surf aces of the lens, or dust either on or
on*object between the components, unless in an aggravated form, do not
glasses. interfere with its performance beyond stopping or scattering a
little light, but the slightest shifting of one of the lenses or the
least smear of grease or moisture will entirely upset the corrections
and ruin its performance. No glass surface should ever be touched
by the fingers, as they always leave a smear of grease. It is,
therefore, of the utmost importance to treat all object glasses,
eyepieces, and condensers with care, and to keep them free from
moisture, dirt, or grease. They will, even with the greatest care,
collect dust from the atmosphere in time, but they should always
be kept in a dry place, especially when in a moist climate.
Dirt on Dirt in the eyepieces shows in the field of view, that on the
eyepieces, object glasses is uot clcarly visible, but may make the image
hazy and indistinct. It is quite readily detected in the eyepieces,
as by revolving the eyepiece in the drawtube the specks due to
dirt in the eyepiece will revolve. If the specks are on the object
or in the observer's eye, they will remain stationary.
Cleaning To icmovc dirt from the eyepiece, the surfaces should be
carefully cleaned with a very soft piece of well- washed silk ; and
if after this any still adheres, the silk should be moistened with a
little xylol or alcohol. For this purpose it is quite safe for the
microscopist to unscrew the cells, which hold the lenses, from
the eyepiece tube, provided that care is taken to replace them at
the right ends. It is best to only unscrew one at a time. It is
inadvisable for the microscopist to attempt to clean the internal
lens surfaces of an object glass ; the interior surfaces do not readily
76
lenses.
OBJECT GLASSES AND EYEPIECES
77
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78
THE MICROSCOPE
Removing
oil from
immersion
lenses.
Keeping
object
glasses.
Corrections
of object
glasses.
become dirtv, and in most cases the dirt will be on the front
surface. This should be cleaned with soft silk or very soft
chamois leather, but it must always be remembered that dust
consists in many cases of hard particles, often harder than glass,
and if these are rubbed upon the surface of the lenses they will
leave fine scratches. Hence the correct method is to wipe very
gently and so to remove the smaU particles and not to grind them
on to the surfaces.
The lens which requires most cleaning is the front of an oil
immersion, which is necessarily continually covered with cedar-
wood oil. The oil should always be removed when the lens
is put away after use. Oil can be removed with xylol,
benzol, or spirits of wine, but care should be taken not to
use too much of this liquid, so that there is no danger of
its getting into the interior of the lenses. A piece of filter
paper or blotting paper moistened with xylol or benzol and
lightly wiped over the front surface will remove the oil with-
out rubbing.
It is advisable to keep object glasses in the dust- tight metal
boxes in which they are supplied when they are not in use. They
will be safe except in a very moist atmosphere in a dust-tight
nosepiece or in the boxes of the Sloan object glass changer. If
object glasses show dirt on the interior surfaces or any other
defects, they should be returned to the manufacturers, who alone
can satisfactorily put these matters right and see that the lenses
are in adjustment. High-power object glasses can be put out of
order by the slightest error in putting the component lenses
together. If a piece of dirt prevents one of the cells from screwing
quite home, it is sufficient to destroy its performance. An object
glass on the table when not in use should always be stood with
its front lens upwards to prevent dust from accumulating on its
back surface.
The reasons for constructing an object glass out of a number
of separate lenses in order to correct its aberrations will be
discussed in a more complete treatise referred to in the preface,
but one characteristic of a corrected lens should be thoroughly
grasped. Any lens or combination of lenses can be made to form
an image of an object at many different positions. If a lens, such
as a bulFs-eye, be put in front of a lamp, it can be moved to and
fro from the lamp till a position is found where it will form a
picture of the lamp on a wall ten feet away. If a card be now
interposed at a distance of only two feet from the lamp, a slight
movement of the lens away from the lamp will form the picture
upon the card instead of the wall. In the same way, if the length
of the drawtube of a microscope be altered, a slight movement of
the object glass wiU bring the image which it forms to the correct
position for the eyepiece to render it sharply defined. Any two
positions where an object and its image are situated are called
OBJECT GLASSES AND EYEPIECES 79
a pair of conjugate foci. The important characteristic of an conjugate
object glass is that it can only be absolutely correct for one pair »"^^-<^-
of conjugate foci which, as applied to the use of the microscope,
means that at one length of drawtube (160 mm.) the image will
be clearer than in any other position. It is a point that is
sometimes considerably exaggerated by writers on the microscope,
who give the impression that if the wrong length of drawtube is
used, the object glass is almost as bad as an uncorrected lens.
The truth is that, especially with high-power lenses used with high
eyepieces, for examining the finest details it becomes a factor of
importance; but with moderate power eyepieces for general
observation, a considerable variation of the length of tube makes Tube length
no noticeable deterioration in the sharpness of the picture and ^^"'^^ction.
forms an exceedingly useful means of altering the magnifying
power. Low-power lenses, on account of their smaller aperture,
are much less sensitive to a change in the length of the drawtube,
and when using the IJ-inch (40- nam. and 32-mm.), 2/3-inch
(16-mm.), or even 1/3-inch (8-min.) object glasses with eyepieces
not higher in power than X 10, a variation of 30 or 40 mm. in
the drawtube is difficult to notice. With 1/6-inch (4-mm.),
1/8-inch (3-nmi.), 1/12-inch (2-mm.) object glasses, it is best
to use the standard 160- mm. tube length, and if a 1/3-inch (8-mm.)
is being used with high eyepiece, the drawtube should be set
at its correct length.
The other important factor in the best performance of an Thickness of
object glass is the thickness of the cover glass which is between ^^^"^ ^'*^'
the object and the front lens. With high-power lenses, the
thickness of this cover glass has a far greater efiect on the quality
of the image than the length of tube.
If a cover glass be used which is incorrect it can be corrected correction of
to some extent by making an alteration in the length of tube, thidLl^y
and the following table gives the approximate amount of altera- drawtube.
tion in the tube length required to correct a variation in the
thickness of the cover glass with different powers. An oil-
immersion lens is not subject to this variation because it has
nothing but glass, or its optical equivalent, between the object
and the front lens. If the cover glass is thicker, the cedar-wood
oil is proportionately thinner. For this reason the table only
refers to dry lenses. It will only enable corrections to be made
for cover glasses which vary between certain limits. For in-
stance, a 1/6-inch cannot be corrected for an uncovered object
by its means. The low-power lenses are so insensitive to small
amounts of variation that the table is chiefly of use in indicating
that the thickness of cover glass is unimportant except when
working through thick troughs of water. It should be remembered
that water has only about two-thirds the effect of glass, and,
therefore, where a trough is used, one-third may be added
to the figures given.
Table of
drawtube
corrections.
80
THE MICROSCOPE
Length of
140 mm
150 mm.
160 mm.
180 mm.
200 mm.
240 mm.
Drawtube.
Corresponding Cover Glass Thicknesses.
in.
mm.
in.
mm.
in.
mm.
in.
mm.
in.
mm.
in.
mm.
IJ in. (32 mm.)i .
2/3 in. (16 mm.)i.
1/3 in. (8 mm.)i .
1/6 in. (4 mm.) .
•200
•100
•018
•008
5
2^5
•46
•2
•140
•085
•012
•0074
35
21
•31
•19
•100
•070
•007
•007
2^5
1-8
•17
•17
•070
•050
•004
•006
b8
1-3
•1
•15
•040
•030
•0053
•1
•08
•135
•004
•1
1 The lower power lenses selected for the purpose of compiling this
table are not as usually supplied, but are specially corrected for longer
tubes to show the variation better.
Canada balsam acts in a similar manner to glass, and a layer
of Canada balsam between the cover glass and the object has the
effect of increasing the thickness of the cover glass. It must
be allowed for unless the object is mounted in contact with the
under-surface of the cover glass.
Correction of It will occur to the reader that the correct tube length can
tS^nes^by ^^ ascertained by observing the image through the microscope.
obserration. ^]^jg jg very readily done under dark-ground illumination, and
with more difficulty with transmitted light. Under dark-
ground illumination there will always be fine specks of dust
illuminated as brilliant points ; one which is so small as to show
no apparent outline or shape should be selected and be placed
near the centre of the field of view. It should then he focussed
backwards and forwards, and the image examined on each side
of the sharpest focus. If the light is all coming to an exact focus
at any one point, the appearance on each side of the focus will be
the same : it will be a small disc or patch of light equally intense
Fig. 89.
on either side. If it is out of adjustment, one side will be clear
and the other side hazy. Fig. 89 shows that if all the light
OBJECT GLASSES AND EYEPIECES 81
from the object glass is not coming to the same point, the illu-
mination will not be equally distributed at the two positions
(A and B), It is important to select a very small point for the
purpose, because many of the objects seen through the micro-
scope are partially or completely transparent, and often globular,
and act as small lenses themselves, which interferes with the
phenomenon unless they are extremely minute. With trans-
mitted light on a bright field the same plan may be adopted,
but it requires much more careful observation because the image
of a fine speck of dust when out of focus appears as a faint patch
on a bright field, and is not so easily observed as a bright
patch on a dark background. High- power eyepieces should
always be used for making these observations. By the examina-
tion of insects' scales and diatoms much greater accuracy can be
obtained in this adjustment, but the explanation of their use
requires further discussion of the theory of the microscope
and is not attempted in this book.
All high- power dry object glasses are made for use with a
cover glass -006 inch (-15 mm.) or -007 inch (-17 mm.) thick
unless specially ordered to be made for use without cover
glass for polished metal specimens. The l/6th-inch (4-mm.) is correction
sometimes made with a correction collar, which is an adjustment ^^^-
which, by altering the distance between the component lenses,
enables the object glass to be corrected for any thickness of cover
glass between 0 and -01 inch (-25 mm.). Such a lens in the hands
of a beginner should always be used with its correction collar
set at about -007 inch (-17 mm.) unless no cover glass is being
used. It may be a positive disadvantage to use such an object
glass unless the microscopist is practised in the method of making
the adjustment.
The colour correction of an object glass is referred to
later, but it should be remembered that an achromatic object
glass can sometimes be slightly improved by the use of a
colour screen, as such lenses always give a slight indication
of faint colour. The faint colour efiects often seen in trans-
parent objects are, however, frequently due to the objects acting
as small uncorrected lenses themselves, or to diffraction effects.
The flatness of field of a microscope depends on the object
glass and the eyepiece combined. The edge is generally not
in focus at exactly the same position as the centre of the field.
Such a defect cannot be entirely cured in the best lenses,
because to do so would sacrifice the finest definition in the centre ;
consequently, for the most exact examination the object must
always be brought near to the centre of the field. With low-
power lenses the defect is not so apparent, but with high powers
there is always a marked superiority in the performance near
the centre.
The following table gives the approximate sizes of the field
82
THE MICROSCOPE
Sizes of
field.
of view with various object glasses at the standard tube length,
and the working distance of each lens.
standard
screw of
object
glasses.
Variety of
object
glasses
required.
Eyepieces,
Working Dis-
Object Glasses.
X 6.
X 10.
X 15 Field
of View.
X 17.
X 25.
tance, X 10
Eyepiece.
in.
in.
in.
in.
in.
in.
l^in. (40mm.) 1 .
•25
•16
•115
[•11
•1
2^05
U in. (32 mm.) .
•2
•11
•09
•085
•075
•88
2/3 in. (16 mm.).
2/3 in. (16mm.)i
}-08
•045
•035
•032
•03 1
•25
•11
2/3 in. (14 mm.)i
•07
•04
•03
•028
•025
•07
1/3 in. (8 mm.) .
1/3 in. (8mm.)i.
}^045
•025
•02
•019
•015 1
•06
•037
1/6 in. (4 mm.) .
•02
•01
•0085
•0083
•007
•024
1/8 in. (3 mm.) .
•015
•008
•0075
•0065
•006
•016
1/12 in. (2 mm.).
•0085
•005
•004
•0038
•0035
•01
1 Those marked ^ are apochromatic. The other object glasses are
achromatic.
All object glasses are now made with a screw standardised
by the Royal Microscopical Society ; its specification is as follows :
Thread, Whitworth Screw, 36 to the inch, length ^125 inch.
Plain fitting on object glass above screw, •! inch long, ^759
inch diameter.
Diameter of thread of object glass at top of thread, -7952 to
•7982 inch.
Diameter of thread of object glass at bottom of thread, -7596
to -7626 inch.
Diameter of thread of nosepiece at top of thread, '7644: to
•7674 inch.
Diameter of thread of nosepiece at bottom of thread, -8 to
•803 inch.
The stem of all the Beck object glasses are of a standard
diameter, '65 inch.
The series of object glasses mentioned in this book is sufficiently
large to cover all the requirements of microscopy, with the possible
exception of a very low power for examining unusually large
objects. The highest power is a l/12th inch. It has sufficient
magnifying power to show all the detail that the maximum aperture
will resolve; and although object glasses of higher power can be
made, they cannot be made with this maximum aperture because
the lenses must be much smaller, and as a fixed distance must be
allowed for a cover glass they cannot collect as large an angle.
Object glasses of imtermediate sizes have also been made
from time to time, but the magnifying power given by an inter-
mediate size can be so readily obtained by a change in the eye-
piece that they are of little advantage. The manufacture of
OBJECT GLASSES AND EYEPIECES 83
object glasses has, therefore, been limited to a somewhat more
restricted number of sizes than was customary some years ago,
with considerable advantage, as concentration on the smaller
number of lenses has tended to improve their quality.
There are two types of object glasses, achromatic and Achromatic
apochromatic. Both types are excellent, and although there gfaS/
is no doubt that the apochromatic series possess qualities which
render them of greater service where the most exacting scientific
investigation is being carried out, for the more general
work this extremely high quahty of optical construction is not
required. Hence the achromatic series fill the requirement for
most purposes, and are in more universal use, on account of the
fact that they can be made to a simpler formula and with a less
number of component lenses and less expensive materials. The
resolution of the achromatic lenses is of a very high order. As
an example of their good performance, the diatom " Pleuro-
sigma Angulatum " has dots in its structure which are approxi-
mately 1/48000 of an inch apart. The theoretical aperture which
will show these as separate dots is -5 N.A. This can be done with
a Beck 8-mm. achromatic object glass which has this aperture
(•5 N.A.), showing that an ordinary achromatic object glass, if
properly constructed and adjusted, is so perfectly corrected for its
zonal and other aberrations that it will resolve up to its theoreti-
cal limit. On a Grayson's ruling this lens will resolve 45,000
lines to the inch with a green screen and 50,000 with a blue screen.
For visual purposes with a colour screen, achromatic lenses can
be made almost optically perfect, but the apochromatic series
described later are more perfect as regards their colour correc-
tion, are better for photography, have somewhat larger aperture,
and will, therefore, stand the use of higher eyepieces, giving a
slightly better defined image, especially when a colour screen is
not employed.
The chief feature of the apochromatic series is that different Apochro-
glasses are employed and other materials substituted, and that this passes! ^^'^
combined with a different formula, involving the use of a large
number of component lenses, produces an object glass in which
there is more perfect correction for chromatic aberration. In the
achromatic object glasses the correction is made for two colours
of the spectrum, but in apochromatic lenses this correction is
made for three colours. For very fine markings undoubtedly
apochromatic object glasses give superior results ; the difference is
slight, but the perfection of the colour correction enables certain
objects to be seen with a greater crispness than is possible with
achromatic lenses. Although the achromatic are suitable for
photographic work, the apochromatic series has an advantage.
For those interested in the optical construction of these lenses,
we append a somewhat technical note on the theory of their
construction.
84
THE MICROSCOPE
It is a well-known fact that glass refracts* the various colours
by a different amount, and consequently a single lens will not give
an image free from colour because it has different foci for different
colours, the focus for red in a positive lens being further from the
lens than the focus for blue. This property of refracting colours
by a different amount is called the dispersion of the glass. From
the time of Newton to that of Dolland it was supposed that the
dispersion of different glasses was proportionate to their refrac-
tive powers (/W.-1), and therefore proportionate to their foci.
In other words, it was thought that any positive and negative
lenses which had the same effect on the colour must have the
same effect on the focus whatever glass they were made from,
and that combining two such lenses would give the effect of a
plain piece of glass without any focus. Converselyj they supposed
that any combination of lenses which had a focus must have a
colour aberration equal to a single lens of that focus.
In the middle of the eighteenth century Dolland discovered
that this was not so, and that whilst the flint then is use had as
compared with the crown glass a refractive power of 60 to 50,
it had a dispersive power of 60 to 36.
If we take a crown glass of 1-5 refractive index, the difference
in focus between the pale yellow or C-rays, and the green or F-
rays, is about 1/60 of the focus ; but if we take a flint glass of about
1'6 refractive index, the difference between the corresponding
rays is about 1/36 of the focus. Therefore, if we take a positive
lens of crown glass which is 36 inches focus, and a negative
lens of flint glass which is 60 inches focus, the colour will
be corrected for these two rays when the two lenses are put
together, and the result will be an achromatic lens of 90 inches
focus.
Although in this combination the two rays C and F would
be correct, it would not give a perfect correction for the other
parts of the spectrum, because the refractive power of the two
glasses is not quite regular for the different colours.
For instance, the four coloured rays known by the
spectrum lines C, D, F, and G do not have proportional disper-
sions ; if we caU the dispersion from C to F 1,000, we find that in
a hard crown the distance from C to D is 295, and from F to G
568, whilst in a medium flint the distances are respectively 285
and 608. This may be expressed in the following manner.
-to-
-D— to-
-F-
-to — G
Hard crown
Medium flint .
295
705
285
715
568
680
OBJECT GLASSES AND EYEPIECES
85
In this figure the lines at C and F coincide, but those at
D and G do not, and the want of coincidence at D and G gives an
idea of the secondary error.
If we make a pair of lenses out of these two glasses which
when combined together give an achromatic lens of 1 inch focus,
and correct for C and F, we find that the foci for the different
rays are :
C 1-00000
D -99963
E 1-00000
G 1-00165
In all kinds of optical glass with high dispersion the relative
dispersion from F to G is higher than those of low dispersion,
but in some the difference is slight, and telescope lenses with
reduced secondary spectrum can be made from these. By com-
bining three glasses together apochromatic telescope object glasses
can be made, but in these the lenses have to be of relatively
short focus, and consequently only small apertures compared to a
microscope object glass can be obtained; //6 is a very large aperture
for a telescope object glass, but only corresponds to '07 N.A.
in a microscope object glass. No apochromatic microscope
object glasses have yet been made satisfactorily by this means.
The peculiar mineral fluorspar, however, has totally different
properties. It has a low refractive index and an extremely low
dispersion about 1/95 on the focus. The great peculiarity,
however, is that it has a larger proportional dispersion from
F to G than many of the glasses with higher dispersion ; the
corresponding figure being 583. Now if we make a similar diagram
to the last, but of fluorspar and light baryta flint, we get :
-to
D
-to-
F-
-to — G
Fluorspar
Light baryta flint
296
704
583
296
704
570
A combination made of these two materials and achromatic, of
1-inch focus, gives the following results for foci of different colours :
C
D
F
G
1-00000
1-00000
1-00000
-99947
Now it is quite evident that combining with this combination
another combination such as the first, in which the aberration for
F to G is in the opposite direction, it is possible to produce a lens
in which all the foci for the four rays are the same.
86
THE MICKOSCOPE
In practice the matter is more complicated than it appears,
because the thickness of the lenses not only alters their foci
but also slightly alters the ratio of their partial dispersions, and
this has to be allowed for to get the corrections accurate.
In achromatic object glasses it is usual to correct for about
C to F ; this means that when correct visually there is a very
slight error for D ; but these can be neglected for most work, but
for photographic purposes the error in G might be appreciable
unless a coloured screen is used.
In apochromatic object glasses the colour is corrected for at
least three parts of the spectrum, and also the spherical aberration
is much more fully corrected for aU colours ; this means the lower
power lenses have decidedly larger apertures than the correspond-
ing achromatic lenses and that with all apochromatic object
glasses higher eyepieces can be used. Also, when it is important
to distinguish between objects with slight differences of colour,
these lenses are much to be preferred.
object glass.
2/3-inch
object glass.
1/3-incli
object glass,
Chaeacteristics of Object Glasses of Different Powers
The IJ-inch (40-mm. or 32-mm.) object glass gives a maximum
field of view -2 inch (5 mm.) and has a working distance of
•88 inch (22 mm.) in the achromatic, or 2*05 (50 mm.) in the
apochromatic series. It is, therefore, specially useful for obtain-
ing a general view of large entomological and botanical specimens.
It is the only object glass with which opaque vertical reflectors of
the type of the Sorby flat silvered mirror, or the parallel flat
glass mirror, can be used between the front of the object glass
and the specimen, and is, therefore, useful for low-power metallur-
gical specimens. Its aperture (-16 or -17 N.A.) gives a theoretical
resolution of 15,000 to 18,000 lines to the inch, according to the
colour of the light employed.
The 2/3-inch (16-mm. or 14-mm.) object glass gives a maximum
field of view of '08 inch (2 mm.). The achromatic 16 mm.
has a working distance of '25 inch (6^ mm.). The apochromatic
16 mm. and 14 mm. have working distances of -11 inch (3 mm.)
and '07 inch (If mm.) respectively. This is the power that is
the most useful all-round low-power lens for every purpose. It
forms a useful finder for searching specimens to be examined
later with a high power. It can be used with the parabolic
reflector for opaque objects, and is probably used in larger numbers
than any other lens. The aperture of the achromatic ('28 N.A.)
gives a theoretical resolution of 25,000 to 30,000 lines, and of the
apochromatic -35 N.A., 30,000 to 36,000 lines to the inch.
The 1/3-inch (8-mm.) object glass gives a maximum field
of^view of -045 inch (IJ mm.). It has a working distance of
about -06 inch (1| mm.). It is a medium- power lens of the greatest
use for many purposes, and is not sufficiently appreciated. For
OBJECT GLASSES AND EYEPIECES 87
bacteriological and pathological investigation it can be used to
do a great deal of the work for which two object glasses are
usually employed. With a low-power eyepiece it has a field of
view large enough for searching, and with a high- power eyepiece
it can be used for blood counts and recognising micro-organisms
such as trypanosomes, malaria parasites, and bacteria, and for
pond life it is perhaps the most useful all-round power. For
metallurgy it is an excellent lens for photography, and with this
lens and an oil-immersion lens a microscopist can frequently do
all his work in certain branches of research. This lens is par-
ticularly useful for dark-ground illumination with a substage
condenser and patch-stops. The achromatic with an aperture
of 5 N.A. gives a theoretical resolution of 48,000 to 52,000 lines,
the apochromatic of '65 N.A. about 62,000 to 67,000 lines to
the inch.
The 1/6-inch (4-mm.) object glass gives a maximum field of i/e-inch
view of -02 inch (1/2 mm.) and has a working distance of -024 inch ° ^^^ ^ ^^'
(1/2 mm.). It is the universal high-power, and when only two lenses
are supplied, for cell- structure, histology, and all general high-
power purposes, it is more popular than any other lens, and being
made in large quantities is moderate in price. The achromatic
has an aperture of -85 N.A., the apochromatic -95 N.A., and
theoretical resolving powers of about 81,000 to 88,000 lines and
90,000 to 100,000 lines to the inch respectively. The very
large aperture of the apochromatic lens of this focus and its perfect
corrections renders it specially valuable for use with high eyepieces
when an oil-immersion cannot be used. In this case the 1/6-inch
with a correction collar should be selected, and the microscopist
should become familiar with the correct adjustment of this
collar (see page 80).
The 1/8-inch (3-mm.) oil-immersion object glass has a maxi- i/8-inch
mum field of view of -015 inch (-STS-mm.) and a working distance of °^J^^*s ass.
•016 inch (•4-mm.). Being an oil-immersion lens, it is not affected
by the thickness of the cover glass used, and is thus always
working at its best. It is introduced not because a power between
a 1/6-inch (4-mm.) and a 1/12-inch (2-mm.) is often required, but
because a special lens with a maximum aperture that can be
used with dark-ground illumination is urgently required for this
work. As explained in the description of the use of the dark-
ground illuminator, a large aperture oil-immersion lens such as a
1/12-inch (2-mm.) 1^3 N.A. must be stopped down to the aperture
of a 1/6-inch (4-mm.) dry lens to enable it to be used with dark-
ground illumination, and resolving power is thus lost. The
1/8-inch (3-mm.) object glass can also be used to do most of the
work with one object glass that is generally done with the 1/6-inch
(4-mm.) and the 1/12-inch (2-mm.). It has an aperture of -95
N.A. and a theoretical resolving power of 90,000 to 100,000 lines
to the inch.
88
THE MICROSCOPE
1/12-inch
object glass.
The 1/12-incli (2-inin.) oil-immersion object glass has a maxi-
mum field of view of -OOSS inch (-2 mm.) and a working distance
of -01 inch (1/4 mm.). This is the high-power lens which must be
used, if it is necessary to see the finest detail which can be observed
with any microscope. It has an aperture of 1-3 N.A. and a
theoretical resolving power of 125,000 to 135,000 lines to the
inch. It is the object glass that reaches the highest limit yet
obtained in microscopic vision, and is a necessary portion of a
complete outfit. The apochromatic, being slightly better than
the achromatic, is worth the extra cost even if all other lenses
are of the achromatic series. If structure of an object is just
beyond the limit of vision of a low-power, a higher power object
glass can be used ; but this does not apply to a 1/12-inch, as no
higher power will show more ; and if the quality of the highest
power lens is such that even slightly higher power eyepieces can
be employed, the scope of the instrument is extended.
Eyepieces
Huyghenian Eyepieces
No.
Focal Length. Magnifying
3260
42 mm.
X 6
3261
25 mm.
XlO
3262
17 mm.
Compensating Eyepieces
X15
3266
45 mm.
X 6
3267
30 mm.
X 8
3268
22 mm.
Xll
3269
15 mm.
X17
3270
10 mm.
X25
Standard
size.
Best
eyepieces
for general
work.
All eyepieces are made to the diameter of the Royal Micro-
scopical Society's No. 1 Standard Drawtube, 'Ql? inch diameter.
They are made to drop in loosely, so that they may be changed
without any tendency to alter the adjustments of the microscope.
They are designated by their focal length, and their magnifying
power is given for the distance of distinct vision — 10 inches
(250 mm.) — and is engraved on each eyepiece.
The best eyepieces to use for general work are those of the
lowest powers, 42 or 45 mm. and 25 or 30 mm. The eyepoint
(T, Fig. 1, page 9) is large with low-power eyepieces, and fine
specks of dust on the surface of the eye or in any part of the
instrument do not readily show. The higher the power of the
eyepiece used, the smaller is the diameter of the eyepoint, and any
such minute obstacles to the passage of the light become more
apparent. It is, however, of the utmost value to be able to
slip in a high- power eyepiece for occasional examinations, in
order to increase the power without altering the adjustment of the
OBJECT GLASSES AND EYEPIECES
89
eces
instrument, and for this purpose a 17- or 15-mm. or a 10-mm.
eyepiece is required. A 5-mm. eyepiece magnifying 50, and a
2-5-mm. magnifying 100, are made to order for special testing
purposes, and have their uses.
Compensating eyepieces are specially corrected to work with Compensat
apochromatic object glasses, and when of a higher power than ^'^'^''P'
15 mm. are the best for use with achromatic object glasses. The
difierence in performance of the Huyghenian and the compensating
eyepieces is not very marked.
Huyghenian eyepieces consist of two plano-convex lenses, Huyghenian
one at each end of a tube, with a diaphragm between them, ^^^p'^'^®*
It is an eyepiece that has many advantages for visual work, but
it is not the best for photography. It is also not quite so perfectly
corrected as the compensating eyepieces which are specially
made for work with apochromatic object glasses. The lower of
the two lenses is called the field lens, because it increases the size
of field while the upper one does the
magnifying. The two lenses are of such
powers and placed in such positions that
they are achromatic and give a fairly flat
field for visual purposes, but do not do
so for photography, where the microscope
has to be re-focussed in such a manner
that an actual image is formed behind
the eyepiece instead of a virtual image
projected in front of the observer's eye.
The corrections of an eyepiece need
not be of so perfect a character as those
of an object glass, because the individual
bundles of rays from each point of the object are very narrow
beams of light as they emerge from the eyepiece, and the defects
of an eyepiece are reduced in a similar manner to those of an
object glass when it is stopped down by a pinhole aperture.
The aperture which limits the size of the beams of light is not a
pinhole, but the same effect is produced by the narrow angled
cones of light which come from the object glass.
The magnifying power of the eyepiece is never very great
compared with that of the object glass, and it is only in those of
high power that the corrections are of such importance that the
extra quality of the compensating series are very noticeable,
except for photography or where entire freedom from colour is
essential.
The so-called projection eyepieces are no better for projection
and photography than the compensating, and are far more difficult
to adjust.
Eyepiece micrometers or plates of glass ruled with squares
or cross lines may be dropped upon the diaphragm in the tube
of the eyepiece by removing the cell holding the upper lens. By
Fig. 90.
90
THE MICROSCOPE
screwing the upper lens cell up and down into tlie tube they may
be sharply focussed (see page 67).
An eyepiece with a movable pointer in the field of view, an
erecting eyepiece and a polarising eyepiece are described in
pages 72 and 73.
Histology
and
pathology
The Selection or Object Glasses and Eyepieces
Advantage Where pricc is not an object, it is advisable to have a complete
of complete ggj^ Qf either achromatic or apochromatic object glasses, including
only one of the 2/3-inch apochromatic and one of the 1/6-inch
apochromatic. Each size has its special uses as previously described,
and they do not overlap. It is best to have apochromatic
object glasses and a complete set of compensating eyepieces.
At a time when many intermediate sizes of object glasses were
made, a selection was always necessary ; now that they have been
reduced to a smaller number of standard sizes, they are all of
great assistance to any observer. It is not, however, every
microscopist who can afford to buy a complete set, and in this
case the selection becomes a matter of importance. For work
on all subjects, all sizes will probably be eventually required,
though they need not be purchased at once.
For histological and pathological work, the student is advised
by his teacher to purchase a 2/3-inch (16-mm.) and a 1/6-inch
(4-mm.) object glass and two eyepieces magnifying X6 and XlO,
adding a 1/12-inch oil-immersion for pathological work at a later
date. It is a question whether in some cases he might not do
better to start with a 1/3-inch (8-mm.) and an 1/8-inch (3-mm.)
oil-iramersion, adding a IJ-inch (32 -mm.) for very low- power
work if required. In such a case he should have three eyepieces,
X6, XlO, and Xl5, as the highest power eyepiece enables a
great deal of work to be done with the 1/3-inch (8-mm.) that would
generally be done with a 1/6-inch (4-mm.). The 1/8-inch (3-mm.)
oil-immersion is a very useful power. The 1/3-inch (8-mm.) is
only slightly affected by the thickness of the cover glass, and the
1/8-inch oil-immersion is not affected at all, so that the unskilled
observer is more likely to get the best out of his instrument with
these two powers. The addition at a later date of a 1/12-inch
apochromatic object glass and a high-power compensating eye-
piece makes a very perfect outfit. Where price is of great
importance, the cheapest outfit will be a 2/3-inch (16-mm.) and
1/6 -inch (4-mm.), as usually recommended.
For biological work the same remarks apply to a great extent,
but the 2/3-inch (16-mm.) has the great advantage that, used with
an erecting eyepiece, it turns the instrument at once into a dis-
secting microscope. It is also sometimes troublesome to use an
oil-immersion with an unmounted specimen examined in water
under a cover glass, and a high- power dry lens is often preferred.
Biology.
OBJECT GLASSES AND EYEPIECES 91
The difficulties of using an oil-immersion lens, however, cliiefly
apply to cases where it is necessary to search with a low power,
change to a high power, and then rapidly search again, as in the
latter process the oil must be wiped off the cover glass with a
piece of filter paper dipped in benzol or xylol before the low power
is used. The 1/6-inch dry apochromatic with a cover glass
adjustment is a very useful lens for such work, because it has a
very large aperture and resolving power, and if carefully adjusted
for the cover glass thickness, owing to its very perfect corrections,
can be employed with very high eyepieces to do much of the
work that would otherwise be done with a 1/12-inch.
For botanical work the 2/3-inch (16-mm.) and 1/6-inch (4-mm.) Botany,
are generally used by the student. A great deal of the work
could be better done with IJ-inch (32-mm.) and 1/3-inch (8-mm.)
with a higher eyepiece. A 1/12-inch oil-immersion may be added
for cell structure, such as Karyokinesis.
For metallurgical work the best three lenses are the IJ-inch Metallurgy.
(32-mm.), 1/3-inch (8-mm.) and 1/12-inch (2-mm.) oil-immersion.
For photography the apochromatic series have a marked advantage,
as colour screens need not be used ; compensating eyepieces
should always be selected for photomicrography as the Huyghenian
eyepieces are not satisfactory for this purpose. For chemical
and industrial purposes it is difficult to make any recommendation.
The objects examined are so varied and the conditions so different
that nothing but a complete series will meet every requirement.
It is best to study the capabilities of each object glass as given
on page 86, and select according to circumstances.
For petrology the best two lenses are the 2/3-inch (16-mm.) Petrology,
and the 1/6-inch (4-mm.) ; the high-power dry lens being essential
for observing interference figures, as a large aperture is necessary
for this work. A low-power IJ-inch (32-mm.) is very useful.
The apochromatic series must be used with caution for this
purpose, the fluorspar of which they are made may render them
unsuitable in some cases.
For general recreation the whole series will appeal to the General
microscopist who wishes to dip into a large number of subjects, ^^^^eation.
If only two object glasses are required he should begin with a
IJ-inch (32-mm.) and a 1/3-inch (8-mm.) and three eyepieces.
CHAPTER VI
THE MICROSCOPE STAND
Essential In all microscopes certain characteristics are important. The
qualities. quality of the optical portions is the essential, but the stand
requires to possess good adjustments and rigidity of construction
to enable the optical qualities to be made full use of. Those
who are not competent to judge of the optical performance may
be sometimes tempted to criticise small details of mechanical
construction which are of no importance, but certain main points
are worthy of consideration.
Base and The basc and pillar of the microscope have been a subject of
piUar. j^jjg discussion among microscopists. If the instrument is
supported in a rigid manner, their shape and construction is not
of great importance. Two chief types have been made, one of
which has three projecting legs of varying shapes, the other
consists of a flat slab with a pillar fixed upon its upper side.
The former is generally known as the English model, and the latter
as the horseshoe base. It was originally introduced more or less
of the shape of a horseshoe, but has since been altered in its
outlines. Both stands rest upon the table on three toes and,
provided the distance apart of these toes is the same, the two
models are equally rigid. The horseshoe pattern relies for its
stability slightly more upon its weight than its size. It has the
advantage that it can be used rather nearer the edge of the table
when the microscope is in a vertical position, and that all the
adjustment of the substage can be more readily got at than in
the English model, where the side projecting legs are more or
less in the way of the hands. It also requires a rather smaller
case or bell-glass cover. Most microscopes are now made with
the piUar and slab form of base known as the horseshoe, or with
one piece that has a shape which approximates to a horseshoe
base and pillar combined. Far too much time has been wasted
in the past by arguing on the relative merits of the two forms.
The microscope should stand rigidly and be free from any tremor
in its parts. In cities or near machinery where constant vibration
is present it is sometimes worth while to take special measures
to overcome this. A slab of slate supported on a layer of cotton
wool an inch thick will generally damp out vibration. In a
92
THE MICEOSCOPE STAND 93
factory with rapidly- moving macliinery, the microscope table
may be placed on a stone which rests on an inflated motor-car
tyre. Under ordinary circumstances such precautions are not
necessary, and any firm table is satisfactory.
The stage should be firm and its upper surface should not be stage.
less than 4| inches above the table. An ebonite covering makes
a better surface than metal for giving a smooth motion to the
slide. It is less likely to be damaged by reagents, but a brass
stage with a surface ground flat is very satisfactory. There
should be a horizontal distance of not less than 3 inches
between the optic axis or centre of the stage aperture and the limb
to enable Petri dishes and large culture plates to be examined.
The body tube must be of a variable length. The early Body tube.
microscopes were generally made with a 9- or 10-inch tube, but
have been entirely superseded by the more compact type which
has a tube length of 140 mm., which, by means of a drawtube,
can be increased to 200 mm. The shorter tube length has an
advantage in addition to the reduction in the size of the microscope
of which it admits. In the previous chapter it has been explained
how the variation in the thickness of the cover glass can be
largely compensated by a variation in the length of the drawtube.
A body tube of great length must be moved to a great extent to
produce much alteration, while a short body is far more sensitive
in this respect, and a much greater range of correction can be
obtained. It is also possible by an extra tube to further lengthen
a short body, while it is not feasible to shorten a long tube.
The drawtube should always be graduated in millimetres, Drawtube.
which give the length at every position, and it should work with
great smoothness, so that when the microscope is in use the
length of the tube may be altered without exerting any force
which is likely to upset the adjustment of the instrument. The
sliding fitting should always be in cloth or other fabric which will
ensure a smooth motion. A metal-to-metal slide is not so satis-
factory ; such a slide may be perfect when it leaves the makers'
hands, but the slightest film of tarnish or oxidisation ruins its
working and gives a jerky, uneven motion. Due to the elasticity
of a thin cloth or a fabric slide, it cannot be quite as stiff and rigid
as a metal slide, but this is a matter of no practical consequence,
as a slight movement of the eyepiece out of the optic axis has
no effect on the quality of the image. The drawtube must be
provided with a diaphragm to prevent reflections at the inner
sides of the tube, and the upper portion of the drawtube should
be slightly smaller in diameter than the lower part, so that
pushing the eyepieces in does not tend to polish the tube below
the position where the shortest eyepiece fits. The lower end of
the drawtube should have a screw fitting for the use of a low-power
object glass.
The coarse and fine focussing adjustments must be well made Adjustments.
94
THE MICROSCOPE
Joint.
Limb.
and must work with a smooth, even motion that allows of the
most delicate setting for focus. The coarse adjustment should
be capable of focussing with a 1/6-inch object glass, although
after the focus has been found the slow motion will generally be
used. It is an advantage to have a series of divisions on the slow
motion by which the thickness of a cover glass or a section can
be ascertained (see page 53). The value of the divisions is given
under the description of different microscopes. The two adjust-
ments should work in fittings which are made with the utmost
precision. These fittings should be solid metal slides without
any adjusting screws. The wear in the fittings of a microscope
is infinitesimal compared with those of running machinery, and
slides well fitted in the first instance will wear for a lifetime
without adjustment if properly used. All kinds of adjustable
fittings have been tried, but have been abandoned. The adjusting
screws work loose, the slides do n6t have to be so well fitted
originally, and nothing is so good as a solid slide well fitted in
the first instance.
The milled heads of both the adjustments should move in the
same direction, so that the upper portion of the milled heads is
going away from the observer when the body tube is going down
or approaching the object. Mistakes made by turning the milled
heads in the wrong direction may result in breaking the slide or
damaging the object glass. If all the milled heads in a microscope
move in one direction, such mistakes need not be made.
A microscope should have a
joint for inclination. The instru-
ment may have to be used occa-
sionally in a vertical position, but
it is so much more convenient in
any other position that an in-
clining joint should not be omitted
from the stand. The writer does
all his most difficult tests and ex-
aminations with a microscope on
an optical bench in an almost hori-
zontal position, the axis pointing
down only about 15°. The eye-
piece is at the eye-height of the
observer when in a sitting position.
Prolonged observation of several
hours ceases to be tiring with the
microscope thus arranged. An
ordinary microscope, however, in-
cUned to about 45° is very com-
fortable for prolonged work.
A microscope should have a limb that can be readily grasped
by the hand for lifting. It must never be lifted by its body or
Fig. 91.
THE MICROSCOPE STAND 95
any of the adjusting milled heads. The only other portion of
the instrument by which it may be lifted is the base or pillar.
Valuable instruments may be badly damaged if lifted carelessly.
Adjustment slides may be ruined, pinions and screws bent, or the
entire instrument may be dropped if these instructions are not
followed.
^A suitable size for the mirror of a microscope depends upon Minor,
how far it is placed below the stage ; a large mirror close to the
stage is of no advantage. A 2-inch diameter mirror 3J inches
from the stage will converge a beam of light at approximately
30°, and this is more than is ever required. When a substage
condenser is employed, the mirror need not be much larger than
the back lens of the condenser, which never exceeds IJ inches.
A 2-inch mirror is, therefore, more than sufficient for all ordinary
types of microscopes. A mirror should preferably be on a fitting
by which its distance from the stage may be varied. It is not only
of advantage for focussing the concave mirror, but enables very
long apparatus to be used in the substage by sliding it farther
from the stage than its usual position. It is convenient that it
should be capable of swinging to one side for inserting substage
apparatus or for using light direct from a source of illumination,
but in use it must always be placed in the axis of the instrument.
All the microscopes illustrated in this book possess the features
here described as being of importance ; the following brief notes
explain their special characteristics.
Except the special metallurgical and petrological microscopes,
all the instruments illustrated are suitable for every branch of
work. The highest class of research work calls for a mechanical
stage and the best substage adjustments. The rack and pinion
adjustment to the drawtube and the rotating stage are of con-
siderable advantage, but are not essential.
The Standard London Microscope is illustrated in six forms, standard
The first three of these forms are the same except as regards their ^^'^oscopes.
substages. These microscopes fulfil the conditions given in
previous pages as to the essential features which a serviceable
microscope must possess. They have also many smaller advantages
and refinements. The base consists of an iron casting of suitable The base,
weight to give rigidity to the instrument, encased in a covering
of vulcanite which gives it a durable finish. It is of such a spread
as to prevent the instrument from tipping, and is also made of
such a shape that it can easily be fixed down to a bench when used
for photomicrography when the instrument is used horizontally.
This may be done with advantage, as it prevents the microscope
from being moved during the process of attaching the camera.
The joint of the instrument is stopped at the exact vertical and
horizontal position. The stage consists of a brass core completely The stage.
embedded in vulcanite. This method is more satisfactory than
the usual method of fixing on thin vulcanite plate on the top of
96
THE MICROSCOPE
The limb.
The
mechanical
stage.
The body.
The
a brass stage, as it is less subject to warping, is not easily chipped
or broken. The stage has four holes for the accommodation of
stage clips. The limb is drilled with a hole by means of which
a mechanical stage can be attached, held in position by a strong
bolt with clamping milled head. This mechanical stage can be
fitted by the user of the microscope without returning the
instrument to the maker, although it is best to do so if possible,
as in this case a steady pin is also put in to ensure that the mechani-
cal stage is in its exact position, and thus to make certain that
the finder divisions read correctly. The finder divisions read from
the left-hand side and the bottom of the 3x1 slip. The standard
body of the microscope is of rather larger diameter than is usual
with the ordinary small body tube. The coarse adjustment is
adjustmentsi actuated by the upper milled head, and the fine adjustment by the
lower. The fine adjustment is of rigid type and gives a very
sensitive and smooth motion. It has two speeds, the left-hand
milled head travelling the body at half the speed of the right-
hand milled head. The substages are all interchangeable, and
all the microscopes are supplied with the holes necessary for the
fitting of any of the various forms of substage which can be attached
by the user with the aid of a screw-driver. Thus a microscope with
a plain tubular substage may first be bought, and if the micro-
scopist at a later date feels the need of a substage with focussing
and centring adjustments, this substage may be purchased
separately, and he is able to fix it himself, without sending the
microscope to the maker. The following are the dimensions of
these microscopes :
The
substage
Dimensions
of the
microscope.
4f
>>
m
>>
6i
>>
H
»»
7
■§■
j»
3i
>>
•01
mm.
6
>>
No. 3210.
Size of base . . . . . . 6| X 4 X 1 in.
Distance of upper surface of stage from table
The height of the microscope in use when vertical .
The height of its optical centre when horizontal
Diameter of coarse adjustment milled heads .
Diameter of fine adjustment milled heads
Travel of coarse adjustment ....
Each division of the fine adjustment moves the body
Travel of fine adjustment .....
Diameter of object glass screw, Royal Microscopical
Society's Standard ......
Diameter of drawtube, Royal Microscopical Society';
Standard No. 1 ..... .
Diameter of substage, Royal Microscopical Society'
Standard .......
Diameter of mirror ......
Focal length of mirror .....
Vertical travel of mirror .....
Tube length 140
Outside diameter of upper portion of drawtube
Diameter of object glass stem ....
Distance from optic axis to inside of limb
No. 3210, page 97, is the simplest form of the microscope,
and has a plain tubular substage into which slides a fitting with
•8 in.
•917
to
1-527
>>
2
»>
3-5
*»
1-5
>>
)0
mm
1-05
in.
•65
it
3
*t
Fig. 92. — ^No. 3210, Standard London Microscope, with plain tubular
substage and dust- tight double nosepiece.
7 97
Fig. 93. — No. 3211, Standard London Microscoije, with screw
focussing swing-out substage, dust-tight double nosepiece.
98
Fig. 94. — No. 3213, Standard London Microscope, with rack and
pinion and centring swing-out substage, triple nosepiece, mechanical
stage.
99
100 THE MICROSCOPE
an iris diaphragm. In the upper cell of this a small Abbe con-
denser can be fitted, and also a series of patch-stops or a coloured
or ground glass. The fitting can be moved up and down in the
tube to a limited extent for focussing.
No. 3211. No. 3211 (page 98) has a substage which focusses by means of
a screw actuated by a milled knob. When this screw reaches the
end of its travel in a downward direction the whole fitting carrying
the iris diaphragm and condenser swings aside, so that it is a
very simple matter to entirely dispense with the condenser
when it is not required. The substage is held rigidly in the
optic axis imtil the fitting is focussed down to its lowest position,
when a further turn of the milled head swings it out of position,
thus the addition of this motion does not in any way afiect the
rigidity of the substage. This substage is suitable for use with
either the small or large form of the Abbe condenser. Centring
motions cannot be fitted, and it is consequently not suitable
for use with an achromatic condenser. A high-power dark-
ground illuminator can be used with it, but must be in a fitting
that is provided with centring screws.
No. 3213. No. 3213 (page 99). This microscope has a substage with full
adjustments — namely, focussing by rack and pinion, swing-out
motion, and centring motion. The focussing is actuated by a large
milled head on the right of the instrument travelling in the same
way as the coarse adjustment milled heads. When at the bottom
of its travel the substage may be completely swung aside. Here
again, as this substage is held in position by a guiding pin, until
it is in its lowest position, there is no tendency to lose rigidity
by the addition of the swing- out motion. The centring is actuated
by two screws with milled heads. A modified form of this stand.
No. 3212, is made which is the same as No. 3213, except that the
substage has not a centring adjustment. This is suitable when
it is not desired to use a higher class condenser than the Abbe
form, but for all more exacting work the No. 3213 is preferable. The
substage on this stand No. 2313 enables the achromatic condenser
and the high-power dark-ground illuminator to work to their
full advantage. The illustration, page 99, shows this microscope
with a detachable mechanical stage attached by bolt and nut,
as mentioned previously.
The Portable Standard London Microscope
Portable This microscope has been designed for the use of the micro-
No. 3221. ' scopist whose work requires that he should have an instrument
of the usual rigid construction, with all the movements neces-
sary for the highest forms of research work, but to whom porta-
bility is also an advantage. For travellers engaged in critical
work, and bacteriologists in foreign countries, this microscope is
especially suitable. The stand is the same as the standard
THE MICROSCOPE STAND
101
pattern No. 3213 except as regards the base and stage. The
former is folding, and has the same spread as that of the standard
model. The stage with the substage attached removes for
packmg into the case. It is so made that when in position it
IS even more rigid than the standard form. It is attached on a
bracket and is held in position by a taper bolt. The substage
has all the_ adjustments of the No. 3213, including focussing
rack and pinion, centring by screws and a swing-out motion^
The instrument is packed in a case which only measures 1L\ x
8 X 2J inches. It is as perfect an instrument in every way
Fig. 95. — No. 3221, Portable Standard London Microscope
in case.
as jthe ordinary model. The incase will carry two eyepieces, three
object glasses, substage condenser, dark-ground illuminator,
a detachable mechanical stage, triple nosepiece, and a bottle
of oil, together with a supply of slips, cover glasses, and sundry
small apparatus. It does not weigh much less than the
ordinary model. The small dimensions of its case render
it specially suitable for travelling where a bulky instrument is
inadmissible. It has no disadvantages due to its portability,
and most standard apparatus can be fitted to it. For the ordinary
microscopist who takes his instrument^from place to place it is
very convenient.
Fig. 96. — No. 3221, Portable Standard London Microscope, with
rack and pinion, and centring substage, mechanical stage, triple nose-
piece, and condenser.
102
Fig. 97.
No. 3216, Standard London Microscope, with large body, circular
stage, and complete substage adjustments.
No. 3217, Rack and pinion focussing and double^extension draw-
tube,
103
104 THE MICROSCOPE
The Standard London Microscope with Circular
Rotating Centring Stage
This instrument is made in four forms. No. 3214 has a screw
focussing substage and is the same as No. 3211, page 98, with
the addition of the circular rotating and centring stage.
No. 3215 has a rack and pinion focussing and centring sub-
stage and is the same as No. 3213, page 99, with the addition
of the circular rotating and centring stage.
No. 3216 is the same as No. 3215, but with a large 2-inch
body instead of the standard size. Both the nosepiece and the
drawtube end of the body can be unscrewed, and a photographic
lens can be slid into the centre of the tube for photographing
large specimens. The large size body does not cut off the angle
of view given by such a photographic lens. Also, if the drawtube
end of the tube be unscrewed and the nosepiece left in position,
low- power lenses with a large angle of view may be used in the
nosepiece for a similar purpose.
No. 3217 is the same as No. 3216, but with a rack and
pinion adjustment to the drawtube, and is provided with a
second drawtube, enabling the length of the tube to be varied
from 140 mm. to 250 mm. The drawtube is very large in diameter,
and can be provided with extra large eyepieces, 1*41 inch diameter,
of the No. 1 R.M.S. standard size.
The circular mechanical stage illustrated on page 52 fits
any of these four models.
This microscope, with an interchangeable binocular body
described later, makes a very perfect research microscope.
The Massive Model Microscope
No. 3201. There are certain cases in which most small microscopes give
dissatisfaction for very delicate work, and this model was first
made for The National Institute for Medical Research, who gave
valuable assistance in the design and construction. It is intended
for those who feel the want of a very perfect instrument. It has
been made throughout on a very heavy and stiS design. It does
not stand much higher than the standard model, but it is unusually
strong and stiff, so that no vibration or flexure can take place.
The limb consists of a massive brass casting which extends in
one piece from the body to the mirror. The tail-piece and fine
adjustment slide are planed out in one continuous cut so as to
ensure the perfect alignment of the substage with the focussing
adjustment. The stage, which is strengthened below by side
ribs, is rigidly fixed on to the limb, so that it is as strong as a solid
piece. It is of great advantage to have a stage so solid that it
does not show movement under the highest powers by the weight
Fig. 98. — No. 3201, Massive Model Microscope.
105
106 THE MICROSCOPE
of the hands placed even heavily upon it. The pillar and base
are equally heavy and free from any spring. The fine adjustment
is exceptionally delicate — one revolution of the milled head moves
the body only '1 mm. Each division on the milled head is equal
to "OOl mm. The coarse adjustment milled heads are very large,
enabling a finer adjustment to be made. The stage is square,
measuring 4 J X 4f inches. In the simple model it is flat, with a
gap cut out in front, and the standard mechanical stage can be
attached to it at will in a similar manner to that of the standard
model, being clamped to the limb through an aperture left for
the purpose. "When a mechanical stage is supplied at the same
time as the microscope, it is fitted with a steady pin entering a
second hole in the limb, so that it cannot be attached in an in-
correct position. In the best form of instrument, as illustrated
on page 105, the square stage has two dovetailed grooves planed
in its surface, and the mechanical stage racks up and down
these grooves or can be removed at will. This mechanical stage
has its actuating milled heads projecting laterally on the right-
hand side. The upper one moves the slide laterally and has
3 inches (75 mm.) travel, the lower one moves the slide vertically
and has a travel of IJ inches (30 mm.). The latter motion is
provided with a clamp screw, so that it can be locked to prevent
any chance of the slide moving when the instrument is in a
horizontal position. This prevents any settling down of the
object during photomicrography. Verniers reading to 1/10 mm.
are provided to both movements in convenient positions for
reading.
The substage racks up and down on the lower portion of
the limb, which is accurately in the optic axis of the microscope,
and the mirror fits by means of a sliding fitting on the same slide.
The substage has centring adjustments and is of the standard
size, but at its upper end is fitted with a dovetailed fitting to
receive the condensers or dark- ground illuminators. All the
illuminators are mounted on dovetailed slides which slide easily
into the dovetailed fitting, and are held accurately in position
by a clamping milled head. Each illuminator is accurately
centred and of the same length, so that they can be rapidly
interchanged while the object is under observation. The front
portion of the stage is cut out to enable this to be done, and even
an oil-immersion condenser can be changed for a dark-ground
illuminator while the slide is under observation. While these
illuminators are in use, the tubular portion of the substage is
free to receive apparatus which can be used in conjunction with
them. The back of the foot of the microscope carries a short
vertical post, and when the microscope is placed in a horizontal
position for photomicrography this takes the weight of the limb
and makes a rigid support under conditions where a slight tremor
might ruin the sharpness of a photograph. The body is of the
THE MICROSCOPE STAND 107
large 2-inch diameter, with a drawtube giving a variation in
length from 140 mm. to 200 mm. A rack and pinion double
extension drawtube, as illustrated on page 103, can be fitted if
desired. An interchangeable binocular body can be fitted to
the instrument, and a circular rotating stage can be made in
place of the square stage ; but in this case a gap cannot be cut
out in front, and some of the advantages of the interchangeable
substage apparatus are lost. The apparatus can be interchanged,
but only after racking down the substage by the amount of the
thickness of the stage.
With, this massively made microscope, the body and apparatus
can be relied upon to be always truly in the optic axis, the
manipulation of one part of the instrument does not tend to
upset the adjustment of the other parts, and when using the very
highest power lenses it is a pleasure to work on account of its
stability and the delicacy of all its adjustments.
The Binocular Microscope
Hitherto binocular microscopes have not been used for
research work, except in special cases. For practical purposes
the monocular has for many years held the field, and the use of
binoculars has practically been restricted to workers who only
use low powers, or for exhibition purposes. The reason of this
is simply explained : the one advantage of using two eyes did not
outweigh the loss of the many advantages possessed only by the
monocular stands.
Binocular microscopes may be divided into three types, and
a brief description of each type will set forth their
respective merits and demerits.
Type 1, best represented by the " Wenham,"
bisected the beam of light that emerged from the
object glass (0, Fig. 99) and directed the right-hand
half into the one eye, the left-hand half into the
other.
Binocular vision with this was not equal to
monocular, because by reducing the size of the
beam of light which formed each image it reduced
the resolving power of the microscope. It could not be used
with high-power object glasses, because the prism could not be
placed sufficiently close to the back lens of the object glass to
properly bisect the beam of light into two separate halves
before the rays had intermingled. Efforts to accomplish this
by mounting high- power object glasses in special short mounts
only partly overcame the difficulty, and rendered the use of
revolving nosepieces impossible.
This type of instrument involved long tubes and con-
sequently bulky instruments, and could only be satisfactorily
108
THE MICEOSCOPE
Fig. 100.
employed when the illumination was specially arranged so that
the whole of the object glass was equally illuminated. If, for
instance, a fine oblique feather of light was employed, it would
only enter one side of the object glass and consequently only one
eye would receive the light, the other eye seeing no image. In
like manner, if more light happened to be entering one half
of the object glass than the other, the illumination of the two
eyes was different, often to the extent of
making binocular vision inoperative.
It will be seen that none of these dis-
advantages exists in the new instrument here
described.
The second type of binocular was in one
respect on the right principle. In all models
of this type the beam of light is not bisected
into two halves, but the entire beam is filtered
into two portions, so that some light from
every part of the object glass goes to each eye.
The Powell and Lealand (Fig. 100) shows the
earliest form, the whole light from the object
glass (0) impinges on a glass plate (1) and the
major part passes through this thick glass plate,
emerging in a direction parallel to and almost continuous with its
original direction ; but a percentage is reflected at the first surface
and proceeds to the prism (2), which reflects it up a second tube,
placed at an angle with the optic axis of the direct beam.
This type of instrument gives equal resolution to that of a
monocular microscope because the size of the beam which forms
each image is not reduced.
With the Powell and Lealand form, however, the tubes of
the microscope must be long and the instrument bulky, and it
suffers from the very grave defect that
the light that is reflected is so feeble as
to be insufficient for satisfactory vision.
The light in one eye is only of about
one-sixth the intensity of that of the
other.
The Abbe binocular eyepiece, which
acted optically on the same principle in
not bisecting the beam into two halves,
but in filtering the light by a reflected
and a refracted beam, improved the light distribution, but only
made the relative illumination in the two eyes about 1 to 2|,
and, while not curing this defect, introduced a farther dis-
advantage. The general plan of this eyepiece is shown in Fig. 101,
and it will be seen that the light which is split up into two by
reflection and transmission at the surface is resolved into two
beams, one (A D) which is transmitted, the other (A B C) which
/
Fig. 101.
THE MICROSCOPE STAND
109
is reflected, and the reflected light at the time it emerges has
travelled a path that is longer than the direct light by the
amount A B. This difficulty was overcome by making a special
pair of eyepieces whose focal points were different in position,
but it limits the use of the instrument to the use of special
eyepieces.
Beck
Fig. 102. — Diagram showing
(A)
Eyepiece.
(B)
Drawtube.
(D)
Body.
(E)
Coarse focussing adjustment
(F)
Prism box knob.
(&)
Sliding prism box.
(H)
Object glass.
(I)
Stage.
(K)
Substage condenser.
W
Iris diaphragm.
paths of light tlirough the Beck Binocular
Microscope.
(M) Substage focussing adjustment.
(N) Mirror.
(0) Pillar.
(P) Base.
(x y) Object.
(a/ y') Image formed by object glass.
(x' y") Virtual image formed by eyepiece.
(« «') Ramsden discs — conjugate images of
back equivalent plane of object
glass.
no THE MICROSCOPE
The second type of instrument therefore, while giving good
resolution, was not free from other defects, and was not there-
fore equal to the monocular.
The third type of binocular, which consists of two
microscopes set at an angle to one another, both pointing at
the focal point, while quite satisfactory in its performance, is
limited to the use of low powers and requires specially mounted
and accurately adjusted pairs of object glasses.
It will be gathered from this description of the properties of
previous instruments what were the difficulties that had to be
overcome in making a really satisfactory binocular, and in the
following description of the properties of the Beck binocular we
treat each point separately, explaining how the objections have
been removed.
Eeaoiation. The resolving power of a microscope is a measure of the
fineness of detail that it will depict in the image which it forms,
quite apart from the magnifying power. The microscope must
have sufficient magnifying power to render such detail visible to
the eye, but no amount of extra magnifying power is of use
unless the resolving power is
sufficient to produce an image
containing the requisite detail.
Resolving power depends upon
the size of the cone of light
Fig. 103. - which forms each point of the
image. Suppose the lens 0
(Fig. 103) represents the object glass forming an image of the
central point of the object C at a point D in the centre of
the image ; the resolution for a given magnifying power will
depend on the diameter A B of the cone of light A D B which
forms the image ; this cone of light has an exact ratio to the
angle A C B of the light which enters the lens from each point
of the object, and it is by means of the angle A C B that
the resolving power is generally and more conveniently expressed
as numerical aperture (N.A), but it might be expressed with
reference to the angle A D B. It is evident that if the cone
of light A D B be bisected and the complete half 0 D A be
used to form the image received by one eye and the complete
half 0 D B used to form the image received by the other
eye, the cone of light forming each image is only half the size,
and the resolution or power of depicting fine detail is reduced
thereby ; thus this method of making a binocular microscope
reduces its power of resolving fine detail.
The new Beck binocular acts on a different principle. Above
the object glass is a prism shaped as shown in Fig. 104. The
whole of the light from the object glass 0 passes through the
surface of the glass B A to a surface E A, which is coated with a
semi-transparent surface of silver. This allows part of the light to
THE MICROSCOPE STAND
111
Fig. 104.
pass througli and part to be reflected into tlie second tube of the
microscope as shown by the dotted lines, thus the full size beam
goes to form each image and no lack of resolution occurs ; two
perfect pictures are produced with maxi-
mum detail, one in each eye.
It may be expected by those who
have not followed the vast improvement
that has recently taken place in optical
manufacture that the effect of light
passing through the prisms woidd injure
the quality of the image. This is not
the case ; the flat surfaces can be polished
without an error of one-millionth of an
inch, and no optical designer now hesi-
tates to make use of prisms in optical
instruments even of the most exacting
requirements.
As the transparency and reflecting power of the surface E A Equal
(Fig. 104) can be regulated according to the amount of silver that tio™
is deposited, the relative intensity of each image can be made
identical, and the right- and left-hand images are equal in brilliancy.
As to the intensity of the mental impression, it has been urged
that when an initial body of light is divided into two brilliant
parts and one part is sent into each eye of the observer, the effect
of brilliancy is the same as if the whole light be directed into one
eye only. Certainly there is some reason for this argument,
though it may be an over-statement of the case. It is, however,
no disadvantage if a slightly stronger light is required with a
binocular than a monocular microscope. The monocular ob-
server, in order to more readily concentrate his attention on the
employed eye, is apt to use an illumination that is far too brilliant,
to the detriment of his eyesight. In the use of the binocular,
both eyes are equally stimulated, and there is no temptation to
use excessive illumination, and theory goes to show that a low
illumination is more efficient for displaying fine detail.
The diagram of the binocular prism (Fig. 104) shows that the Equal
distance from the surface E A, where the beam of light is divided p^thTfor
into two portions, to the two eyepieces is not of equal length, both
the light on the right-hand side has to ^°^^^
travel a distance G H farther than the
light that passes directly through. It
would, therefore, not be possible to focus
both beams of light to the same points in
the two eyepieces ; if this were not com-
pensated, one image would be out of
focus when the other was sharp. Fig. 105 shows how placing
a plate of glass in the path of a beam of light converging to
a focus at A has the effect of extending the focus to B, and
A B
Fig. 105.
112 THE MICEOSCOPE
is a means of overcoming what would otherwise be a serious
error. It is corrected in the Beck binocular by combining
a parallel plate of glass of the required thickness with the
right-hand prism, thus equality in the focus and in the magnify-
ing power of the two images is ensured. The binocular prism is
carried in a sliding box in the body of the microscope (Fig. 102).
By sliding it out of the optic axis the microscope is converted
into a monocular instrument, or by unscrewing the knob (F,
Fig. 102) it can be slid completely out of the microscope for
cleaning or dusting. It is quite safe to remove the prism complete
in its box, as it returns with accuracy to its exact position, and
the adjustment will not be interfered with. Dust may be removed
from the prism with a camel's-hair brush or it may be carefully
wiped with a silk handkerchief or leather ; but glass should never
be touched with the fingers, a greasy smear damages the defini-
tion more than a considerable amount of dust.
The fact that when the prism moves to one side the instrument
becomes absolutely the same as a monocular microscope renders
this microscope equally useful for photography, drawing,, mi-
crometry, or any other purpose.
Many believe that eventually the binocular will be almost
universally used, but we recognise that at present this opinion
may not be shared by all, and that an opportunity of using
either monocular or binocular should be provided.
Short The construction of this binocular renders it possible to retain
le^th. ^^^ short tube of the compact monocular microscope. This
binocular body, indeed, can be fitted to most of the various
recent models of monocular microscopes. When the drawtubes
are partially extended, the tube is of the standard 160 mm.
length, the binocular microscope is thus rendered as compact
and serviceable as the monocular type. In the older types of
binocular microscopes a tube of about 9 to 10 inches in length
was required in order to extend the eyepieces to the necessary
interocular distance, but examination of the diagram (Fig. 102)
shows that, owing to the peculiar construction of the prism, the
tubes, instead of converging towards the prism, converge to
an apex about 3J inches below it ; thus, although the standard
angle of normal convergence is retained, the tubes need not
be long to give the required separation for the eyes. The
tubes converge at an angle of about 14°. This will be found
in practice to give absolute comfort for either long or short
periods of working. The eyes are in exactly the condition
required for reading a book.
Binocular telescopes which are used with the eyes looking out
horizontally at distant objects generally and correctly have their
two tubes parallel, but this is unsuitable for a microscope. The
microscopist who uses his instrument alternately with examining
objects on the table on which it stands would find it difficult
THE MICROSCOPE STAND 113
and tiring to constantly change the direction of his convergence,
such is the force of habit that the mere action of bending the head
downwards induces the convergence of the eyes necessary for
examining near objects.
Any make of object glass or eyepiece of the standard size Any make
can be used. There are absolutely no special requirements— a °^, ^^^^^*
revolving nosepiece, an objective changer, or any form of apparatus eyepiece
can be employed.
The interocular distance is varied by turning the milled head The
on the direct tube of the microscope (Fig. 108), this causes both ji'j^^e'*'
drawtubes to move in or out and alters the distance between the
oculars from 2 inches to 2J inches, which, as the observer's eyes
cannot be in contact with the eyepieces, represents interocular
distances of about 2^ inches to 2f inches. The tube length is
the standard 160 mm. at an intermediate position. For those
whose eyes are farther apart than this, tubes can be so con-
structed that they give extra separation.
If the two eyes of an observer are dissimilar, the necessary lens
to render them equal can be supplied in a cap to fit over the
eyepiece. This is a better plan than the separate focussing
adjustment provided in a binocular telescope, because to effect
an alteration in focus by means of the microscope eyepiece
requires such a large amount of motion.
The advantages of binocular vision are not only that a Binocular
stereoscopic relief can be obtained: the rest to the eyes prevents
fatigue and improves the quality of the vision ; not only is more
seen, but the perceptive faculties are much more constant. It
is frequently found that after a quarter of an hour's examina-
tion with a monocular microscope, the perception of fine detail
goes and does not return till after a pause. This does not seem
to occur with binocular vision, or at least to only a slight degree.
A further and somewhat more serious consequence of mon-
ocular vision is that the employed eye generally loses its visual
intensity of light. In order to concentrate the attention
upon the employed eye, a stronger light than is wise is often
used, and by degrees an illumination that appears white to the
unemployed eye is only grey to the other. Most microscopists
who do not force themselves to use the two eyes alternately will
find that the perception of light is less with the eye which has
been most used.
Doubt has been at times expressed as to whether a microscope stereo? cop ic
looking at an object with a single object glass can under any
circumstances give a really stereoscopic relief. Those who have
worked with a binocular microscope do not retain such a doubt, and
the explanation of the phenomenon is quite satisfactory. Suppose
that 0 (Fig. 106) represents the objective and that an object
at X consists of a fine blade of material placed on end, all the light
from the left-hand of this blade which enters the object glass at all
8
vision.
vision.
114 THE MICROSCOPE
readies the left-hand of the lens only, and from the right-hand side
of X reaches the right-hand side only. If the light from the
lens 0 is geometrically divided and passed to one eye at A, and
the other at B, a perfect stereoscopic picture will result, as though
the eyes were looking on both sides of a card held
in front of them in the well-known experiment on
binocular vision. A microscope inverts the image, and
consequently to pass the correct image to the eyes to
obtain the stereoscopic relief, the light from the right-
hand side of the object glass must be passed to the
left eye, and vice versa.
The first kind of binocular microscope described
(Fig. 99) bisected the beam of light at the back of the object glass
and passed one beam to each eye, and for long it was supposed
that unless the beam were thus divided immediately behind the
back lens of the object glass, no microscope could be made which
would give stereoscopic relief. By examining the diagram of the
rays passing through a microscope as indicated in Fig. 102, it will
be seen that the rays of light intermingle after they leave
the object glass, and at no other place between the lenses could
the right-hand half of the rays entering the object glass be
separated from the left half. It might be done for any particular
bundle like that indicated by the shaded portion, but not for all
such bundles ; a diaphragm placed, for instance, over half the
field half-way up the tube would obliterate almost all the light
from one side of the object, and allow all to pass from the other
side of the object. It would not obliterate all the rays that
enter from one side of the object glass, but would obscure half
the object.
It will, however, be noticed in Fig. 102 that all the rays of
light, after passing through the microscope, pass through a small
area called the Kamsden circle [zz') just above the eyepiece.
This circular disc is a picture formed by the eyepiece of the aper-
ture of the object glass. At this place the light may be divided
just as if it were the back of the object glass, and if in this place
a complete circular bundle of light is received from each eyepiece
of a binocular microscope it is possible, by placing suitable dia-
phragms at these points, to exclude from the right eye all light
that enters the object glass from the right-hand side of each
point on the object, and from the left-hand eye all light that
enters the object glass from the left-hand side of the object
points. Thus, two D-shaped diaphragms placed at the positions
of the Ramsden circles exclude from each eye the correct portions
of light and give the stereoscope relief with the same efficiency as
the first kind of binocular microscope, except for the loss of light.
There is, however, a practical objection to this procedure. The
proper use of the microscope is dependent on the eyes being so
placed that these discs are within the eye very near to the pupils,
THE MICROSCOPE STAND
116
Fig. 107.
and therefore such suggested diaphragms cannot be placed in the
correct positions— in fact, due to the eyelids and eyelashes of the
observer, they cannot even be placed near the correct position.
But there is another method of stopping out the portions
required to give a stereoscopic picture.
If the eyepieces be placed at a slightly
incorrect interocular distance, the pupils
of the observer's eyes cut ofi the edges of
the two Ramsden discs (Fig. 107), and
as the stereoscopic effect with a high-
power object glass is generally exag-
gerated, a very small movement is
sufficient to give perfect depth of vision.
The tubes of the new microscope
are, however, inclined, and there is no
necessity to vary the interocular dis-
tance. The observer naturally * places
his eyes so that the whole of the Ramsden
discs (Fig. 107) enter the pupils of the eyes,
and obtains all the advantages as to aperture, resolution, and illu-
mination of a monocular microscope. Then, by moving his head
either forward or backward, he cuts off with his pupils the one
or other side of the Ramsden discs and obtains either stereoscopic
or pseudoscopic relief instantly. The movement required is scarcely
over an eighth of an inch, and the result is that all the advantages
of stereoscopic relief are obtained without sacrificing anything.
The result of the movement of the head is very astonishing :
if objects are being examined which lie on different levels, one
point appears either in front of or behind another at will, and the
position of the observer's head indicates which is the stereoscopic
or pseudoscopic picture.
The Beck high-power binocular body can be suppHed on any
of the microscopes illustrated, either in place of the ordinary
body or as an extra interchangeable body.
Metallurgical microscopes require certain special features Metai-
because almost all objects for which they are used require mfSoscopes.
illumination from above. A great deal of their examination
is done with high powers with one or other of the vertical illu-
minators mentioned on page 41. It is, therefore, important that
the beam of light for the use of these illuminators, having once
been adjusted, should be allowed to remain in a fixed position.
If the body tube of the microscope to which these illuminators
are attached is focussed up and down to examine specimens of
different thickness or to enable different object glasses to be used,
the illuminator cannot be kept opposite to the illuminating beam
of light. Metallurgical microscopes must, therefore, be made in
a manner that will overcome this difficulty. The three following
forms of microscopes show three methods of accomplishing this.
Fig. 108. — The Binocular High- and Low-power Microscope as
appUed to Stand No. 3213.
116
Fig. 109. No. 3227, Metallurgical Microscope, with rack and pioion
focussing stage, object glass, and vertical illuminator.
117
118
THE MICEOSCOPE
The Beck-Kowley Metalliirgical Attacliment (Fig. 110)converts the
Standard London Microscope into a metallurgical instrument by the
use of prisms. The Standard Metallurgical Microscope (Fig. 109)
is a model of the Standard London Microscope in which the stage of
the microscope is not fixed to the limb of the instrument, but is
carried in a strong slide, and can be focussed up and down by
means of a rack and
pinion, so that the
focussing can be done
by the stage and not
by the body. It does
not detract from the
performance of the in-
strument for other pur-
poses, and when racked
up to the correct posi-
tion will work with the
mechanical stage of the
standard microscope.
It can also be supplied
with any of the standard
substages, although for
purely metallurgical
purposes a substage is
not required.
The third method
(Fig. Ill) has an elec-
tric light fixed to
the body tube of the
microscope which moves
up and down with the
illuminator as it is
focussed.
The Beck - Rowley
Metallurgical Attach-
ment converts an
ordinary microscope
into an efficient metal-
lurgical instrument
The attachment may be readily attached or removed without
any alteration to the microscope.
With this illuminator the light is projected along the tilting
axis of the microscope, and from thence by means of prisms into
the vertical illuminator ; when this method is employed the
microscope tube can be racked up and down for focussing in the
ordinary way, and the inclination of the microscope can be
effected without in any way interfering with the original accuracy
of iUumination.
The Beck-
Rowley
Metal-
Khment.FlG. llO.-No. 3225 ^^ . ^ ^ ^
Microscope, and Metallurgical Attach-
ment.
Standard London
THE MICROSCOPE STAND 119
It will be seen from the illustration tliat the attachment con-
sists of two pieces, one screwing on to the body-tube of the
microscope, and including the vertical illuminator (D), and the
other fitting into the standard substage or understage and held
in place by the screw H.
Light from any desired source is projected into the prism A
and reflected into the prism B, and from thence to the prism
C and again into the vertical illuminator, where the light is
reflected, by the thin glass reflector D, downwards through the
object glass to the metal surface to be examined.
A removable lens is fitted at G which can focus the iris dia-
phragm (F) upon the object and enables all extraneous light to
be cut ofi; a holder (E) for light filters and ground glass is
placed immediately behind the iris diaphragm and enables the
diaphragm to be used as the light source. The iris diaphragm and
ground glass can be moved so that it can be focussed upon the
object, thus giving so-called " critical" illumination.
The reflector in the vertical illuminator is readily removed for
cleaning or replacement. A thin glass and a thicker parallel
glass, a green glass and a ground glass are supplied with each
instrument.
For geology and mineralogy the illuminator will also be found
of value in the examination of polished specimens of ores and rocks.
The fact that objectives of different powers can be used and
focussed without interfering with the adjustment of the light is
of special importance in the examination of opaque metalliferous
minerals.
The bench metallurgical microscope (Fig. Ill) has no pillar and The bench
base. It has a limb carrying the body with the usual coarse and fine ^r^!^i
adjustments fixed to a large square stage. This stage is carried on microscope.
four levelling screws one at each corner of the stage. The micro-
scope can be stood upon a table or bench and used in the ordinary
way with specimens placed on the stage, or it may be placed on a
large metal or other surface, and the surface examined by focussing
the object glass down through the aperture in the stage. To render
this microscope convenient for metallurgical work a metal filament
electric lamp is attached to the illuminator and is provided with a
pair of light-tight tubular covers. It moves up and down as the
microscope is focussed, thus allowing the instrument to be focussed
without interfering with the illumination. The prism illuminator
is supplied with this microscope because it is more suitable for
low powers and almost equally good for high powers. This in-
strument is useful for many other purposes, including the Brinnel
test, in which case a scale is fitted into the eyepiece. The electric
filament lamp can be used on any voltage from 100 to 250 volts,
and on direct or alternating currents. It is troublesome to use a
lamp of low voltage which requires accumulators, but for those
who have 6- or 12-volt accumulators suitable lamps can be supplied.
120
THE MICROSCOPE
In the tubular portion connecting the lamp to the illuminator there
are two slots into which colour screens, ground glass, or a focussing
lens can be dropped, and a ground glass, a green glass, and a
lens are supplied with the microscope for the purpose.
Fig. 111. — No. 3226, Metallurgical Bench Microscope with prism
illuminator and electric lamp and fittings.
Pefcroiogicai A pctrological microscopc is essentially an ordinary microscope
microscopes. pjQyj(jg(j ^j^j^^ ^ number of special adjustments and appliances
for the study of rocks and crystals. The most important of these
additions are a polarising apparatus and a rotating stage. A
polarising apparatus consists of a Nicol prism made of Iceland
spar which must be placed below the object to be examined and
a similar but somewhat smaller prism which must be placed
above the object. At least one of the prisms and the object
must be capable of rotation, and the amount of the rotation
determined on a scale. There must be a means of rapidly throwing
THE MICROSCOPE STAND 121
out of the axis one, or preferably both prisms, so that an
immediate change from polarised to ordinary light may be made.
The eyepiece must be provided with cross lines for measuring
the angles of crystals by setting first one and then the other
edge against one of the lines and measuring the angle of rotation
of the stage required to effect such setting. As the various
object glasses and their mountings are never perfectly inter-
changeable, a rotating stage must be provided with centring
adjustments, so that the axis of rotation can be made to exactly
coincide with the optic axis. The Sloan object glass changer
described on page 20 is a very useful appliance for adjusting
individual object glasses, and is far preferable to a double or
triple nosepiece for rapidly changing them. The prisms should
be provided with spring clips so that as they are rotated
the positions when the prisms are " crossed " may be felt. The
lower prism, called the polarising prism, should be large enough
to enable the back of the condenser or the object to be fully
illuminated, but its size is determined to some extent on the supply
of Iceland spar, which cannot always be obtained in large crystals.
The upper prism, called the analysing prism, need not be so large,
and may be fitted in one of three positions — either immediately
over the object glass, in the interior between the two lenses of the
eyepiece, or over the top of the eyepiece. If it is immediately
over the object glass it makes a slight change in the exact focus
of the microscope when pushed in and out unless furnished with
a compensating lens or block of glass, which is seldom fitted, as
most observers object to the introduction of an extra optical
element, when there is no real necessity. With the analyser in
this position the use of a quartz wedge is less convenient than in
the other two forms. In both the other forms the quartz wedge
fits through a slot which is in the focus of the upper lens of the
eyepiece. The analysing prism in the interior of the eyepiece
is probably the most convenient form, because if fitted above the
eyepiece its considerable thickness prevents the eye from being
placed in the eyepoint of the microscope (see Fig. 1, page 9)
and seriously restricts the field of view.
The interference rings and brushes of crystals are formed if
a wide-angle cone of light be made to pass through the object
by a small, specially made substage condenser and if this wide-
angle cone of light be collected by a wide-angle object glass. A
1/6-inch (4-mm.) is generally used for this purpose. The image
of these rings and brushes is formed at the back focus of the
object glass very close to the back lens of the latter, and there are
three methods of observing them. In the microscope in which
the analysing prism is immediately over the object glass, the eye-
piece may be taken out and the eye placed two or three inches
away from the upper end of the tube of the microscope. The
image will then be seen, but it will be very small. If the analysing
122 THE MICROSCOPE
prism is not in this position this method is not permissible, because
removing the eyepiece removes the analysing prism, which is an
essential to the image being formed. When an eyepiece analysing
prism is used, a lens known as a Bertrand lens may be screwed into
the lower end of the drawtube or placed into the body through
a special slot made for the purpose, and the drawtube pushed up
and down until the image is clearly focussed. The Bertrand
lens converts the drawtube into a low-power microscope which is
focussed to give a sharp image of the back focal plane of the object
glass, and a magnified image is obtained. This method sufiers
from the inconvenience that the drawtube must generally be
removed to put in the Bertrand lens, and that it is troublesome
to use a sliding drawtube in a petrological microscope, as it may
interfere with the accuracy of the crossed position of the prisms.
The best method of observing the rings and brushes is by means
of a small microscope called a Becke lens, which fits on to the
top of the eyepiece and gives a highly magnified image of the
eyepoint or Ramsden circle of the microscope (Fig. 1, page 9).
The image of the rings and brushes is, as previously mentioned,
in the back focal plane of the object glass, but this is reproduced
by the eyepiece in the eyepoint, and it may be examined equally
well in this position. The use of the Becke lens does not interfere
with any of the adjustments of the instrument, and is to be
preferred to any other plan.
Petrological microscopes cannot be thoroughly explained
without considerable discussion of the theory of polarised light,
which is not attempted in this book. There are excellent books
on Petrology to which the student is referred, and to whom
the following technical description of a petrological microscope
will then appeal,
standard The Standard London Petrological Microscope is made in two
m1cro3c?pe! ^^rms (Nos. 3222 and 3223). Both forms have the rack and
pinion spiral coarse adjustment and the double- speed fine adjust-
ment of the Standard London Microscopes ; both have a circular
rotating stage divided in degrees and cross-finder divisions on
the surface with centring screws to set the axis of rotation in the
optic axis ; they both have cross-wires to the eyepieces, a polariser
in a swing-out fitting below the stage, and a wide-angle series
of converging lenses in a sliding fitting in the stage. This
condenser can also be fitted in an independent swing-out and
focussing arm, which enables the condenser to be thrown out of
the optic axis in a manner similar to the polariser. The polariser
is provided with spring clicks at positions of crossed prisms and
lines at parallel positions. No. 3222 has an analyser in a push-
out fitting above the object glass at the lower end of the body.
Below this is a slot covered by a revolving tube when not in use,
for the insertion of mica or quartz plates. A Becke lens slides
over the eyepiece for examining the rings and brushes of crystals.
No. 3223.
Fig. 112.
No. 3222, Petrological Microscope Nosepiece Analyser.
No. 3223, Petrological Microscope Eyepiece Analyser.
123
124 THE MICROSCOPE
No. 3223 has the analysing prism in a revolving fitting within the
eyepiece. It is a form of the Abbe prism, devised by Mr. E. M.
Nelson, which pushes in and out of position. Its great advantage
is that it gives the full field of view. Its only disadvantage is
that in certain circumstances a faint second image of the cross-lines
can be observed, but this is of no practical disadvantage. It is
provided with spring clicks at positions of crossed Nicols and lines
at parallel positions. Below the analysing prism a slot is pro-
vided for the insertion of a quartz wedge or mica plate. The top
lens of the eyepiece is provided with an adjustment for focussing
to either the quartz wedge or the cross- wires, and a Becke lens is
provided, fitting over the eyepiece, for examining the rings and
brushes of crystals.
The whole eyepiece pushes into the drawtube with a pin
fitting into a slot so that the position of crossed Nicols may be
correct when the prisms are set in their clicked position. On
either side of the clicked position a line is marked on the flange
of the eyepiece which is 2J° away from the true position for the
total extinction. By setting the analyser to these positions a
better determination of the extinction can sometimes be obtained.
A shutter with a series of apertures is provided which can be
introduced into the field of view to cut ofi all parts of the field
except the centre. A slot is provided at the lower end of the
polariser fitting for the insertion of a plate with a fine aperture
and a slit, for the testing of refractive index by the Becke shadow
test.
Oircuiar ^ A circular mechanical stage (page 52) can be fitted to either of
the above instruments, and all apparatus of standard microscopes
can be supplied, but the substa.ge apparatus is supplied in slightly
longer mounts to accommodate for the extra thickness of the
mechanical stage.
mechanical
soage.
CHAPTER VII
THE MICROSCOPE AS A RECREATION
Science owes more to the discoveries made with the microscope
than to those made with any other instrument, but it is not
always appreciated what a fund of enjoyment is available to all
by making use of the addition to one's eyesight that the microscope
affords. The reader may have met an enthusiast who devotes
hours at a time to gazing down the tube of this instrument, and
have wondered what could so engross his attention. If questioned ,
such an enthusiast might have explained that in the stagnant
ponds and ditches he had discovered numbers of curious and
amazing animals — creatures that had been unobserved for thou-
sands of years because they were small — creatures more varied
than the inmates of the Zoological Gardens, and of types of
astonishing originality and beauty.
A visit to a weedy pond with a few bottles, the collection of
some of the water, weed, and mud, and their examination under
the microscope will be convincing proof that the enthusiast was
correct.
For some time an observer may be content to watch these
new-found animalcula and wonder at their curious diversity of
appearance, but the time will probably arrive when he will
desire to know more of their habits ; he will then discover that
during the last sixty or seventy years books have been written
about them. The first glance at such books may fill him with
dismay ; they are filled with long words and terrible names, and
it would almost appear that a new language has been evolved
to describe these minute creatures.
Further examination, however, will show that the terminology
is but a thin veneer and that a method is discernible in the
apparent madness of these writers. They state that they have
discovered a history of existence, which they call development,
which shows how in the ages that have gone, great and complex
animals— perhaps man himself — have grown from simple and
minute beginnings. The more enterprising of these simple forms
have, they say, from time to time, altered their characteristics
and grown through gradual stages to more complex forms. Some
have advanced while others have remained in their original con-
125
126
THE MICROSCOPE
Fig. 113.—
Amoeba.
dition. These steps have not been obliterated, and amongst
the denizens of our ponds and ditches are to be found specimens
of many of these early phases of life — specimens so nearly alike
that it is quite possible to follow the lines along which one form
has developed into another. Such a concep-
tion leads one to examine the ponds and
ditches with a connected idea.
The class of creatures which represents this
least complex form of existence is called
Protozoa, quite as simple a name as kangaroo
when you become accustomed to it, and to
those who remember their classics a much more
descriptive one.
In almost any pond with weed, a careful search will produce
a creature called an Amoeba, which is the least elaborate piece
of living animal matter known. One calls it a piece of living
matter, for it is nothing more than a morsel of jelly, which changes
its shape every minute. This jelly has no case or skin, but, as
it does not dissolve, it remains separate from the water like a
bubble of oil. It can move its contents to one end of itself,
thus increasing for the time being that end and diminishing the
other, and so it flows about in any direction, altering its shape
to an indefinite extent, forming itself either into a long projection
as a tiny trickling stream, swelling out into circular knobs, or
doing both at the same time. In this way it slowly moves about
without, so far as can be seen, any fixed intention ; and, as the jelly
of which it is made is filled with fine particles, the flowing of the
fluid creature can be easily watched. Besides these tiny particles
there are much larger things rolling about within its substance.
These are often recognisable as shells of diatoms and of other
tiny creatures that are to be met with alive swimming about
in the neighbourhood of the Amoeba. If the Amoeba be care-
fully watched, it will be seen that when it comes across something
which appears suitable it begins to pour itself out in three or
four streams all around the desired object,
and these streams, as they meet round the
victim, join together. The object thus
caught and enclosed remains in the jelly,
where it is slowlv dissolved.
The Amoeba feeds by literally putting
itself outside its food. When the victim
has been dissolved, the hard and insoluble
parts are allowed to escape back into the
water, and the portion that is assimi-
lated goes to increase the size of the jelly. It is not, how-
ever, correct to say that this creature consists of nothing but
the granular jelly filled with the remains of the things it has
absorbed. It has two primitive organs — one, a small spot of
Fig. 114.— Villous
Amceba.
THE MICROSCOPE AS A RECREATION
127
Fig. 115.~Difflugia.
somewhat darker and harder material, is always present and is
essential to life. What part it plays is unknown ; it appears
to be a kind of vital spark, and is called the Nucleus. The other
organ is notliing more or less than a good-sized bubble, called
the Vacuole.
The Amoeba, the simplest form
of animal that exists, is so colour-
less and so transparent that every-
thing going on in its interior is
visible. Its structure can be
understood at a glance, and start-
ing from this simple form we can
find creatures varying from each
other but slightly, which show
step by step an almost com-
plete series of stages of development up to elaborate organisms.
For instance, there is one species of Amoeba which has one
end of its body hardened into an unchanging shape — just one
corner only around which some of the jelly has hardened up at the
edge, showing the commencement of the development of a
covering, while the rest of the creature is exactly like its simpler
brother, having, with the exception of this little corner, no fixed
shape, but pouring about as before.
The next shape is reached in the Difflugia. It is an Amoeba
and possesses the same curious means of engulfing food ; but when
in the course of its meals it gets outside pieces of sand or similar
indigestible material, it retains them, fixing them around the surface
of its body until a cap is formed and only a small portion of the
jelly is left free. These particles are cemented together with
some of the hardened jelly, and form a rough shell in the shape of
an egg with one end broken off. From this open end the creature
flows in irregular projections of jelly to catch food, and crawls
about carrying the shell on its back.
It seems to have a power of selection
as to the size and shape of the grains
that will form a satisfactory shell,
and, although there is not a perfect
regularity in its construction, it is
evidently not left entirely to chance.
A further development in the
direction of producing a protective
covering is shown in the beautiful
Heliozoa. In this case a spherical
shell is deposited, perforated with
tiny holes, through which fine rays of jelly exude in the
form of delicate filaments. Here the shell is not built up
of pieces of sand, but is probably formed of the products of
digestion.
Fig. 116. — Heliozoa.
128
THE MICROSCOPE
Fig. 117. — ^Foraminifera.
The Foraminifera are from a structural point of view similar
to the Heliozoa, being morsels of jelly having the power of forming
round themselves shells of chalk
extracted from their food and the
water in which they live. These
shells take myriads of different
forms, but have one thing in com-
mon : they are perforated with
multitudinous holes through which
slender threads of jelly exude. To
this family belong the shells which
form chalk. Innumerable numbers
of these tiny creatures fall, as they
die, to the bottom of the ocean,
forming there, in the course of ages,
a layer of chalk which may later
be raised by volcanic action above the sea-level.
Such examples illustrate the gradual development of a shell,
the creature in every other respect retaining its original simplicity.
We can now trace development in a different direction
leading to more complex creatures endowed with
locomotion. The jelly or protoplasm of which the
living animal is formed appears to slightly harden
all round its borders, and a creature of a more or
less definite shape is produced, still very elastic
and capable of retracting or extending itself to per-
haps three times its normal length.
It has a somewhat pointed end, and the margin fig. 118.
of its body is still sufficiently soft to enable it to Trypano-
feed by absorbing into its substance through any some,
portion of the surface small particles of food, but
it cannot get outside such large things as the Amoeba. This
is the creature which, if it finds its way into the blood of
animals or men, causes in one case the tzetze-fly disease and in
the other the dread sleeping-sickness, and it is known as the
Trypanosome.
A further stage shows the de-
velopment of a fiagellum, or whip,
which is formed by the drying up
and hardening of the pointed end of
the body. The fiagellum vibrates, and
by its aid the creature can swim
about with considerable rapidity. In-
numerable forms of these Flagellata
are found in all decaying matter, and
their activity is surprising. Some of
them have further extended their cell wall into a sucker, by
which they attach themselves to some fixed object, and whole
Fig. 119.
-Flagellata.
THE MICROSCOPE AS A RECREATION 129
colonies of such Monads, as they are called, are to be found on
weeds, ceaselessly lashing the water with their flagelia, causing
a current which brings particles of food within their reach.
Fig. 120.—
Monad.
Fig. 121.— Collarerl
Monad,
Fig. 122.— Collared
Monad in Shells.
A further elaboration of this cell wall is found in the Collared
Monads, which are possessed of a transparent cup made from an
extension of the hardened margin of their body. In the centre of
this the flagellum vibrates, bringing a steady flow of water into
this cup or collar. This is the simplest form, but in a more
complicated one these Collared Monads have provided themselves
with transparent shells of most elegant forms, to the bottom of
which they anchor themselves. They retreat right into them
for protection from danger, but are found extended when engaged
in finding their food.
Thus a series of creatures are met with which possess a shell
of the same simple type, consisting of nothing but a piece of jelly
with a nucleus and a bubble, but showing great diversity of^form
as regards the struc-
ture of the wall of the
cell in which the jelly
is contained.
The development of
a single Flagellum has
been traced, but now
we come to the Ciliata,
which have rows of hairs.
If we imagine the soft,
jelly-like exudations of
the Heliozoa to be
hardened and given a
vibratile motion, we have the simplest form of Ciliate, just a tiny
ball with rapidly vibrating hairs all over it, these giving it a con-
tinuously rolling movement. Myriads of such creatures in different
forms exist, some briUiantly coloured, some perfectly transparent.
Fig. 123.— Ciliata.
Fig. 124.—
Vorticellae.
130
THE MICKOSCOPE
c
Fig. 125.—
Stentor.
The Vorticellse are a particularly lovely family resembling
groups of dainty lilies. They have a circle of vibrating hairs
around the mouth of a bell-shaped body, and are anchored down
by a long stalk. If there is a sudden shock and they are alarmed,
the stems shut down like corkscrews, and down they go in a
flash, taking refuge till the danger is over, and
coming out slowly and carefully a few moments
later.
A somewhat similar species, the Stentor, has
a horn-shaped body, with a powerful ring of
hairs around its upper surface. It is a most
voracious animal and eats almost anything that
is brought to it by the strong current of water
which its vibrating hairs set up. One species
somewhat like the Stentor has a brown shell in
which it lives. This is fitted with a trap-door
attached to the body of the creature in such a
manner that when it retreats into its cell or case
the shell closes like the nest of a trap-door spider.
The Protozoa, therefore, display in their living representa-
tives what looks like a fairly complete history of their original
development as regards external structure, indicating the creation
of a shell or covering, and the creation of cilia and of swim-
naing apparatus. It is now interesting to examine the question
of their feeding from the same point of view.
The Amoeba simply pours itself round and engulfs any object
it meets and wishes to feed upon, the object being then gradually
dissolved. Some of its constituents mix with the jelly and
are absorbed, and the creature gradually increases in bulk until,
being too large for comfort or convenience — if such terms can be
applied to such a primitive creature — it splits itself into two parts,
each of which is a perfect animal.
Those portions of the food which are insoluble
are allowed to escape from the jelly, but there are
other portions which, although dissolved, are not
suitable or required for nourishment and growth.
Water is also taken in with the food particles. It
would not do for the Amoeba to be constantly
filling itself up with useless material, neither would
it be satisfactory for it to be continuously diluting -p^^ ^^q
itself. ^ Trap - door
Some means must be found to get rid of these Animalcule,
waste products, and the means employed are ex-
tremely simple. The water and the unnecessary products of
the dissolving process form into a bubble, and as soon as such
a bubble approaches conveniently near to the surface of the
animal, it bursts, discharging its contents into the surrounding
water. This is certainly the simplest form of digestion that
THE MICROSCOPE AS A RECREATION 131
can be imagined, but it fulfils all the necessary functions, and,
moreover, the constant introduction of water into different parts
of the jelly tends to supply the necessary oxygen to keep the
animal healthy.
This Contractile Vacuole, as the bubble is named, is the earliest
germ of both a digestive and a respiratory system, and we shall
now see how from this simple commencement a gradual growth
in complexity can be traced.
The bubble of the Amoeba forms at any convenient position
within its substance, and in some of the Protozoa several of such
bubbles form ; but the second development is to be found in those
allied creatures in which the bubble is always in the same place
in the same species, and the water drains from the rest of the body
to that spot.
In the next stage the bubble is no longer a perfect sphere,
but has one or more extensions, until in the final stage there are
minute channels all over the creature which communicate with
the main bubble, thus creating a complex drainage system, and
this is as far as digestion is developed in the Protozoa.
The development of a mouth presents features of equal
interest.
The Amoeba has no mouth — it does not eat, it engulfs.
The Difflugia is similar, but its area of action is reduced by
the fact that a large portion of its body is enclosed in a shell.
The Heliozoa also have no special feeding organ. When an
object becomes entangled in their fine filaments of jelly, it may,
if very small, find its way for digestion into the interior of the
shell, but as likely as not the fine rays may join up around the
object outside the main body, and digestion will proceed
there just as weU as in the interior.
There is another series of animals called the Acineta, which
somewhat resemble the Heliozoa in that they are provided with
long, ray-like projections which are hard
or leathery except at the tips. Here they
swell out into small knobs of soft material,
through which small portions of food can
be taken in. Such organisms may be con-
sidered as having hundreds of mouths.
Another series shows a much more
direct development. On these the skin is
hard except in patches, where alone food
can enter; and around these soft patches Fig. 127.— Acineta.
there is usually a ring of rapidly vibra-
ting hairs, which create a current, bringing the floating
particles into contact with the absorbent portion or portions of
the body. The Vorticellse (Fig. 124) have a disc-shaped ab-
sorbent surface surrounded by the strong ring of vibrating hairs
The Stentor (Fig. 125), which is larger and a most voracioua
132
THE MICROSCOPE
creature, has also a large disc-like surface where there is prac-
tically no skin to its jelly. The cilia which vibrate around this
disc cause a powerful current to flow, and it is amazing to watch
the smaller kinds of Protozoa being hustled into the creature's
body, where they swim about for a second or two and are then
still.
As a final stage we find creatures with only one small open-
ing where food can be absorbed, and the complete development
of the mouth is here concluded.
One of the earlier figures shows the dainty little Collared
Monad, which consists of a single cell with a vibrating whip, or
flagellum, and a very perfect little cup or collar of transparent
material. They are often found in large colonies on the surface
of weeds. To them we owe our sponges. A microscopic examina-
tion of one of the holes of a growing sponge reveals a colony of
these little organisms, closely arranged aU round the interior
of its surface. These have the power of creating instead of
shells a fibrous material, which forms
the matrix in which they are embedded.
In place of shells they deposit hard, flinty
spines in the substance of this matrix,
and these are called spicules. Thus a
sponge consists of myriads of colonies of
Collared Monads, their vibrating flagellae
causing a current of water to rush through
every cavity of the entire sponge, in order
to provide the food and oxygen necessary
for the support of the community.
The Protozoa show in a series of inter-
esting stages the gradual development of creatures of one cell.
Each cell is complete in itself, though, as in the case of the
sponge, an approach to a more elaborate form is seen. Never-
theless, here each organ eats for itself, breathes for itself, re-
produces itself by splitting in half, and is an individual.
Later stages of development show creatures of more than
one cell, in which some cells perform one function and some
another, and none are complete by themselves ; and the develop-
ment of the simplest form of life into a more complex animalcule
as indicated by a study of the Protozoa is but an indication of
the interest that can be obtained by the use of the microscope.
The more elaborate and highly organised creatures met with
in water have equal charm and variety. The manner in which
they feed upon each other, the manner in which some become
parasites, and the methods of reproduction, are all subjects which
well repay investigation. The development of many of the animal-
cula from the egg to the finished and perfect creature has a special
fascination, because naturalists have discovered that in this
change from stage to stage which certain forms go through
Fig. 128.— Sponge.
Fig. 129. — Holopediura.
THE MICROSCOPE AS A RECREATION 133
there is a history in an abbreviated form of the stages througli
which the species originally developed. The so-called water-
fleas, for instance, are little crustaceans Hke small shrimps.
They are hatched out
from eggs as small oval
bodies with short legs, and
very little else except one
eye. After a time the
young creature casts off
its skin and becomes rather
more elaborate in form.
This goes on stage by stage
till it develops into a
creature with the most
complete series of legs,
antennae, tail, and other
appendages. It has as-
sumed the appearance of a small shrimp. In some species it
goes further, and after having for a short time lived a free and
energetic life it develops into nothing but a bag and suckers,
which attach themselves to fishes and suck their nutriment from
the fish's body.
This points to a degeneration in the development which has
taken place in the history of a race who found it less fatiguing,
if less honourable, to live on other people rather than to fight
their own battle in life.
These small Crustacea, generally known as water-fleas, are
one of the chief foods of fish, both salt
and fresh water. They exist in such
enormous numbers in some parts that
they even satisfy the appetite of the
whale. The sea is sometimes of a blood-
red colour due to the myriads of a
coloured form of these creatures. There
is no pond that has not many varieties,
and they can be best captured with a
collecting net. Certain forms are phos-
phorescent, but all are more or less trans-
parent, and can be thoroughly investi-
gated under the microscope. Fig. 129
shows one form found in the lakes of
Cumberland, which is supposed to be a
delicacy beloved by the salmon trout and
the char. This curious species is em-
bedded in an envelope of jelly much
larger than itself. It is quite transparent. The rolling of its
single eye, the beating of its heart, and the digestion of its food,
can all be watched under quite a low-power object glass.
I
Fig. 130
trepes.
Bytho-
134
THE MICKOSCOPE
Another form with a tail like a long spine and an eye that
fills most of its head is shown in Fig. 130.
The study of the development of the cell structure in vegetable
life is equally fascinating — how cells which in their simplest
forms, having all similar functions, group themselves together
into colonies. Some of the constituents take on certain functions
only, leaving others to accomplish difierent work, until a complex
vegetable growth is built up of cells, all of which have their own
characteristics.
The circulation of the sap in plants can be readily observed.
The breathing apparatus of plants where they absorb carbonic
acid and liberate oxygen can be^ found on the
under-surface of most leaves. The hairs of plants
form a study in themselves. Fig. 131 shows the
hair of the stinging nettle : on the left it is in its
undamaged condition. It has a knob on the end,
and a closed canal can be seen running up the
centre. A light touch knocks off the knob, leaving
a sharp pointed end which will pierce the sldn;
and the canal being opened by the removal of the
knob, the poison that it contains can enter the
prick made by its sharp point.
The seeds and pollen of plants are wonderful
in the elegance of their design and the variety
of their structure.
The spore cases of ferns, with their apparatus like tiny spiral
springs for hurling the spores to a distance when ripe, can be
found as brown patches on the under-surfaces of the fronds.
Perhaps nothing will create more amusement and interest
than the examination of the contents of an open umbrella after
it has been held under the bushes, on a hot summer day, while
the bushes are lightly beaten with a stick. No one could have
imagined what a variety of tiny microscopic insects exist of which
most people are entirely unaware. The eyes, legs, wings, pro-
boscis, and other parts of the insects should be examined, and
the habits of the voracious little creatures will surprise even the
naturalist who is used to the curious manners and customs of
the larger animals.
These few notes on the employment of the microscope for
the less serious subjects than those from which it is a necessary
as a scientific tool, do not do more than indicate a few directions
in which enjoyment can be obtained from its use.
FiQ. 131.—
Nettle Hair.
17 0 0
PRICE OF INSTRUMENTS AND APPARATUS
DESCRIBED IN PREVIOUS PAGES (1922)
MICROSCOPE STANDS
No. Page £ 3. d.
3210 97 Standard London Microscope, with plain sub-
stage, with iris diaphragm; stand only, in
case . . . , . . . 10 10 0
3211 93 Standard London Microscope, with spiral screw
focussing substage ; stand only, in case . 11 15 0
3212 100 Standard London Microscope, with rack and
pinion substage ; stand only, in case . . 13 10 0
3213 99 Standard London Microscope, with rack
and pinion centring focussing substage ; stand
only, in case . . . . . . 14 10 0
3214 104 Standard London Microscope, with standard
body, circular rotating stage, screw focussing
substage ; stand only, in case . . .14 5 0
3215 104 Standard London Microscope, with standard
body circular rotating stage, rack and pinion
focussing and centring substage ; stand only,
in case .......
3216 103 Standard London Microscope, with circular
rotating stage, large 2-inch body, rack and
pinion focussing and centring substage ;
stand only, in case . . . . . 19 0 0
3217 103 Standard London Microscope, with circular
rotating stage, with double extension rack
and pinion drawtubes, rack and pinion
focussing and centring substage ; stand only,
in case . . . . . • . 23 0 0
— 116 Binocular body in place in ordinary body on
Stands 18 0 0
3221 101 Standard London Portable Microscope, with
rack and pinion focussing and centring
swing-out substage and mechanical stage, in
case .......
3227 117 Standard London Metalliu-gical Microscope;
stand only 15 10 0
3225 118 Standard Microscope, with Rowley metallur-
gical attachment ; stand only, in case . 23 2 0
3226 120 Metallurgical Bench Microscope Stand, without
case, but with vertical prism illuminator
lamp, with cover tubes and fittings complete
for plugging into 220- volt or 100- volt circuit. 13 10 0
3222 123 Standard Petrological Microscope, with nose-
piece analyser, eyepiece with cross-wires,
polarising and analysing prisms, converging
system of lenses, Becke lens, mica J-wave
plate, and Klein's quartz plate ; stand only,
in case ...••••
135
28 0 0
29 0 0
136
THE MICKOSCOPE
No. Page
3223 123
— 122
3352 122
3351
122
3350
122
3218
116
116
3219
103
3219a
/103
\ 52
1103
" \104
3200a 104
3201a 104
— 116
— 103
Standard Petrological Microscope, with eye-
piece analyser, with cross-wires, polarising
prism ; stand only, in case ....
Independent swing -out and focussing arm,
which enables the condenser to be swung out
of the axis .... extra
Becke lens and converging system of lenses,
in fitting in stage-plate with small apertures
for Becke shadow test to fit under polariser,
and plate with apertures of various sizes
for limiting the field to fit into eyepiece slot .
Plain quartz wedge, ungraduated .
Quartz wedge, cemented on gypsum plate (red
1st order), graduated in Retardations .
High-power Binocular Micioseope, with square
stage and rack and pinion focussing and
centring substage ; stand only, in case
Stand as No. 3218, but with detachable
mechanical stage .....
High Binocular Microscope, with plain circular
lotating stage and centring adjustments ;
stand only, in case .....
Microscope as No. 3219, with addition of
circular rotating mechanical stage ; stand
onlj', in case ......
Interchangeable extra monocular large body,
with rack and pinion drawtubes extending
to 260 mm. ......
Massive Model Microscope, with plain, square
stage and attachable mechanical stage, as
on page 99, Abbe condenser, dark-ground
illuminator in interchangeable mounts, and
one extra substage slide ; in case
Massive Model Microscope, with mechanical
stage as illustiated on page 105, with dry and
immersion achromatic condenser and focuss-
ing dark ground illuminator and one extra
substage slide ; in case ....
Intel changeable high-power binocular body
extra
Double extension drawtube, with rack and
pinion adjustment . . . extra
£ s. d.
24 7 0
1 5 0
3 15 6
2 15 0
4 10 0
32 10 0
38 10 0
35 0 0
47 0 0
7 0 0
44 10 0
3300
3301
3280
3281
3282
3230
3231
3232
3234
3236
NOSEPIECE AND OBJECT GLASS CHANGERS
20 Dust- tight double nosepiece ....
20 Dust- tight triple nosepiece ....
20 Sloan object glass changer, adapter, spanner,
and two fittings .....
20 Extra fittings for above . . . each
20 Case to hold three fittings, with object glasses
attached in dust-tight spring holders .
OBJECT GLASSES AND EYEPIECES
77 li-in. Achromatic object glass (32 mm.) .
77 2/3-in. Achromatic object glass (16 mm.)
77 1/3-in. Achromatic object glass (8 mm.) ♦
77 1/6-in. Achromatic object glass (4 mm.) .
77 1/8-in. Achromatic object glass (3 mm.), Oil-
immersion ......
70
2
6
20
0
0
5
0
0
S
1
1
7
10
6
0
1
0
7
5
6
0
0 11 6
2 5 0
1 10 0
4 5 0
3 15 0
6 17 6
PRICE OF INSTRUMENTS AND APPARATUS 137
No.
Page
3235
77
3240
77
3241
77
3241a
77
3242
77
3244
77
3245
77
3248
77
3260
88
3261
88
3262
88
3266
88
3267
88
3268
88
3269
88
3270
88
3275
68
3263
73
3264
73
3273
73
1/12-in. Achromatic object glass (2 ram.), Oil-
immersion .....
li-in. Apochromatic object glass (40 mm.)
2/3-in. Apochromatic object glass (16 mm.)
2/3-in. Apochromatic object glass (14 mm.)
1/3-in. Apochromatic object glass (8 mm.)
1/6-in. Apochromatic object glass (4 mm.'i
1/6-in. Apochromatic object glass (4 mm.), with
correction collar ....
1/12-in. Apochromatic object glass (2 mm.), Oil
imm.ersion .....
42-mm. x 6 Huyghenian eyepiece .
25-mm. x 10 Huyghenian eyepiece
17-mm. X 15 Huyghenian eyepiece
45-mm. X 6 Compensating eyepiece
30-mm. X 8 Compensating eyejjiece
22-mm. X 11 Compensating eyepiece
15-mm. X 17 Compensating eyepiece
10-mm. X 25 Compensating eyepiece
Beck micrometer eyepiece
Eyepiece with indicator
Eyepiece with cross- wires
Erecting eyepiece ....
£ s.
8 10
0
4 10
0
7 15
0
. 7 15
0
9 10
0
. 11 0
0
. 12 0
0
. 18 0
0
0 12
0
0 12
0
0 12
0
2 2
0
2 2
0
2 10
0
2 10
0
2 10
0
2 2
0
0 18
0
0 17
0
1 10
0
LARGE SIZE EYEPIECES 1-41 INCH DIAMETER OF TUBE
3257
85
3253
88
3254
88
3255
88
Eyeshade .....
42-mm. X 6 Huyghenian eyepiece .
25-mm. X 10 Huyghenian eyepiece
17-mm. X 15 Huyghenian eyepiece
0
2
0
2
5
0
2
5
0
2
5
0
APPARATUS FOR ILLUMINATING OBJECTS
3285 26
3285p 33
3286 26
3286p 33
3287 27
3288 27
3288p 33
3291 28
3284 33
3295 34
3296 34
Small Abbe condenser, fitting with iris dia-
phragm on Microscopes No. 3210 and 3211 .
Set of patch-stops for above ....
Large Abbe condenser in fitting with iris dia-
phragm and swing-out tray for colour screens
with coloured and ground glass .
Set of three patch-stops for above .
Beck dry achromatic condenser, 1 N.A., lenses
only in mount, with standard object glass,
screw thread ....••
Beck dry achromatic condenser, 1 N.A., com-
plete in fitting with iris diaphragm and swing-
out tray, grey and green glass .
Set of three patch-stops for No. 3288 or 3290 .
Beck dry and immersion achromatic and apla-
natic condenser, 1-3 N.A., in mount, with iris
diaphragm and tray for patch-stops and \yith
ground and green glass double-wedge light
moderator ...•••
Traviss expanding iris patch-stop . . .
High-power dark-gromid illuminator, optical
portion only in mount, with standard object
glass thread . . • • • •
Illuminator as above in plain substage fitting .
1
0
5
7
0
6
2
0
10
7
0
6
4 5 0
5 15 0
0 7 6
9 15
0
0 12
6
2 0
0
2 10
0
No.
Page
3297
34
3298
34
3293
35
3294
35
3215
38
3216
38
3360
40
3361
40
3362
40
3363
41
3364
41
3328
32
3366
43
3335
44
3330
46
£
B.
d.
3
5
0
0
2
6
5
7
6
6
2
6
2
10
0
1
5
0
2
2
0
2
17
6
1
7
6
1
7
6
1
17
6
3
17
6
2
10
0
2
15
0
138 THE MICROSCOPE
Uliuninator as above in centring substage fitting
Stop to fit 1/12-in. oil-immersion object glass to
reduce aperture for dark-ground illumination
Beck patent focussing dark-ground immersion
illuminator in plain substage fitting
Ditto, in centring substage fitting .
Bull's-eye condenser, 2|^-in. diameter, on heavy
stand, with full adjustments
Bull's-eye condenser, H-in. diameter, on
smaller stand with full adjustments
Parabolic reflector .....
Parabolic reflector with Sorby reflector .
Thin glass reflector .....
Thin glass vertical illuminator
Prism illuminator ......
Double-wedge light moderator
Colour trough ......
Paraffin lamp ......
Beck electric lamp, complete with bull's-eye
condenser, ground glass, signal-green glass,
and metal filament 60 candle-power lamp . 8 10 0
3331 46 Beck electric lamp as No. 3330, but with 100
candle-power half- watt lamp . . .900
3332 46 Beck electric ]amp as No. 3330, but with
" Pointolite " electric lamp, with resistance to
work off direct current from 100 to 250 volts 14 15 0
3333 42 Set of 9 Wratten & Wainwright's colour
screens, for use with above lamps, com-
prising : light blue, to give daylight colour.
No. 78 ; dark blue, dominant wave-length,
5,000 ; blue-green, dom. wave-length, 4,700 ;
light green, dom. wave-length, 5,500 ; green,
dom. wave-length, 5,350; yellow, dom. wave-
length, 6,000 ; orange, dom. wave-leng-th,
6,300 ; red, dom. wave-length, 6,500 ; neutral
tint, passing 1/10 .
— 42 Single filters, as above . . . each
— — Stand to hold two filters, as above
— Circular screens to fit substage rings can be
supplied ..... each
3337 48 Incandescent gas lamp ....
— Extra mantles ......
3338 48 Incandescent spirit lamp ....
— Extra mantles ..... each
3336 45 Electric lamp on stand to take ordinary bulb .
APPARATUS FOR HOLDING SPECIMENS
3307 51 Sliding ledge 0 14 0
3305 99 Detachable mechanical stage, as illustrated on
Microscope No. 3213 (page 99) . . .600
3306 52 Concentric rotating mechanical stage, as illus-
trated 12 0 0
3400 52 Glass slips, 3x1, best quality, ground edges,
approximate thickness 1 mm. per doz.
per gross
3401 62 Ditto, second quality, ground edges per doz.
per gross
3
12
6
0
6
6
1
10
6
0
5
6
2
5
0
0
1
0
2
17
6
0
1
0
1
10
0
0
0
9
0
8
6
0
0
7
0
6
6
PRICE OF INSTRUMENTS AND APPARATUS 139
No.
3405
Page
65
3390 63
3391
3392
3393
3394
3395
3388
3406
3409
3410
3412
3413
3414
3415
3416
3420
3321
3421
3422
3425
53
63
53
53
53
53
64
55
55
56
56
56
56
56
57
57
57
57
68
3386 58
3325a 59
3325b 59
3325c 59
3325s 61
3222
61
3223
69
3224
59
3274
63
3384
62
64
1290
64
1292 64
Glass slips, 3x1, ground edges and excavated
hollow ..... per doz.
Cover glasses, No. 1, average thickness -006,
circular ..... per oz.
Ditto, square .... per oz.
Cover glasses. No. 2, average thickness -008,
circular . . . ' . . per oz.
Ditto, square ......
Cover glasses. No. 3, average thickness -01,
circular .......
Ditto, square ......
Micrometer screw gauge for measuring thickness
of cover glass slips, etc. ....
Glass slips with ledge .....
Cells, metal, circular . . . per 100
Cells, glass. ^ . . . per doz.
Troughs on 3 X 1 slips, small
Troughs on 3 X 1 slips, larger
Troughs on 3 X 1^ slips
Tirough, 1| X U X ^ in.
Beck's glass trough
Live box
Rousselet's live box
Beck compressor .
Stage forceps
Case of apparatus for holding objects, including
3 slips, 2 slips with hollow, slip with ledge,
trough on 3 X 1 slip, Beck's glass trough.
Beck's compressor, stage forceps, live box
and thin glass ......
Xylol 2 oz.
Benzol . . . . . . 2 oz.
Canada balsam, pure, in benzol or xylol 25 gr.
Hollis glue per bottle
Turntable . . • • _ •
Thoma-Hawksley Hsemacytometer, with two
pipettes and covers in case ....
Ditto, but with Zappert, Turck, Fiichs and
Rosenthal, or Breuer ruling
Ditto, Biirker model .....
Hsemacytometer chess-board pattern squared
glass plate to drop into eyepiece, 25 squares of
1 mm., 9 squares of 2 mm., or squares over
entire field either I mm., J mm., 1 mm., or
2 mm each
Cotmting chamber for use with chess-board
Pipette for red corpuscles
Pipette for white corpuscles .
Microspectroscope eyepiece .
Simple warm stage
S.I.R.A. wax, per stick 6 X | in
Beck grinding and polishing machme for
preparation of metallurgical specimens;
standard machine without motor, with one
polishing disc, cover, catcher, disc-lifter, 1 2
feet connecting wire plug adapter, and tin
of grease ...••••
Extra poli shing discs .••.••
Motors fitted at makers' current prices.
£ B.
0 1 6
0
7
6
0
6
0
0
6
0
0
4
6
0
4
6
0
3
6
3
3
0
0
0
6
0
6
0
0
5
0
0
1
0
0
2
0
0
2
6
0
4
6
0
9
0
0
8
6
0
17
6
1
1
0
0
14
0
3 15 0
0
1
6
0
1
6
0
2
6
0
1
0
1
I
0
o
10
0
2
12
6
3
10
0
0 12 6
0 2 6
0 7 6
0 7 6
0 7
0 1
1 15 0
6
6
21 0 0
1 15 0
140
THE MICROSCOPE
No.
Page
3429
65
3430
65
Pipettes
Pipettes, glass, with teat
each
£ s. d.
0 0 3
0 0 6
3431
3432
3433
3434
3435
3436
3437
3438
3439
3446
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
0
0
0
3
0
6
6
6
6
6
DISSECTING INSTRUMENTS
Blow-pipes with stiletto . . . .016
Razor, hollow ground one side, flat the other . 0 2 9
Scalpels, with ebony handles, in three sizes of
blade ...... each
Needles in wooden handles with ferrule .
Needles in bayonet shape in wood handles
Platinum needles in glass handles .
Scissors, curved .....
Scissors, elbow .....
Scissors, straight, 4- inch, with fine points
Scissors, straight, 4^ inches .
Scissors, straight, 6 inches
Seekers in ebony handles
Chain hooks .....
Forceps, without guide pin, 4| inches
Forceps, without guide pin, 6 inches
Forceps, with guide pin, 2h inches and 4 inches
Forceps, Comett's cover glass
Section lifter, copper ....
Section lifter and seeker combined, nickel
plated . . . . . . .010
Metallic hone, for sharpening razors, scalpels, etc. 0 2 6
Case of dissecting instruments, consisting of
three pairs of scissors, two scalpels, razor,
two pairs of forceps, combined seeker and
section lifter, blow-pipe, two needles, pipette
with teat, magnifier, in walnut case . . 2 5 0
0 2 9
0 0 6
0 0
0 2
0 6
0 5
4
3
5
0 0 9
0 3 0
0
2
6
0
5
0
0
5
9
0
2
6
0
0
6
COLLECTING APPARATUS
3452 66 Tow-net for collecting marine specimens, 11
inches diameter, with fine muslin bag and
bottle attached, with 24 yards stout cord on
wood frame ......
3453 66 Ditto, made of bolting silk ....
3454 — . Dredge for bottom sea collecting, canvas and
net bag, 10 inches
3455 — Ditto, 14 inches .
3456 — Ditto, 16 inches .
3457 — Ditto, 18 inches .
3458 — Ditto, 24 inches .
3459 66 Collecting stick with inner lengthening rod,
total extension 5 feet 6 inches, polished cane
with crook handle .....
3460 66 Net ring for attaching to above, 5 inches
diameter, with bolting silk net and bottle
3461 66 Ditto, but with 6-inch diameter net .
3462 66 Flanged tube for collecting nets .
3463 — Cutting hook for cutting weeds
3464 — Collecting bottles, 6 inches X 1 inch per doz.
3465 — 4 inches x 1 inch . . . per doz.
0 15
0
1 12
6
0 18
6
1 2
6
1 10
0
1 17
6
2 5
0
0 13 6
0
5
6
0
7
6
0
0
6
0
3
6
0
3
6
0
3
0
PRICE OF INSTRUMENTS AND APPARATUS 141
No. Page
3466 — Collecting bottles, 3 inches x 1 inch ,
3467 — 3 inches X J inch
3468 — 2 inches x f inch
3469 — If inch x i inch
£
8. d.
per doz.
0
2 9
per doz.
0
2 0
per doz.
0
1 0
per doz.
0
1 0
SUNDRY APPARATUS
3279 67 Glass plate ruled in squares .
3277 67 Stage micrometer with engraved scale. 1/10 and
1/100 of a millimetre
3278 67 Stage micrometer with engraved scale, 1/100
and 1/1000 of an inch
3276 68 Eyepiece micrometer, glass plate to fit into
eyepiece .......
3480 67 Glass scale with 100 divisions etched on in milli
metres for use in making drawings
Cross- wire for eyepiece ....
Beck micrometer eyepiece
Beck horizontal camera lucida
Beck vertical camera lucida.
Abbe camera lucida ....
Simple type of Abbe camera lucida
Drawing table .....
Iris diaphragm to fit between object glass and
nosepiece. .....
Polariscope for use on microscope .
Photomicrographic camera, vertical type, with
one dark slide, J-plate size
— — Extra double plate-holders .
3340 75 Photomicrographic camera, horizontal type
J-plate size, with one dark slide .
— — Extra double plate-holders .
3343 75 Focussing glass .....
3483 ' 63 Handcentrifugewithtwotest-tubesinaluminium
covers .......
3265
73
3275
68
3368
69
3369
70
3370
71
3371
71
3375
71
3358
3345
72
3342
74
0 10 6
0 12 6
0 12 6
0 10 6
0 10 G
0 5 0
2 2 0
1 17 6
2 10 0
4 5 n
3 3 0
0 15 0
1 15
3 10
0
0
3 3 0
0 13 6
8 15 0
0 13 6
0 15 0
3 3 0
INDEX
Abbe camera lucida, 7 1
,, condenser, 26
Achromatic condenser, 27
,, object glasses, 76
Acineta, 131
Adjustable patch -stop, 33
Adjustment, focussing, 18, 93
,, of vertical illuminator, 41
Amoeba, 126
Angle of aperture, 14, 16, 17
Aperture, 14, 15, 16, 17, 77
Apochromatic object glasses, 83
Apparatus for holding objects, 60
,, sundry, 67
Base of microscope, 92
Beck binocular, 107
,, compressor, 57
„ glass trough, 56
Bench metallurgical microscope, 119
Binocular, Abbe, 108
„ Beck, 107
„ illumination with, 111
,, optical path. 111
„ Powell and Lealand, 108
„ resolution with, 110
„ short tube, 112
„ Wenham, 107
Biology, best object glasses for, 90
Blood counts, 68
„ films, 64
Body of microscope, 8, 93, 103
Botany, best object glasses for, 91
Box for Sloan object glass changer, 21
,, live, 57
Bull's-eye condenser, 24, 38
Bythotrepes, 133
Camera lucida, 69
„ photomicrographic, 74
Case of apparatus for holding objects,
68
Cedar-wood oil, 17
Cells, 65
Cements, 58
Centrifuge, 63
Centring of dark -ground illuminator, 34
ofhght, 22
,, of object glass, 21
,, of rotating stage, 52
,, of substage condenser, 29
Ciliata, 129
Circular stage, 52, 120
Cleaning cover glasses, 54
Cleaning object glasses and eyepieces,
76
Coarse adjustment, 18, 93
Collecting net, 66
Colour screens, 42, 81
Concave mirror, 23
Condenser, Abbe, 26
,, achromatic, 27
bull's-eye, 24, 38
,, for dark -ground illumina-
tion, 27, 33
,, immersion, 28
,, substage, 24, 26
„ use of substage, 28
Cone of illuminating light, 27
Conjugate images, 78
Construction of eyepieces, 14
Convergent light, 23
Compressor, 57
Cork holder for objects, 67
Correction collar, 81
,, for cover glass, 79, 80
„ of object glasses, 78
Counting blood corpusc'es, 68
Cover glass corrections, 79, 80
,, „ thickness, 16, 63, 64
Cross-lines in eyepiece, 73
Cultiire plates, 62
Dark -ground illuminator, 34
Depth of focus, 18
Diameter of eyepieces, 88
Diatom periodic structure, 26
Difflugia, 127
Dimensions of microscopes, 96
Direction of light, 22
Dirt on lenses, 76
Dissecting instruments, 73
Divergent hght, 23
Double wedge light moderator, 32
Drawing table, 7 1
,, with the microscope, 69
Drawtube, 14, 93, 103
Dry mounted objects, 36, 58
Electric lamp, 46
Entomostraca, 133
Erecting eyepiece, 73
Eyepiece, 11, 13, 15, 88
compensating, 89
erecting, 73
for photography, 89
Huyghenian, 89
micrometer, 68
142
INDEX
143
Eyepiece, projecting, 89
}, with cross-lines, 73
» ,, indicator, 73
Eyepoint, 13
Eyeshade, 73
Field, flatness of, 18, 81
„ ofview, 14, 82
Films, blood, 58
Finder divisions, 51,72
Fine adjustment, 19, 93
Flagellata, 128
Flat mirror, 23
Flatness of field, 18, 81
Flattening objects, 54, 57
Focal length, 12
Focussing adjustment, 18, 93
,, best method of, 19
„ dark -ground illuminator, 35
„ glass, 75
,, substage condenser, 28
Foraminifera, 128
Forceps, stage, 57
Freehand drawing, 67
Gas lamp, 48
Glass cover, 53
„ focussing, 75
„ plate ruled in squares, 6 1
,, slip with ledge, 54
,, shps, 62
Grinding machine, 64
Haemacytometer, 58
Hair of nettle, 134
HeHozoa, 127
High-power illuminator, dark -ground,
34
Histology, best object glasses for, 90
Holding specimens, 50
Holopedium, 133
Illumination, 21, 23, 28
„ intensity of, 37, 43, 44
„ opaque, 38, 39
,, with condenser, 26, 38
,, ,, dark -ground, 34
,, ,, mirror, 23
Illuminator, parabolic, 40
,, vertical, 40
Image formed by eyepiece, 9, 13
,, ,, ,, object glass, 9, 11
„ incorrect, 31
Immersion condenser, 28
,, fluids, 17
„ object glasses, 17, 34
Inclined position of microscope, 50
Indicator eyepiece, 73
Initial magnifying power, 68, 77
Instrument, dissecting, 73
Intensity of light, 44
Iris diaphragm, 23, 26
Joint of microscope stand, 94
Lamp, electric, 45, 46
„' gas, 48
Lamp, paraffin, 44
,, spirit, 48
Ledge, sliding, 51
Length of body, 79, 93
Light, centring, 15
,, cone of illuminating, 26
,, convergent, 23
,, correct direction, 22
„ divergent, 23
,, intensity, 44
,, moderator, 32
,, scattered, 24
Limb of microscope stand, 94
Linear magnifying power, 13
Live box, 57
Living specimens, 55
Lucida camera, 69
Magnifying power, 13, 77, 88
Measuring cover glass, 53
,, specimens, 67
Mechanical stage, 52
Metallurgical microscopes, 115
,, specimens, 63
Metallurgy, object glasses for, 91
Micrometer, 67
Microscope as a recreation, 125
„ binocular, 107
,, massive model, 104
,, metallurgical, 115
,, petrological, 120
„ portable, 100
„ Standard London, 95
,, stands, 9, 92
Mirror, 23, 28, 95
Moderator, light, 32
Monad, 129
Mounting cells, 55, 58
,, specimens, 58
Net collecting, 66
Nosepiece, 20
Numerical aperture, 15, 17, 70
Object glass, 11, 12, 17, 76
„ centring, 21
,, ,, changer, 21
,, ,, characteristics of differ-
ent, 82
,, ,, examination of back lens.
29
,, ,, selection of, 87
Object, scattering of light by, 24
Objects, flattening, 54, 57
Oil-immersion, 78
Optical quahty of substage condenser,
26
Parabolic illuminator, 40
Patch-stop adjustment, 33
Pathology, best object glasses for, 87
Penetration, 18
Periodic structure, 25, 31
Petrological microscope, 120
Photomicrography, camera for, 74
,, eyepiece for, 89
Pillar of microscope stand, 62
Pinhole aperture, 12
lU
INDEX
Pipettes, 59, 66
Plates, cvilture, 62
Podura scale, 31
Pointolite lamp, 46
Polarising apparatus, 72
Polishing machine, 64
Portable microscope, 100
Power, magnifying, 13, 16
Preparing metallurgical specimens, 63
Prism illuminator, 41
Protozoa, 126
Quality of substage condenser, 26
Ramsden circle, 13
Recreation, microscope as a, 125
Reflection from objects on dark ground,
37
Reflector, Sorby's, 40
,, thin glass, 40
Requirements of microscope stand, 92
Resolution, 15, 31, 37, 86
,, binocular, 110
Rotating stage, 52
Rowley metallurgical attachment, 118
Rulings for blood counts, 60
Scatteredlight, 24, 31
Screens, colour, 42, 81
Screw, object glass, 82
Seeds and pollen, 134
Selection of object glasses and ^eye-
pieces, 90
Shape of limb, 94
Size of mirror, 95
Sliding ledge, 51
Slip, glass, 52
,, with ledge, 54
Sloan object glass changer, 20
Specimens, how to hold, 50
living, 55
,, mounting, 58
Spirit lamp, 48
Sponge, 132
Spores of fern, 134
Squares ruled on glass plate, 67
Stage clips, 50
,, forceps, 57
Stage, mechanical, 51
,, rotating, 52
,, warm, 62
Stand of microscope, 9, 92
Standard London microscope, 95
„ object glasses, 77
„ of magnification, 13
,, screw, 82
„ thicknessof cover glass, 53, 79
„ tube length, 15
Stentor, 130
Stereoscopic vision, 113
Structure revealed by dark ground, 37
Substage, 8
„ condenser, 24, 25, 26
Table, drawing, 66
,, of drawtube corrections, 80
,, ,, field of view, 77
,, ,, object glasses, 70
Thickness of cover glass, 53
„ „ slip, 52
Traviss patch-stop, 33
Troughs, 56
Trypanosome, 128
Tube length, 16,79,93
Tubercle bacillus, 31
Tiu-ntable, 58
Use of dark-ground illuminator, 34
„ „ substage condenser, 28
Varying magnifying power, 15
Vernier, 72
Vertical camera lucida, 69
,, illuminator, 40
,, photomicrographic camera,
74
,, position of microscope, 50
View, field of, 14
Villous amoeba, 126
Virtual image, 13
Vision of natural objects, 22
Vorticella, 129
Warm stage, 62
Wedge, light moderator, 32
Working distance, 12
Printed by Hazell, Watson & Viney, Ld., London and Aylesbury