e.  AHE

| MICROSCOPE
ae CONRAD
BECK

 

 
 

 

CORNELL .
UNIVERSITY
LIBRARY

 

FROM

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TTT
THE MICROSCOPE
THE MICROSCOPE

A SIMPLE HANDBOOK

CONRAD BECK

FIRST EDITION

TWO SHILLINGS AND SIXPENCE NET

LONDON
R. & J. BECK, LTD.

68 CORNHILL, E.C.
1921
Re
PREFACE

Tus 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 : . dl
CHAPTER IT

ILLUMINATING APPARATUS AND SOURCES OF ILLUMINATION . 26

CHAPTER ITI

APPARATUS FOR HOLDING SPECIMENS j 50
CHAPTER IV

SUNDRY APPARATUS ‘ . 3 ‘ j c . 67
CHAPTER V

OBJECT GLASSES AND EYEPIECES . 16
CHAPTER VI

THE MICROSCOPE STAND 2 A . ‘ , . 92
CHAPTER VII

THE MICROSCOPE AS A RECREATION . 3 - ‘ . 125

INDEX . ‘ ‘: : . ‘: : ‘ . 143
ZAEP AS ee Bo 8 tp

a

H@pOmMO

DESCRIPTION OF FIG. 1

Base to support instrument.

Pillar to support instrument.

Joint for inclining instrument.

Stage for reception of object to be examined.
Hole for attaching mechanical stage.

Mirror for illuminating transparent objects.
Substage for carrying illuminating apparatus.
Substage focussing adjustment.

Substage condenser for regulating the illumination.
Iris diaphragm for varying the light.

Limb for holding the body and stage.

Body for carrying the observing lenses,
Drawtube for lengthening body.

. Drawtube stop to prevent reflections from tube entering eyepiece.

Eyepiece, combination of lenses nearest observer's eye.
Fine adjustment for delicate focussing of body.

Coarse adjustment for rapid focussing of body.

Object glass, combination of lenses nearest the object.
Nosepiece with universal screw for carrying object glasses,

Eyepoint position where all emergent light passes through a small
area and where observer’s eye should be placed.

Position where the primary image formed by the object glass is
produced. ‘

Position where final virtual image formed by eyepiece is produced.
A

 

 
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 construction 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
(O), 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 obiect glass and the eyepiece,
and a diaphragm (N1) 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.

ll
12 THE MICROSCOPE

The object The object glass (R) in the earliest instruments was a single
: double convex lens (Fig. 2); it oo an oe but very im-
erfect picture of small objects, the outlines were

Firecandel by coloured fringes, and the details
were fuzzy and indistinct. Such lenses were made Fic. 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
of object” 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,

i i ction of the 2/3-inch (16-mm.),
Fig. 3 shows the optical construct: I /6-inch(4-mm.),
ge and 1/12-inch
3 (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
oly, ordinary single
Fie. 3.—f = focal length; w = working distance. Jeng of 92-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.

Working Object glasses are not single lenses, but are composed of
distance. + 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

   

Ss4
|
1

—- sem wan
ho Bl

 

 
A SIMPLE DESCRIPTION OF THE;}MICROSCOPE 13

the formation of the images. An enlarged picture 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
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
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 O 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 E o Oo oO”
object being examined. In this Fie. 4,
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-
cation may be produced by different methods. The object
glass may magnify the object twenty times in the primary image,
and the eyepiece increasing the primary image five times will
give a total of a hundred. This magnification may also be pro-

Eyepiece.

Virtual
mage.

Magnifying

power.

Different
methods of
obtaining
magni
power.
Field of
view.

Aperture.

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 slight 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.

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.

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
A SIMPLE DESCRIPTION OF THE MICROSCOPE 15

enter the instrument (see Fig. 5). It was soon found that
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 4
the object is called the aperture SmallAperture Large Aperture
(a, Fig. 5). It is expressed aan cee
either by the angle of the cone F!4. 5.—@ = angular aperture.
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 rimit ot
of what can be seen with the microscope does not depend upon yeoman
what magnifying power can be obtained, but upon what size on aperture,
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, Best method
therefore, by changing the object glass. Most object glasses Senne
have sufficient aperture to allow of the use of an eyepiece of as power.
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 Standard
magnifying power upon extending the drawtube of the micro- pg?
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

 
Thickness of

cover glass.

Apertures
suitable for
different
powers.

Table of
apertures
and powers.

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.

The thickness of the cover glass used over the object has no
effect with an immersion 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 :

 

 

Initial
- F
mens eae nai ee | ogee
Power.
: 42mm. | 25mm. | 17 mm.
lfin. =40 mm. . 19° 16 3 20 34 50
1jin. = 32mm. . 17° 15 4 25 45 65
2/3in. = 16mm. .j{ 32° 28 10 62 110 155
1/3in. =8mm, . 60° 5 18-5 115 200 285
1/6in, =4mm. . | 116° +85 40 285 490 690
1/8 in. oil immer-
sion =3mm, . —_ 95 60 427 735 (| 1,015
1/12 in. oil immer-
sion =2mm. . — 1:3 90 530 900 |1,275

 

 

 

 

 

 
A SIMPLE DESCRIPTION OF THE MICROSCOPE 17

The 1/6-inch is receiving from the object, cones of light Angle in air
of 116°, as shown in Fig. 6. It could not be made to collect compared.
a very much larger angle of light because it cannot be used in lass.
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
passes through the object. It
is spread out by refraction as it
enters the air between the cover
glass and the lens, If the air
space between the cover glass
and the lens could be filled up
with glass, this spreading out of
the cone would not occur, and
the cone of light would remain
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 ope.
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. Itis 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

2

 
Immersion
fluids.

Flatness of
field.

Depth of
focus.

Coarse
focussing
adjustment.

18 THE MICROSCOPE

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 allowed in the microscope.

The penetration or depth of an object glass or the number of
different 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 DESCRIPTION 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
coarse adjustment, but the movement is far more delicate:
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 correct focus, care is required to prevent the front of the
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 cedur-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 may be focussed first, and thefineadjustmentlowered
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

Fine
focussing
adjustment.

The best
method of
focussing.
Revolving
nosepiece,

20 THE MICROSCOPE

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
Sy 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
Fie. 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.

 

Sloan object An object glass changer is an apparatus for rapidly changing

changer.

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.

Tt 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 ye, 8,—No. 3280, Sloan Object

it up. Glass Changer.
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

 

!

 

 

 

 
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 10mm., wg, 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 of tumin- |.

almost as much importance as the quality of the lenses. I¢ is 4
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 recognise
the true appearance.
22 THE MICROSCOPE

Yasenot The same holds true of opaque objects examined with the

objects. microscope, but the greater number 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. | 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

—L- 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
LMP 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
Obliquelight. Directlight. be visible unless the light is passing

Fie. 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

Direction of
the light.

 

 
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 uot in itself sufficient to make the
final adjustment.

The nature of the illumination may be varied according to Matarsot
whether it is parallel, divergent, or convergent. If the flat side"™™™*4°™
of the mirror be used and the
source of light is at a consider-
able distance, a beam of nearly
parallel light is obtained (a,
Fig. 11). If the source of illu-
mination is very close, a diver-
gent beam is obtained (c, Fig. 11).
If the concave mirror is used, it
will be a slightly convergent Fre.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).

 
Tilumina-
tion for
transparent
objects.

Scattering
of light by
object.

24 THE MICROSCOPE

As regards the best kind of illumination for transparent
objects, the light may be a nearly parallel beam from the flat
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

a b objects it is distributed or scattered

Fie. 12. 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
illumination, 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 (b).

All small objects spread the light slightly by diffraction,

though in the case of a single dark object on a white field, the
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

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 different 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),

  

Fie. 13.—No. 3286, Abbe Condenser,

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 (O) coincides with the object being examined.
The object is by this means brilliantly illuminated. 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 O. 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 O, 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
an opaque patch or stop is placed below
the condenser so that the object is being

 

Fic. 15.—No. 3288, Achromatic
Condenser.

[A

SSSHpggy]
LAM

LMM

   
   

 

ABCODEF
Fia. 16.

 

Fie. 18.
Immersion
achromatic
condenser.

Use of a
Bubstage
condenser.

To focus a
substage
condenser,

28 THE MICROSCOPE

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. i
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 1}-inch (32-mm.) a condenser
is not required. The aperture of
such a low-power lens is small,

and the angle obtained by the

ee on Dry and —_yse of a concave mirror is usually

Puaeran Condenser: sufficient 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 by 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 APPARATUS 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 {supsiase
generally sufficient 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, naect ot
the eyepiece of the microscope should be removed, and the See aae
effect of opening and closing the iris diaphragm be observed by aisohseu vt
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
Hffect of
focussing a
substage
condenser.

Best
aperture of
illuminating
cone.

30 THE MICROSCOPE

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 coverging 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 different 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
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
TLLUMINATING 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 often

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 oY ett,
whole surface as at the spot where the direct light passes through, Py 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. Ifthe 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), showing 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 AB
the full cone of light is passed through the object by the ina a
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

different apertures when it is required to observe very fine struc-
ture.
Variation There is a difficulty in making use of different apertures because

intecuty the light varies so greatly by the change in the aperture of the

when ins condenser, that the variation in intensity becomes a serious incon-
iaphragm

of condenser Vernience. The observer should never be tempted to overdo the

peters 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.

Doe The most satisfactory appliance for critical work is an adjust-

moderator, @ble pair of neutral tint gluss 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,

 

Fic. 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- Dark-ground
denser in which the object is illuminated by light that is so oblique *we
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—14-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 high-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
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. Fie. 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
by means of which a very small image of the source of illumina- inlet aie
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

Fe} Adjustable
patch-spot,

 
34 '  . 4THE MICROSCOPE

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
Fig, 23.—No. 3295, High-power Dark- slide. It must be used
ground Illuminator. 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 illuminator 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
Useofoil- object, enters the microscope (see Fig. 24). A special oil-
immersion immersion 1/8-inch (3-mm.) focus, with an aperture of °95 N.A.,
ea is made for work with this illuminator; or an immersion lens
‘with a 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 all (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 brilliantly. 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. Slides of this thick-

 

 
a

TLLUMINATING APPARATUS 35

ness can be selected for examining living specimens, but mounted
objects can seldom be examined.

To overcome this difficulty, R. & J. Beck, Ltd., have denied Focussing
the dark-ground illuminator oa 24 and 35) with a focussing {uk erqued
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 4 mm.
to 14 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- Fig. 24.—No. 3294, Focussing Dark-
nator, Fig. 25 shows ground Iluminator.
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 slip. 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

Fie. 25.—No. 3294, Focussing direct light. Such objects, however,

Dark-ground Illuminator. 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.

 

 

 

 

 

  

i my

ST oT

   
36 THE MICROSCOPE

To centra It is therefore necessapy that a centring adjustment should

dark ground D€ 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 slip. 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

Get pesana With the non-focussing illuminator, although a very small move-

illuminator. 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
thoroughly 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. ttensity
It is referred to under the heading of ‘“ Jlluminants.” If the f lest.

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
eee ee
: . ‘ . 2 dark-groun,
by its means in certain classes of objects. The spines or illumination.

realise the large amount of extra structure that can be recognised

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

f..

gone

\
38 "dE MICROSCOPE

position where it actually reaches 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.
ieee 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.
oe On an optical bench a condenser will
Fre. 26.—No. 3215, transfer the light source to a position nearer
Bull’s-eye Con- the microscope. Except for these purposes,
denser on Stand. it should not be employed with a good
substage condenser. It does not fi

   

additional light, and it generally ruins the performance of a
perfectly made condenser due to its own lack of correction.
sirens 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 (O) 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 LO is
half LS, and the image is half the size of the luminous source ;
in the lower diagram LO is one-quarter LS, 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 O, 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-

 
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 Fie, 28.—No. 3332.
of the shadows.

A suitable illumination of opaque objects is required for Ulumination
botanical, entomological, and general work, and is of paramount oppo with
importance for metallurgy. When low-power object glasses are bull’s-eye
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,
neatly parallel with the direc-
tion of the light, as in Fig. 29,
the light enters the curved
surface and is condensed ; when
Fie. 29. 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

 

 
Parabolic
reflector.

Sorby
reflector.

Glass
reflector.

Vertical
iuminator.

ad

40 , THE MICROSCOPE

even when a moderately high power, such as a 1/6-inch (4-mm.)
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 lamine,
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
1}-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
ee ee object glass having been kee 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 14-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- om
power object glass and the object (see Fig. 31). re ee
This has the advantage that it does not reduce Reflector.
the aperture of the object glass. It can be
used with either a 1}-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

 

 
ILLUMINATING APPARATUS 41

e light reflected through the object glass itself upon the
object.

This illuminator is generally known as a vertical illuminator. vse of
It is screwed into the nosepiece of the microscope, and the object (fiat
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, qm '
which can be turned round, should be (sg :
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
Migs = by small adjustments in the position of the

= lamp or reflector ‘so that such reflections
Ra rear 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 ‘uminator.
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

 

  
Colour
screens.

42 THE MICROSCOPE

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. 111 (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
a Zi | very useful for increas-
er aoe \ ing colour contrasts,
ec ae both for visual and

EES photomicrographic work.
If a specimen of
bacteria is stained

Fra. 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—

Blue, ared filter should be used.
Green, ared

Red, a green
Yellow, a blue

Brown, a blue _,, ie aR 8s
Purple, a green ,,
Violet, a yellow ,,

 

KODAK.

2? 23 ” »”
2? ” 2 3?

2? EPy a” oy

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 5
Wratten & Wainwright gelatine Fie. 35.—No. 3366, Monochro-
filters. matic Light Trough.

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 ttn eee
the worst form for accurate observation, but that when it is used
a screen or card with a hole in it about 23 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

   
 
 
 
   
44 :

THE MICROSCOPE

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.

Relative The relative intensities of a similar size small area of different

intensities

ofditerent illuminants are approximately according to the following table
fources of | taken from Mr. A. P. Trotter’s book on Illuminating Engineering :

illumination.
Candle . . 24
Daylight (blue sky) : , 2
Paraffinlamp . ; . 4to9
Incandescent gas mantle. ; 50
Carbon electric filament * 300
Metal electric filament ‘i . 1,000
1/2-watt electric bulb . 3 é . 5,250
‘** Pointolite ’? electric bulb . 7 . 12,000
Arc lamp . ‘ ‘ 80,000 to 110,000
Direct light from the sun . z - 800,000

From the above table it is evident why ordinary daylight is

 
  
 
 
 
 
 
 
 
 
   
 

Fie. 36.—No. 3335,
Lamp.

 

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

Paraffin

light should be cut down. Just enough light to show the
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 paraffin lamp for microscopic Parafin
work, It has a single flat wick 5/8 inch wide. The burner *™”
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 1l-inch microscope slip carried in a thin metal
chimney. The burner is insulated from the reservoir by a fibre
ting, 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.

   

Sen

Fria. 37,—No. 3336, Electric Lamp on Stand.

 
46 THE MICROSCOPE

class of illumination; and the Beck electric lamp is a convenient
form which takes either kind of electric bulb.

a The lamp has adjustment so that the beam of light can be
placed at any height
ye 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
off 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
Fie. 38.—No. 3332, “‘ Pointolite” Lamp. 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 Fre.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 APPARATUS 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” —
oe baie Fie. 40.—No. 3332, ‘“ Pointolite’’ Lamp.
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.

 
48 THE MICROSCOPE

Incandes- An incandescent gas mantle, either of the ordinary or inverted
ares 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
a = having been filled with spirit,

Fra. 41.—No. 3337. 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

   

7

 

Fie. 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.
Stage clips.

Vertical
position of

microscope.

Sliding
ledge.

CHAPTER III

APPARATUS FOR HOLDING SPECIMENS FOR
EXAMINATION

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 different 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 difficult 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
clips”’ (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
Fic. 43.—Stage Clips. 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 a3 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

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-
tatus which holds
a slide or object-
holder, and by
means of two
racks actuated by
milled heads moves
it in a delicate 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 slip, 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 24 inches (65 mm.) and a vertical travel of 1 inch (25 mm.)

Fig. 45 shows a concentric rotating stage, with a mechanical
stage built into its surface. In this case the slide moves longitudin-
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

em me we
‘
\
t
2

 

ere mm ee ee Ot Ow

Fic. 44.—No. 3307, Sliding Ledge.

™, Mechanical

stage.

Rotating
mechanical
stage.
Centring
rotating
stage.

Glass slip.

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.

 

Fie. 45.—No. 3306, Rotating Mechanical Stage.

The ordinary mechanical stage is made to take only the 3-inch
standard length slide. It has a lateral travel of 23 inches
(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 14 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. Fre, 46.—No. 3400, Glass Slip.

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

 

 

 

 
APPARATUS FOR HOLDING SPECIMENS 53

of placing the object between a glass slip and a thin 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 } to
14mm. Most forms of illuminating apparatus
can be adjusted to focus EOE alps of
such thickness, but apparatus which cannot be
focussed is cohstructed 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 58
diameter, or can be cut to any particular size
required. It is made in three thicknesses :

No. 1. Average thickness. °006in. °15 mm.
2 2. ” ” . *008 ” 2 38 Fie. 47. — Thin
» 3 ” 55 « SOL gy: 228: 5; 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 (hides of
be observed, the microscope may be focussed to the dust on the of 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

Cover glass.

 
Cleansing
cover
glasses.

Thickness
of cover
glass,

Glass slip
with ledge,

Blood films.

54 THE MICROSCOPE

much added to it. Ifthe 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—14 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 slip 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.

Ii 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
[cits || angle in such a manner that the

Fie. 48.—No, 3406, Slip with cover glass touches one side of
Ledge. 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

 

 

 

 
APPARATUS FOR HOLDING SPECIMENS 55

with 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 $f opiects in
glass should be removed with blotting or filter paper, and capillary
attraction will 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- azz
ties, and are known as slips with
hollows. They are used in the ;
same way as ordinary slips, the water which
| Ee fills the cavity holding the cover glass in
. position by capillary attraction.

Cells or rings of vulcanite metal or glass Cells.
O eg may be cemented to 3 x l-inch slips with
Hollis glue, forming deeper cavities for the
oO 0 00 reception of large specimens (see page 58).
Fra. 51.—Cells, When such objects are in fluid, the removal
ie ’ of the superfluous water is sufficient to make
the cover glass adhere to the cell.
If insects are to be seen dry, a
the cover glass may be made to —
adhere to a alls by placing a = FG. pape with Cell
smear of grease or vaseline around Pe aa
the upper edge. é
wer the salen of aquatic weeds, alge, and animalcula Trough.
with low powers, a trough is a useful apparatus. Fig. 53 shows a

 

   

be
Adjustable
trough.

Live box.

56 THE MICROSCOPE

convenient form mounted on a3 X 1 slip; it has usually a space
for water about 2 mm. thick. 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 1} x1} x} inches,

and is made of glass about
Fie. 53.—No. 3413, Slip with 1mm. thick. It is only suitable,
Trough. 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 screws
and milled nuts, each holding a clamping plate. A half-circle
of indiarubber made from an elastic band is laid on the 3 xX 1
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 uy
illumination or with high powers. Itisa fy, 54.—No. 34165,
very convenient appliance also for the Trough.
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 sufficient fluid dropped in.
If a small circular cover glass be cemented in the centre of the
3 xX l-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,
ere ) A live box consists of a plate

a », 3X1 inches, with an aperture

cee eta in the centre of which is fixed a
short brass tube carrying at the

top a glass plate. Over this tube slides a cap, in the top of which
a cover glass is held by a screwed cell. The object to be examined

 

 

 

 

 

   

   
APPARATUS FOR HOLDING SPECIMENS 57

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 A
high powers. The principle is that pre, 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 22 x 1} 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 fixed. This pillar carries an arm
which holds a thin cover glass 3/4 x 18 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
OBES rsrae ernest carrying the thin glass can be
Frc. 57.—No. 3421, Beck's SWung to one side for placing the
Compressor. 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 ‘res.
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 ™ m
pair of spring forceps opened Fie. 58.—No. 3422, Stage Forceps.
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,

 

 

    

 

oy
a
58 THE MICROSCOPE

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 slip
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 l-inch size.

Mounting Mounting permanent specimens for the microscope is a subject
“poem that is beyond the scope of this book. The microscopist should
be provided with a bottle of Canada balsam dissolved in benzol
or xylol, which is a transparent cement, and a bottle of Hollis 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 a3 Xx 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.
Turntable. 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
Fie. 59.—No. 3386,  camel’s-hair brush, which has been dipped
Turntable. 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.

Hema- The hemacytometer is an apparatus for counting the blood

cytometer. ‘ : ats fs
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

 
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

 

 
 
 
 

   

oT oe Tae
<SEEEEEEEEEX 0 See Fe

Fie. 60.—No. 33254, 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 :
+ Lbs = aan |
{_ 8
Fie. 61.

 

Depth

10m.n. THOMA
ah. HAWKSLEY
400 SGM.m.

 

 

Fic. 62.—No, 33254, 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 hemacytometer this portion is ruled with a
diamond into squares 1/400th of a square mm. each in area. It
 

 

 

 

 

 

 

 

 

 

 

Tiirck.

 

Elzholz.

 

 

Zappert. Centre Ruling of Thoma, Biirker.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

Nebauer. Fiichs and Rosenthal. Breuer,

Fria, 63,—Rulings.

60
APPARATUS FOR HOLDING SPECIMENS 61

will therefore be seen that the amount of liquid resting upon
each square has a cubic capacity of 1/4,000th of acubicmm. The
liquid 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
in 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 (1/4,000th
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 ruled 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 ruling and also the
counting are different in each case.

The use of a mechanical stage greatly assists the counting.

A simpler form of heemacytometer 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- Fia. 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 ‘l 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 slip 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-mm.

 
62

THE MICROSCOPE

chess-board plate be used, then thirty-six squares, six each 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 hemacytometer,
and this method is preferred by some apart from the question of
the cost of the apparatus.

Culture
plates.

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 14-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 l-inch slip with a drop of Canada balsam.

Warm stage’ A warm stage is an apparatus for applying warmth to a speci-
men under continuous observation. A simple form consists

 

Fie. 65.—No. 3384,
Warm Stage.

of an oblong copper plate 3 x 1 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 l-inch portion. It is placed on the
stage of the microscope and held like an
ordinary 3 X 1 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
APPARATUS FOR HOLDING SPECIMENS 63

cover glass should be smeared round its edge with olive oil
or vaseline.

A centrifuge is a small hand machine for revolving test tubes Centrituge.

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 S&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/glass be used,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 metallurgical specimens for examination metal-
under the microscope consists of cutting off a small piece of the Dim.
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 Mounting
lump of wax placed on the slide. The best wax for this purpose with wax.
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 §.1.R.A. wax. The specimen should be attached so
that its surface is parallel 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 MICROSCOPE

and about 1 inch diameter. Lay one of these at each end of a
3X 1 glass slip, and lay the metal specimen face downwards
on the glass slip in the space be-
wy + tween the two pieces of wood.
= 3 Take another 3 x 1 slip with a
EEA __ CO _EZA lump of wax aahacne to the
Fie. 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).

Grinding The grinding and polishing is generally done on a machine
Sane 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.
Grinding It is com-
polishing plete and self- aa
e contained, and Fie. 67.—No. 1290.

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
furnished with a speed cone (F), with pulleys of varying
diameters, and is driven by means of a belt from the driving

 

Fia. 68.—No. 1292.
APPARATUS FOR HOLDING SPECIMENS 65

cone (G), which 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
ting (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 Fia. 69, —
material is placed on the disc (B) and the ring Pipettes.
(D) pressed over the paper until the ring (D) is .
about half-way down the edge of the disc (B). The spring
garter is stretched over the edge. The ring (D) 1s 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 fixed in
the spindle that it cannot be removed by hand, a pair of lifting
levers are supplied, which can be placed resting on the edge of
the catcher with one end under the disc; a steady pressure on
the other end will raise the disc from its fitting in the spindle.

A cover is provided to protect the revolving disc from dust
when it is not in use.

5

 

 

 

 
Pipettes.

66 THE MICROSCOPE

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 (Fig. 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
Fie. 70.—Dissecting Instruments. to a slip, live box,

or compressor. Two

 

Instruments. or three needles, a pair of fine forceps, a pair of scissors, and a

Collecting

net,

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
small calico bag attached to its front edge
which may be filled with stones to enable
it to be towed along when sunk below the ¥
surface of the water.

Most of the free swimming fauna in eonediae Meee”
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.

 
CHAPTER IV
SUNDRY APPARATUS

Tuer drawing of specimens seen under the microscope by free- Drawing
hand suffers from the disadvantage that it is difficult to obtain specimens.
accuracy in,dimensions and relative proportions. Microscope
drawings are seldom required as works of art, but must be
accurate. js The simplest aid to accurate drawing is paper ruled Rulea
with lines in squares used in combination with a glass plate ruled “?"*
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 pg 72, —

latter may be slightly unscrewed till the lines come No. 3279,
sharply into focus. Ruled _Eye-

This method of drawing is popular because the Piece Plate.
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 slip with lines ?°!™-™-
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
1/100ths of a millimetre or 1/1000ths 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 4 x 1 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 millimetre or

67

 
Ruled
micrometer.

" Micrometer
eyepiece.

68 THE MICROSCOPE

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
istdaeintond } lass ruled with a finer series of lines (Fig. 73). It
drops into the eyepiece in a similar manner to the
Fre. 73, Square ruling. An even better method of making
No. 3276, 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-
ae scale (A, Fig. 75) placed in its focus which is outside the
enses.

It is provided with a collar (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
1/10th 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 1/10th 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 object itself, this result has

 

 

 

 

  

 

 

 

 

 

 

 

oe 9

34 a

MH 7

---B 6-4 ‘

3-4 *

me MK 4

¢ 34. 3

lool 2

| | 4d

Stee meee, 4 2

Fie. 74.— No. 3275, Fria. 75. —No. 3275, Fria. 76.—Scale
Micrometer Eyepiece. Micrometer Eyepiece. 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 1/10th 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 tnitial

object glass at a position rather above the stop of the eyepiece. mesnitvine
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 juciia,
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
be made placed upon
the table below the
eyepiece of the :
microscope. The a
camera lucida is a :
small half-silvered Fiea.77.—No. 3368, HorizontalCamera 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

”
a
i
'

oo Bh

 
Vertical
camera
lucida.

Abbe
camera
lucida.

70 THE MICROSCOPE

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
Wollaston 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

 

‘
4 paper is so brilliant
A as to obscure the
A hea one or the other.
a * The relative illumi-
Zo : nation can be easily
regulated by a neu-
Fig. 78.—No. 3369, Beck Vertical Camera tral tint glass placed
Lucida. 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. Aslot 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
SUNDRY APPARATUS 71

used as any of the other forms. With this apparatus the
instrument is placed in a vertical position, and the drawing paper

   

Fic. 79.—No. 3370, 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
made be tilted up so
that the distortion is
corrected, for the image
can then be thrown to
a greater distance to
the side of the instru-
ment. In order to find
Lucida. 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 Modifiea
Abbe type is made in which the Pre
bulky mirror is replaced by a small lucida,

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 Fic. 81.—No. 3375, Draw-

angle which can be ascertained as ex- ing Table.

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
Finders.

Vernier.

Polarising
apparatus.

72 THE MICROSCOPE

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
3 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 133.. 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
Fra. 82, the fourth line—this shows that the true reading
Vernier. is not 134, but is 13-4. 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 itis 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

eegsaceset

Ov:
SUNDRY APPARATUS 73

2 us theory of light, which is not within the scope of this
ook,

An eyepiece with a movable pointer or indicator (Fig. 83) is Demon-
a useful aid to teaching. It consists of an eyepiece magnifying Svtion,
x10, which has a fine movable index ;
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 Eyepiece
(Fig. 84) is necessary for petrology where __ ili||i! | ‘| eee
angles are to be measured by means of a H
pie as aad, ae “usctal fox other din? Zeal

An eyeshade (Fig. 85) which clips Fie.83.—No, 3263, Eye- Eyeshade.
on to the drawtube of the microscope Piece with 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
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

Fra, 94. by extending the drawtube. It gives a very large
No. 3264, field of view and an erect image, so that it at
CrossLines once converts an ordinary microscope into a thor-
of Eye- oughly efficient dissecting microscope; and a slight

ae 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 ™*2™Phy
not require much apparatus or special appliances.

The microscope is first
arranged to give the best

visual image, the particulars A
as regards illumination given (\ je
in the earlier part of the book C2 y
having been carefully followed. J

The eaaaey eyepiece is re- eZ
placed by the 30-mm. focus Fie, 85.—No. 3257, Eyeshade.
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

 
Vertical
photo-
micro-
graphic
camera,

74 THE MICROSCOPE

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. oe

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-
grapbic 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

at V splayed out to give sta-
eer bility. It has a slide
Fia. 86.—No. 3342, Vertical Photomicro- on its upper surface into
graphic Camere. which 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 44 x 3}
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 15

a focussing glass should be used; this consists of a high-power Focussing
lens mounted 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 a
hold sthe lens combination by slightly un-
screwing it, then screw the outer one of Fic. 87,—Focus-
the tubes downwards away from the cell, sing Glass.
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 yee
be placed with its tube in a horizontal position. It enables a staphic
variation in the size of 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 44 X 3} inches
(quarter-plate) size negative. It consists of a heavy steel
hexagonal bar fixed 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
52 and 7} inches, and it can be raised to a higher level for use

TITTIES Steet

Fia. 88.—No. 3340, Horizontal Photomicrographic Camera.

maa PP | H

   

 

   

 

LF

 

 

 

WS)

 

 

 

 

 

 

 

with large microscopes by putting four feet under the levelling
screws.

There are but few purposes for which a larger photomicroscope
than 4} x 3} inches is required, and for this size 30 inches is
ample extension.
Scratches
and dirt
on object
glasses,

Dirt on

eyepieces.

Oleaning
lenses.

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. ach 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 upon the.surfaces of the lens, or dust either on or
between the components, unless in an aggravated form, do not
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 in the eyepieces shows in the field of view, that on the
object glasses is not clearly 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.

To remove 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
77

OBJECT GLASSES AND EYEPIECES

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ssaoetdedAg WITH Jomog SuLsuseyy

 

 

 

 

 

 

SHSSVT) LOULTQ OLLVNOUHOY aUuvaNviLg 10 LSIT
Removing
oil from
immersion
lenses.

Keeping
object
glasses.

Corrections
of object
glasses,

78 THE MICROSCOPE

become dirty, 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 small 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. Ifa 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. Ifa lens, such
as a bull’s-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. Ifa 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 will 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 8°
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.

/Lhe 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 °7tion-
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 1}-inch (40-mm. and 32-mm.), 2/3-inch
(16-mm.), or even 1/3-inch (8-mm.) 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 (4mm),
1/8-inch (3-mm.), 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 effect 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, fover sass.
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.

| 1/6in. (4mm.) .|-008] -2 |-0074

Correction of
cover glass
thickness by

80 THE MICROSCOPE

 

140 mm | 150 mm. 160 mm. | 180 mm. | 200mm. | 240 mm.

 

 

 

 

Length of

Drawtube. Corresponding Cover Glass Thicknesses,
in. | mm. in. |mm.]| in. [mm.| in. |mm.| in. |mm.| in. | mm.
1} in. (32 mm.)1.}200/5 —|+140 | 3-5 }-100| 2-5 |-070]1-8 |-040 |-1_ | — | —
2/3 in. (16 mm.)1.|.100| 2-5 |-085 | 2-1 |-070] 1-8 |-050)1-3 |-080 |-08 | — | —
1/3 in. (8 mm.)1 .|-018| -46/-012 | -31/-007] -17|-004| -1 | — | — | —|—
+19|-007| -17|-006| -15}-0053]-135)-004| +1

cs 1 t

 

 

 

 

 

 

 

 

 

 

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. .

It will occur to the reader that the correct tube length can
be ascertained by observing the image through the microscope.

observation. This is 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

 

Fie. 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 1/6th-inch (4-mm.) is orrection
sometimes made with a correction collar, which is an adjustment 4
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 -Ol 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 effects 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

6
Sizes of
field.

Standard
screw of
object
glasses.

Variety of
object
glasses
required.

82 : THE MICROSCOPE

of view with various object glasses at the standard tube length,
and the working distance of each lens.

 

 

‘Eyepieces, Working Dis-
Object Glasses. x 6. x 10. |x 15 Field) x 17. x 25. | tance, x 10
of View. Eyepiece.
in. in. in. in. in. in.
14in. (40mm.)1. +25 “16 “115 fll ‘1 2-05
3 in, ere : 2 ‘ll “09 085 -075 +88
a3 im t16 Sen }-08 | -o45 | -035 | -o32 | -o3 {| ff
ie in, (14mm.)1 | -07 04 03 028 025 07
1/3 in. (8 mm.) . -06
1/3 im, (8 mama, /Y-045 | 025 | -02 | -o19 | -o15{] “O87
1/6 in. (4mm.) . 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, -1 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 1/12th 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 gps,
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 quality 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-5N.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 ao
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 (-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. Conversely, 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 call 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.

C—to

 

‘D—to

 

F——to—G

 

Hard crown . ‘ ; 295 705 568 |

 

 

 

Medium flint . . . 285 715 680 |

 

 
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:

1-00000
-99963

1-00000
1-00165

QnA

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; //6is avery 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. Nowif we make a similar diagram
to the last, but of fluorspar and light baryta flint, we get:

 

 

 

 

 

Cc to D to——F to——G
Fluorspar : . . 296 704 583 |
Light baryta flint . ‘ 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 :

Oa : . : . 1-00000
Ds. ; ; : .  1-00000
Eos : ; .  1-00000
G. si : . -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.

5

a
j

3
14-inch
object glass.

2/3-inch
object glass.

1/8-inch
object glass.

86 THE MICROSCOPE

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 correctedfor all 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.

CHARACTERISTICS OF OBJECT GLASSES OF DIFFERENT PoweErs

The 14-inch (40-mm. or 32-mm.) object glass gives 2 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 (64 mm.). The apochromatic
16 mm. and 14 mm. have working distances of +11 inch (3 mm.)
and -07 inch (1? 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 (13 mm.). It has a working distance of
about -06 inch (13 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 1/6-inch
view of -02 inch (1/2 mm.) and has a working distance of -024 inch P89.
(1/2 mm.). Itis 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- 1/8-inch
mum field of view of -015 inch (-375-mm.) and a. working distance of °i°t els
-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 atits 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.
1/12-inch
object glass,

Standard
size.

Best
eyepieces
for general
work,

88 THE MICROSCOPE

The 1/12-inch (2-mm.) oil-immersion object glass has a maxi-
mum field of view of -0085, 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 Hyepieces
No. Focal Length. Magnifying Power.
3260 ‘ ; 42 mm. x 6
3261 2 . 25 mm. x10
3262 ; ‘ 17 mm. x15
Compensating Eyepieces
3266 . . 45mm, x 6
3267 : : 30 mm. x 8
3268 ‘ . 22 mm. x11
3269 ‘ 3 15 mm. X17
3270. . 10mm. xX 25

All eyepieces are made to the diameter of the Royal Micro-
scopical Society’s No. 1 Standard Drawtube, -917 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

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 Oompensat-
apochromatic object glasses, and when of a higher power than ™®°¥***.
15 mm. are the best for use with achromatic object glasses. The
difference 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. ee
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 @SqIINNUiIPS=_o% ©
powers and placed in such positions that 4 ir
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 A

that an‘ actual image is formed behind _ ffl

the eyepiece instead of a virtual image

projected in front of the observer's eye. av

The corrections of an eyepiece need ‘
not be of so perfect a character as those Fia. 90.
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
Advantage
of complete
set.

Histology
and
pathology.

Biology.

90 THE MICROSCOPE

screwing the upper lens cell up and down into the 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.

THe SELECTION OF OBJECT GLASSES AND EYEPIECES

Where price is not an object, it is advisable to have a complete
set of either achromatic or apochromatic object glasses, including
only one of the 2/3-inch apochromatic and one of the 1/6-inch
apochromatic. Hach 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 x10,
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-immersion, adding a 14-inch (32-mm.) for very low-power
work if required. In such.a case he should have three eyepieces,
x6, X10, and X15, 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.
OBJECT GLASSES AND EYEPIECES 91

The difficulties of using an oil-immersion lens, however, chiefly
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 14-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 14-inch metaturgy.

(382-mm.), 1/3-inch (8-mm.) and 1/12-inch (2-mm.) oil-immersion.
For photography the apochromaticseries 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 14-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. °#4o™
If only two object glasses are required he should begin with a
14-inch (32-mm.) and a 1/3-inch (8-mm.) and three eyepieces.
Essential
qualities.

Base and
pillar.

CHAPTER VI
THE MICROSCOPE STAND

In all microscopes certain characteristics are important. The
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.

The base and pillar of the microscope have been a subject of
long 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 pillar 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 MICROSCOPE STAND 93

factory with rapidly-moving machinery, 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 44 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 Boay 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

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 not 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-
clined to about 45° is very com-
fortable for prolonged work.
Limb. 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

Joint.

 
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 Mirror.
how far it is placed below the stage; a large mirror close to the
stage is of no advantage. A 2-inch diameter mirror 34 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 1} 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. standara
The first three of these forms are the same except as regards their ™TS°P*s-
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 tne 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
The limb.

The
mechanical
stage.

The body.

The

96 THE MICROSCOPE

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 3 x 1slip. The standard
body ‘of the microscope is of rather larger diameter than is usual
with the ordinary small body tube. The coarse adjustment is

adjustments. 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-

The hand milled head. The substages are all interchangeable, and
substage. all the microscopes are supplied with the holes necessary for the
fitting of any of the various forms of substage which canbe attached
by the user with the aid of ascrew-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 :
Dimensions Size of base 2 64 x 4x1 in,
ot the Distance of upper surface of stage from table z ‘ ”
microscope. The height of the microscope in use when vertical . . 12h,
The height of its optical centre when horizontal . « CF 5
Diameter of coarse adjustment milled heads . - r 13 $9
Diameter of fine adjustment milled heads. ‘ ‘i $ os
Travel of coarse adjustment : 3} 3
Each division of the fine adjustment moves the body ‘ ‘Ol mm.
Travel of fine adjustment 6 i
Diameter of object glass screw, Royal Microscopical
‘Society’s Standard . ‘8 in.
Diameter of drawtube, Royal ‘Microscopical Society’ 3
Standard No. 1 ‘ ‘917 ,,
Diameter of Peeves Royal Microscopical Society’ 8
Standard ‘ < ‘ ‘ 3 1-527 ,,
Diameter of mirror . 2 , is i ‘ s 2 %»
Focal length of mirror 5 : ; ‘ 2 s 85 4
Vertical travel of mirror. 3 - . ; - 15,
Tube length ¥ 140 to 200 mm.
Outside diameter of upper portion of drawtubo : 1:05 in,
Diameter of object glass stem 3 < 65 ,,
Distance from optic axis to inside of limb . . . 8 as
No. 3210.

No. 3210, page 97, is the simplest form of the microscope,
and has a plain tubular substage into which slides a fitting with
 

Fra. 92.—No. 3210, Standard Lo ndon Microscope, with plain tubular
substage and dust-tight double nosepiece.

7 97
 

Fia. 93.—No. 3211, Standard London Microscope, with screw

focussing swing-out substage, dust-tight double nosepiece.

98
 

Fira. 94,—No. 3213, Standard London Microscope, with rack and
pinion and centring swing-out substage, triple nosepiece, mechanical
stage. Ff
99
No. 3211.

No. 3213.

Portable
microscope,
No, 3221,

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 (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 until 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 affect 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 (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 thé 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 MicroscorE

This microscope has been designed for the use of the micro-
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
packing 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 114 X
8 X 24 inches. It is as perfect an instrument in every way

 

}F1a. 95.—No. 3221, Portable Standard London Microscope
in case,

as |the ordinary model. The /case 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,
 

Fie. 96.—No. 3221, Portable Standard London Microscope, with
rack and pinion, and centring substage, mechanical stage, triple nose-
piece, and condenser.

102
 

Fie. 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
No. 3201,

104 , THE MICROSCOPE

Tue StanpDaRD Lonpon Microscope with CIRCULAR
Roratine Centrine STace

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.MLS. 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.

Tue Massive Mopet Microscore

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. Itis intended
for those who feel the want of a very perfect instrument. It has
been made throughout on a very heavy and stiff 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
 

Fic. 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 ‘1mm. Each division on the milled head is equal
to 001 mm. The coarse adjustment milled heads are very large,
enabling a finer adjustment to be made. The stage is square,
measuring 44 x 48 inches. In the simple model itis flat, witha
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 14 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.

Tue BrnocuLaR MicroscorE

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 (O, 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 MICROSCOPE

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
Fra. 100. 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 6 c
to be insufficient for satisfactory vision. : !
The light in one eye is only of about A
one-sixth the intensity of that of the | :

other. es Le

The Abbe binocular eyepiece, which a
acted optically on the same principle in

not bisecting the beam into two halves, Fra. 101.

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 23,
and, while not curing this defect, introduced a further 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
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 AB. 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. ‘

 

 

 

 

 

Fia. 102.—Diagram showing paths of light through the Beck Binocular

Microscope.
(A) Eyepiece. (M) Substage focussing adjustment.
(B) Drawtube. QS) ~~ ‘Mirror.
@) Body. (O) ‘Pillar.
(E) Coarse focussing adjustment. (P) Base.
(F) Prism box knob. (zy) Object.
(@) Sliding prism box. (2’ y’) Image formed by object glass.
(H) Object glass. (z” y”) Virtual image formed by eyepiece.
(I) Stage. (22) Ramsden discs—conjugate images of
(K) Substage condenser. back equivalent plane of object

(LZ) Iris diaphragm. glass,
Resolution.

110 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.

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

Fie. 103. which forms each point of the
image. Suppose the lens O
(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 DB which
forms the image; this cone of light has an exact ratio to the
angle ACB of the light which enters the lens from each point
of the object, and it is by means of the angle ACB 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 ADB, It is evident that if the cone
of light ADB be bisected and the complete half ODA be
used to form the image received by one eye and the complete
half ODB 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 O 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

pass through and part to be reflected into the 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 would 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 Bqual

(Fig. 104) can be regulated according to the amount of silver that maa
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 mquai
distance from the surface E A, where the beam of light is divided optical
into two portions, to the two eyepieces is not of equal length,

the light on the right-hand side has to
Se travel a distance GH farther than the

“[===> light that passes directly through. It

|" AB would, therefore, not be possible to focus
both beams of light to the same points in

Fie. 105. 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

 

Fie. 104,

oth
images.

 

 

 

 
112 THE MICROSCOPE

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. Itis 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

irs, the 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 33 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
can be employed.

The interocular distance is varied by turning the milled head
on the direct tube of the microscope (Fig. 108), this causes both
drawtubes to move in or out and alters the distance between the
oculars from 2 inches to 24 inches, which, as the observer’s eyes
cannot be in contact with the eyepieces, represents interocular
distances of about 24 inches to 22 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
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
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 O (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

of object
glass or
eyepiece

The
interocular
distance,

Binocular
vision,

Stereoscopic
vision,
114 THE MICROSCOPE

reaches 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 O 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 Ramsden 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 115

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 off 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 Fie. 107.
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. Themovement 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 supplied 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
because almost all objects for which they are used require
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,

 

Metal-
lurgical
microscopes,
 

Fie. 108.—The Binocular High- and Low-power Microscope as
applied to Stand No. 3213.

116
 

Fie. 109.—No. 3227, Metallurgical Microscope, with rack and pinion
focussing stage, object glass,,and vertical illuminator.

117
lurgical 4, Fic. 110.—No. 3225, Standard London

118 THE MICROSCOPE

The Beck-Rowley Metallurgical Attachment (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. 111) 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

5 : ordinar microscope

a eseeee and Metallurgical Attach- enito id abident een.

° : 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 meaas 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 illumination.

 
THE MICROSCOPE STAND 119

__ It will be seen from the illustration that 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 off; 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. 111) has no pillar and the bench
base. It has a limb carrying the body with the usual coarse and fine ett,
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. Torender
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.

 

Fie. 111.—No. 3226, Metallurgical Bench Microscope with prism
illuminator and electric lamp and fittings,

Petrological A petrological microscope is essentially an ordinary microscope
microscoPes- provided with a 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 willbe very small. If the analysing
Standard
petrological
microscope.

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 suffers
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.

The Standard London Petrological Microscope is made in two
forms (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 ina 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,
 
  

  
  

al

Ser

 

 

No. 3223.

Fie. 112.

No. 3222, Petrological Microscope Nosepiece Analyser.
No. 3223, Petrological Microscope Eyepiece Analyser.

123
Circular
mechanical
stage.

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. HE. 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 23° 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 off 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.

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 substage apparatus is supplied in slightly
longer mounts to accommodate for the extra thickness of the
mechanical stage.
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

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
J Protozoa, quite as simple a name as kangaroo
Fre. 113.— When you become accustomed to it, and to

Amoeba, _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 Amceba, If the Ameba 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 slowly dissolved.

The Ameeba feeds by literally putting
itself outside its food. When the victim %
has been dissolved, the hard and insoluble ye. 114,—Villous
parts are allowed to escape back into the Ameeba.
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

 

   
THE MICROSCOPE AS A RECREATION 127

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 nothing more or less than a good-sized bubble, called
the Vacuole.

The Ameba, 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 Fic. 115.—Difflugia.
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 Amceba
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

Fra. 116,—Heliozoa. 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.

 

 
128 THE MICROSCOPE

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

. i water in which they live. These

\ \\e shells take myriads of different

WN : forms, but have one thing in com-
SSS mon: they are perforated with
SSN multitudinous holes through which

EER SS. slender threads of jelly exude. To

LE Ii} NS this family belong the shells which
ce f || . form chalk. Innumerable numbers

\ of these tiny creatures fall, as they
die, to the bottom of the ocean,
Fig. 117.—Foraminifere. 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 preg 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 Amceba. 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.

f, A further stage shows the de-
Es velopment of a flagellum, or whip,
Cj &

<Eeg

Wi
eZ
zz

  

{

 

 

which is formed by the drying up

PB and hardening of the pointed end of
—s-————— the body. The flagellum vibrates, and
by its aid the creature can swim

& about with considerable rapidity. In-

“< numerable forms of these Flagellata

Fie. 119.—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

   
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 flagella, causing
a current which brings particles of food within their reach.

 

Fie. 120.— Fie. 121.—Collared Fie, 122.—Collared
Monad. Monad. 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 haverowsof hairs.
If we imagine the soft,

; jelly-like exudations of Fie. 124.—
Fia. 123.—Ciliata. the Heliozoa to be  Vorticelle.
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 brilliantly coloured, some perfectly transparent.

9

  
130 THE MICROSCOPE

The Vorticelle 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

Stentor. 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-
ming apparatus. It is now interesting to examine the question
of their feeding from the same point of view.

The Amceba 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 Amceba to be constantly
filling itself up with useless material, neither would
it be satisfactory for it to be continuously diluting 5, jog
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 Amceba 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 well 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 Fie. 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 Vorticelle (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 voracious

 
132 THE MICROSCOPE

creature, has also a large disc-like surface where there is prae-
tically no skin to its jelly. The cilia which vibrate around this
dise 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 all 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 flagelle
causing a current of water to rush through
every cavity of the entire sponge, in order
to provide the food and oxygen necessary

Fie. 128.—Sponge. 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

 
THE MICROSCOPE AS A RECREATION 133

there is a history in an abbreviated form of the stages through
which the species originally developed. The so-called water-
fleas, for instance, are little crustaceans like 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,
antenne, tail, and other Fic. 129.—Holopedium.
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

Fie. 130.—Bytho- delicacy beloved by the salmon trout and

trepes. 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.

 

 
134 THE MICROSCOPE

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 different 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 skin;
and the canal being opened by the removal of the

: knob, the poison that it contains can enter the
Fre. 131. prick made by its sharp point.
Nettle Hair. 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.

 
No.
3210

3211
3212
3213

3214

3215

3216

3217

3221
3227
3225

3226

3222

PRICE OF INSTRUMENTS AND APPARATUS
DESCRIBED IN PREVIOUS PAGES (1922)

Page
97

93
100
99

104

104

103

103

116
101

117

118

120

123

MICROSCOPE STANDS

Standard London Microscope, with plain sub-
stage, with iris diaphragm; stand only, in
case

Standard London Microscope, with ‘spiral scrow
focussing substage; stand only, in case

Standard London Microscope, with rack and
pinion substage; stand only, in case . :

Standard London Microscope, with rack
and pinion centring focussing substage; stand
only, in case

Standard London Microscope, with standard
body, circular rotating stage, screw focussing
substage ; stand only, in case

Standard London Microscope, with standard
body circular rotating stage, rack and pinion
focussing and centring substage ; stand only,
incase .

Standard London Microscope, with circular
rotating stage, large 2-inch body, rack and
pinion focussing and centring substage ;
stand only, in case

Standard London Microscope, with circular
rotating stage, with double extension rack
and pinion drawtubes, rack and pinion
focussing and eee substage; stand only
incase .

Binocular body in place in ordinary body on
Stands.

Standard London Portable Microscope, with
rack and pinion focussing and centring
swing-out substage and mechanical stage, in
case .

Standard London Metallurgical Microscope ; ;
stand only

Standard Microscope, with Rowley metallur-
gical attachment; stand only, in case

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.

Standard Petrological Microscope, with nose-
piece analyser, eyepiece with cross-wires,
polarising and analysing prisms, converging
system of lenses, Becke lens, mica }-wave
plate, and Klein’s ae plate; 3 stand any,
incase . :

136

10 10
11 15
13 10

14 10

14 6

17 0

19 0

23 0
18 0

28 0

15 10

23 2

13 10

29 0
136

No, Page £8
3223 123 Standard Petrological Microscope, with eye-
piece analyser, with cross-wires, polarising
prism; stand only, i in case, . 24 7
— 122 Independent swing-out and focussing arm,
which enables the condenser to be swung out
of the axis : extra 1 5
3352 122 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 eyepieceslot . 3 15
3351 122 Plain quartz wedge, ungraduated . . » 215
3350 122 Quartz wedge, cemented on gypsum plate (red
Ist order), graduated in Retardations . 410
3218 116 High-power Binocular Mic1oscope, with square
stage and rack and pinion focussing and
centring substage ; stand only, in case 32 10
116 Stand as No. 3218, but with detachable
mechanical stage . 38 10
3219 103 High Binocular Microscope, with plain circular
1otating stage and centring adjustments ;
stand only, in case 35 0
32198 ei Microscope as No. 3219, with addition of
circular rotating mechanical stage; stand
only, in case 47 0
103 Interchangeable extra ‘monocular ‘large body,
104 with rack and pinion drawtubes extending
to 260 mm. 7 0
3200a 104 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 44 10
3201la 104 Massive Model Microscope, with mechanical
stage as illustrated on page 105, with dry and
immersion achromatic condenser and focuss-
ing dark ground illuminator and one extra
substage slide ; in case . wm 2
— 116 Interchangeable high-power binocular body
extra 20 0
— 103 Double extension drawtube, with rack and
pinion adjustment . 3 : extra 5 0
NOSEPIECE AND OBJECT GLASS CHANGERS
3300 20 Dust-tight double nosepiece : 17
3301 20 Dust-tight triple nosepiece 1 10
3280 20 Sloan object glass changer, adapter, spanner,
and two fittings ‘ ‘ 1 7
3281 20 Extra fittings for above : : each 0 5
3282 20 Case to hold three fittings, with object ee
attached in dust-tight spring holders . 011
% OBJECT GLASSES AND EYEPIECES
3230 77 14-in. Achromatic object glass (32 mm.). 2 5
3231 77 2/3-in, Achromatic object glass (16 mm.) 1 10
3232 77 1/3-in. Achromatic object glass (8 mm.) . 4 5
3234 77 1/6-in. Achromatic object glass (4 mm.) . 315.
3236077 1/8-in, Achromatic epee Elase 8 mm.), Oil-
immersion . . 6 17

THE MICROSCOPE

d.

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PRICE OF INSTRUMENTS AND APPARATUS 137

No. Page &s d
3235 77 1/12-in, Achromatic object glass (2 mm. i Oil- .

immersion fs 810 0
3240 77 = 14-in. Apochromatic object glass (40 mm, .) 410 0
3241 77 2/3-in, Apochromatic object glass (16 mm.) 715 0
324la 77 2/3-in. Apochromatic object glass (14 mm.) 715 0
3242 77 1/3-in. Apochromatie object glass (8 mm.) 910 0
3244 77 1/6-in. Apochromatic object glass (4 mm.) . ll 0 0
3245 77 1/6-in. Apochromatic object glass (4mm. ), with

correction collar ‘i 12 0 0
3248 77—«:1/12-in, Apochromatic object glass (2 mm. ei Oil-

immersion : . 18 0 0
3260 88 42mm. x6 Huyghenian eyepiece : 012 0
3261 88 25-mm. x10 Huyghenian eyepiece 012 0
3262 88 l7-mm. x15 Huyghenian eyepiece 012 0
3266 88 45-mm. x 6 Compensating eyepiece 2 2 0
3267 88 30-mm. x8 Compensating eyepiece 22 0
3268 88 22-mm. x11 Compensating eyepiece 210 0
3269 88 15-mm. x17 Compensating eyepiece 210 0
3270 §=88 10-mm. x 25 Compensating a ad 210 0
3275 68 Beck micrometer eyepiece 22 0
3263 73 Eyepiece with indicator 018 0
3264 73 Eyepiece with cross-wires 017 0
3273 73 Erecting eyepiece 110 0

LARGE SIZE EYEPIECES 1:41 INOH DIAMETER OF TUBE
3257 85 Eyeshade . 020
3253 88 42-mm. x6 Huyghenian eyepiece . 2 5 0
3254 88 25-mm, x10 Huyghenian eyepiece 2 5 0
3255 88 17-mm. x15 Huyghenian eyepiece 2 5 0
APPARATUS FOR ILLUMINATING OBJECTS

3285 26 Small Abbe condenser, fitting with iris dia-

phragm on Microscopes No, 3210 and 3211 . 1 5 0
3285p 33 Set of patch-stops for above . 07 6
3286 26 Large Abbe condenser in fitting with iris dia-

phragm and swing-out tray for colour screens

with coloured and ground glass . ». 210 0
3286p 33 Set of three patch-stops for above . 0 7 6

3287 27 Beck dry achromatic condenser, 1 N.A., lenses

only in mount, with standard object glass,

screw thread . 4 5 0
3288 27 Beck dry achromatic condenser, 1 N. Mm com-

plete in fitting with iris diaphragm and Pen

out tray, grey and green glass. 515 0
3288p 33 Set of three patch-stops for No. 3288 or 3290. 0 7 6
3291 28 Beck dry and immersion achromatic and apla-

natic condenser, 1-3 N.A., in mount, with iris

diaphragm and tray for patch-stops and with

ground and green glass poner Mere

moderator ; 915 0
3284 33 Traviss expanding iris patch-stop . ¢ 012 6
3295 34 High-power dark-ground illuminator, optical

portion only in mount, with standard object

glass thread . 2 0 0
3296 34 Mluminator as above in plain substage fitting . 210 0
138 THE MICROSCOPE
No. Page

és 4

3297 34 ITluminatorasabovein centring substage fitting 3 5 0
3298 34 Stop to fit 1/12-in, oil-immersion object glass to

reduce aperture for dark-ground illumination 0 2 6
3293 35 Beck patent focussing dark-ground immersion pie

illuminator in plain substage fitting 5 71 6-
3294 35 Ditto, in centring substage fitting . : 6 2 6
3215 38 Bull’s-eye condenser, 2}-in. diameter, on heavy .

stand, with full adjustments : 210 0
3216 38 Bull’s-eye condenser, 1}-in. diameter, on

smaller stand with full adjustments . 15 0
3360 40 Parabolic reflector : ‘ 22 0
3361 40 Parabolic reflector with Sorby reflector 7 217 6
3362 40 Thin glass reflector 3 ‘ 17 6
3363 41 Thin glass vertical illuminator 17 6
3364 41 Prism illuminator. 117 6
3328 32 Double-wedge light moderator 317 6
3366 43 Colour trough ‘ 210 0
3335 44 Paraffin lamp 215 0

3330 46 Beck electric lamp, complete with bull’s- -eye
condenser, ground glass, signal-green glass,
and metal filament 60 candle-powerlamp .
3331 46 Beck electric lamp as No. 3330, but with 100
’ candle-power half-watt lamp. 9 00
3332 46 Beck electric lamp as No. 3330, but with
“ Pointolite” electric lamp, with resistance to
work off direct current from 100 to 250 volts 1415 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-length,
6,300 ; red, dom. aa 6,500 ; neutral

ao
~
Oo
o

tint, passing 1/10 : 312 6

— 42 Single filters, as above A / each 0 6 6

—_ — Stand to hold two filters, as above ‘ 110 6
— Circular screens to fit euiatese rings can be

supplied ‘ : .each 0 5 6

3337 48 Incandescent gas lamp — : . 2 65 0

— Extra mantles . 01 0

3338 48 Incandescent spirit lamp . 217 6

— Extramantles. < each 0 1 0

3336 45 Electric lamp on stand to take ordinary bulb. 110 0

APPARATUS FOR HOLDING SPECIMENS

3307 51 Sliding ledge . 014 0
3305 99 Detachable mechanical stage, as illustrated on
Microscope No. 3213 (page 99) .  . 60 0
3306 62 Concentric rotating mechanical stage, as illus-
trated 12 0 0
3400 52 Glass slips, 3 x 1, best quality, ground "edges,
approximate thickness 1 mm, per doz 0 0 9
pergross O 8 6
3401 62 Ditto, second quality, groundedges perdoz 0 0 7
pergross 0 6 6
No.
3405

3390

3391
3392

3393
3394

3395
3388

3406
3409
3410
3412
3413
3414
3415
3416
3420
3321
3421
3422
3425

3386
3325a

3325b

3325¢
33258

3222
3223
3224
3274
3384

1290

1292

PRICE OF INSTRUMENTS AND APPARATUS

Page
55

53

61
59

63
62
64
64

64

Glass slips, 3 x 1, a ote and excavated
hollow . - per doz,
Cover glasses, No. 1, average thickness -006,
circular . ‘ per oz.
Ditto, square é per oz.
Cover glasses, No. 2, average thickness -008,
circular . i per oz.

Ditto, square -

Cover glasses, No. 3, ‘average thickness 01,
circular

Ditto, square a

Micrometer screw gauge for measuring thickness
of cover glass slips, etc, “i :

Glass slips with ledge . ; : 3
Cells, metal, circular. : : per 100

Cells, glass. A . per doz.
Troughs on 3 x 1 slips, small ; :
Troughs on 3 x 1 slips, larger

Troughs on 3 X 14 slips

Tirough, 14 x 14 X fin.

Beck’s glass trough

Live box

Rousselet’s live box

Beck compressor .

Stage forceps .

Case of apparatus for hol ding ‘obj ects, including
3 slips, 2 slips with hollow, slip with ledge,
trough on 3 xX 1 slip, Beck’s glass trough,
Beck’s ‘compressor, stage SOReEs, live box
and thin glass %

Xylol . ‘ 5 ‘ ‘ 4 "2 oz,
Benzol_ : . 202,
Canada balsam, pure, in benzol or xylol ‘25 er.
Hollis glue . : ; . per bottle
Turntable

Thoma-Hawksley Hemacytometer, with two
Pipettes and covers in case.

Ditto, but with Zappert, Tiirck,  Fiichs and
Rosenthal, or Breuer ruling ‘

Ditto, Biirker model

Hemacytometer 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 } mm., $mm., 1 mm., or
2mm. . each

Counting chamber for use with chess-board

Pipette for red corpuscles ; .

Pipette for white corpuscles .

Microspectroscope eyepiece

Simple warm stage A

S.LR.A. wax, per stick 6 < tin. ‘

Beck grinding and polishing machine for
preparation of metallurgical specimens ;
standard machine without motor, with one
polishing disc, cover, catcher, disc-lifter, 12
feet connecting wire plug eee and tin
of grease . ‘ .

Extra polishing dises

Motors fitted at makers’ current prices.

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1165

139

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140

No,
3429
3430

3431
3432
3433

3434
3435
3436
3437
3438
3439
3446
3441
3442
3443
3444
3445
3446
3447
3448
3449

3450
3451

3452

3453
3454

3455
3456
3457
3458
3459

3460

3461
3462
3463
3464
3465

Page
65
65

|

ee ae a ee

| |

66

66

66
66

THE MICROSCOPE

Pipettes. . é :
Pipettes, glass, with teat é . each

DISSECTING INSTRUMENTS

Blow-pipes with stiletto 7 :

Razor, hollow ground one side, flat the other :

Scalpels, with ebony handles, in three sizes of
blade 5 . 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, Linch, with fine ‘points

Scissors, straight, 44 inches .

Scissors, straight, 6 inches

Seekers in ebony handles

Chain hooks

Forceps, without guide pin, 4h inches

Forceps, without guide pin, 6 inches 7

Forceps, with guide pin, 24 inches and 4 inches .

Forceps, Cornett’s cover glass

Section lifter, copper

Section lifter and seeker combined, ‘nickel
plated .

Metallic hone forsharpening razors ,scalpels, ete.

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 ‘

:

.

COLLECTING APPARATUS

Tow-net for collecting marine specimens, 11
inches diameter, with fine muslin bag and
bottle attached, with 24 ee stout cord on
wood frame. ‘

Ditto, made of bolting silk

Dredge for bottom sea collecting, canvas and
net bag; 10 inches :

Ditto, 14 inches

Ditto, 16 inches .

Ditto, 18 inches

Ditto, 24 inches

Collecting stick with inner "lengthening rod,
total extension 5 feet 6 sieht polished cane
with crook handle

Net ring for attaching to " above, 5 inches
diameter, with bolting silk net and bottle

Ditto, but with 6-inch diameter net.

Flanged tube for collecting nets

Cutting hook for cutting weeds

Collecting bottles, 6 inches xX 1 inch per doz.
4inches X linch . : -  perdoz.

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18

10
17

13

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SCAAQADMD BD
No.
3466
3467
3468
3469

3279
3277

3278
3276
3480

3265
3275
3368
3369
3370
3371
3375
3358

3345
3342
3340

3343

3483

PRICE OF INSTRUMENTS AND APPARATUS 141

Page

Collecting bottles, 3inches x linch . per doz.

3inches X #inch . . per doz.
2 inches x 5 inch. 4 . per doz.
1g inch x } inch : 7 - per doz,

SUNDRY APPARATUS

Glass plate ruled in squares .

Stage micrometer with engraved scale, 1/ 10 and
1/100 of a millimetre

Stage micrometer with engraved scale, 1/100
and 1/1000 of an inch

Eyepiece micrometer, glass plate to fit into
eyepiece .

Glass scale with 100 divisions ‘etched on in n milli-
metres for use in making drawings

Cross-wire for eyepiece. x

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, }-plate size F :

Extra double plate-holders

Photomicrographic camera, horizontal type,
}-plate size, with one dark slide .

Extra double plate-holders

Focussing glass

Hand centrifuge with twotest-tubesinaluminium
covers ‘ ‘ ‘ :

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10
12
12
10
10
17
10
15
15
10
13
15

13
15

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INDEX

Abbe cameras lucida, 71

x» condenser, 26
Achromatic condenser, 27
es object glasses, 76
Acineta, 131
Adjustable patch-stop, 33
Adjustment, focussing, 18, 93
<3 of vertical illuminator, 41
Amosba, 126
Angle of aperture, 14, 16, 17
Aperture, 14, 15, 16, 17, 77
Apochromatic object glasses, 83
Apparatus for holding objects, 50

% sundry, 67

Base of microscope, 92
Beck binocular, 107
»» compressor, 57
», glass trough, 56
Bench metallurgical microscope, 119
Binocular, Abbe, 108
7 Beck, 107
5 illumination with, 111
oy optical path, 111
oy Powell and Lealand, 108
3 resolution with, 110
a short tube, 112
5 Wenham, 107
Biology, best object glasses for, 90
Blood counts, 58
» films, 54
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 , 7

Camera lucida, 69
» _ photomicrographic, 74

Case of apparatus for holding objects,
58

Cedar-wood oil, 17
Cells, 55
Cements, 58
Centrifuge, 63
Centring of dark-ground illuminator, 34
5 of light, 22 :
6 of object glass, 21
3 of rotating stage, 52
a of substage condenser, 29
Ciliata, 129
Circular stage, 52, 120
Cleaning cover glasses, 54

142

 

Cleaning object glasses and eyepieces,
76

Coarse adjustment, 18, 93
Collecting net, 66

Colour screens, 42, 81
Concave mirror, 23
Condenser, Abbe, 26

35 achromatic, 27

53 bull’s-eye, 24, 38

% for dark-ground illumina-
tion, 27, 33

a immersion, 28

35 substage, 24, 26

a 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, 57
Correction collar, 81

3p for cover glass, 79, 80

5 of object glasses, 78
Counting blood corpusc’es, 58
Cover glass corrections, 79, 80

8 » thickness, 16, 53, 54

Cross-lines in eyepiece, 73
Culture plates, 62

Dark-ground illuminator, 34
Depth of focus, 18
Diameter of eyepieces, 88
Diatom periodic structure, 25
Difflugia, 127
Dimensions of microscopes, 96
Direction of light, 22
Dirt on lenses, 76
Dissecting instruments, 73
Divergent light, 23
Double wedge light moderator, 32
Drawing table, 71

35 with the microscope, 69
Drawtube, 14, 93, 103
Dry mounted objects, 36, 58

Electric lamp, 45
Entomostraca, 133
Erecting eyepiece, 73
Eyepiece, 11, 13, 15, 88
35 compensating, 89
5 erecting, 73
” for photography, 89
” Huyghenian, 89
” micrometer, 68
INDEX

Eyepiece, projecting, 89
a5 with oross-lines, 73
»» indicator, 73
Eyepoint, 13
Eyeshade, 73 |

Field, flatness/of, 18, 81
» of view) 14, 82
Films, blood,
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
53 best method of, 19
5 dark-ground illuminator, 35
55 glass, 75
3 substage condenser, 28
Foraminifera, 128
Forceps, stage, 57
Freehand drawing, 67

ao
Qo

 

Gas lamp, 48
Glass cover, 53
» focussing, 75
» plate ruled in squares, 61
» Slip with ledge, 54
», Slips, 52
Grinding machine, 64

Hemacytometer, 58

Hair of nettle, 134

Heliozoa, 127

High-power illuminator, dark-ground,
34

Histology, best object glasses for, 90

Holding specimens, 50

Holopedium, 133

Tllumination, 21, 23, 28

” intensity of, 37, 43, 44
” opaque, 38, 39
” with condenser, 26, 38

” » dark-ground, 34
»» mirror, 23
Tiluminator, parabolic, 40
vertical, 40
Image ‘formed by eyepiece, 9, 13
at os »» object glass, 9, 11
» incorrect, 31
Immersion condenser, 28
5 fluids, 17
5 object glasses, 17, 34
Inclined position of microscope, 50
Indicator eyepiece, 73
Initial magnifying power, 68,77
Instrument, dissecting, 73
Intensity of light, 44
Tris diaphragm, 23, 26

Joint of microscope stand, 94

Lamp, electric, 45, 46
wt §=—gas, 48

 

143

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
Ss binocular, 107
53 massive model, 104
2 metallurgical, 115
x petrological, 120
3s portable, 100
oS Standard London, 95
stands, 9, 92
Mirror, 23, 28, 95
Moderator, light, 32
Monad, 129
Mounting cells, 55, 58
3 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

Obj. ect, scattering of neh by, 24

Objects, flattening, 54, 5

Oil-immersion, 78

Optical quality 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

Phone eneon Env camera for, 74
eyepiece for, 89

Pillar of microscope stand, 62

Pinhole aperture, 12
144

Pipettes, 59, 66

Plates, culture, 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
55 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

Scattered light, 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
a mounting, 58

Spirit lamp, 48
Sponge, 132
Spores of fern, 134
Squares ruled on glass plate, 67
Stage clips, 50

»» forceps, 57

 

 

INDEX

Stage, mechanical, 51 '
» rotating, 52
» warm, 62
Stand of microscope, 9, 92
Standard London microscope, 95
object glasses, 77
of magnification, 13
screw, 82
thickness of cover glass, 53, 79
is tube length, 15
Stentor, 130 .
Stereoscopic vision, 113
Structure revealed by dark ground, 37
Substage, 8
‘i condenser, 24, 25, 26
Table, drawing, 66
of drawtube corrections, 80
», field of view, 77
» 9» 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
Turntable, 58

”»
”

Use of dark-ground illuminator, 34
»» 9», Substage condenser, 28

Varying magnifying power, 15
Vernier, 72
Vertical camera lucida, 69

<3 illuminator, 40

»  photomicrographic camera,

74

se 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 Hazeli, Watson & Viney, id., London and Aylesbury