John Phin.

How to use the microscope; being practical hints on the selection and use of that instrument, intended for beginners online

. (page 10 of 22)
Online LibraryJohn PhinHow to use the microscope; being practical hints on the selection and use of that instrument, intended for beginners → online text (page 10 of 22)
Font size
QR-code for this ebook


usual sources of unsteadiness than any increase of absolute
weight. Of course, if it is merely desired to make the micro-
scope steady, in the sense that an inkstand is steady that is,
not liable to be tipped over weight is everything. But the
stands ihat are most difficult to tip over are not those that
resist vibrations most perfectly. For the latter a tripod with a



dtf THE Micnoscoi':. 107

Stoall area of support is best; for the former a stand with a flat
base resting over its whole surface on the table should be pre-
ferred.

It is obvious that the causes of unsteadiness are either vibra-
tions transmitted from the floor, or movements caused by the
hand in performing the necessary manipulations and adjust-
ments. The first can never be stopped by weight, unless,
indeed, we make the stand so heavy that its weight will impart
rigidity to the table and floor, and this would require a good
deal more than fifteen pounds, or even twice that. For the
checking of vibrations transmitted from the floor, no device is
better than the stand or table described in a subsequent sec-
tion. So far as movements transmitted by the hand are con-
cerned, if a stand of three or four pounds will not resist them,
the observer should set himself about learning delicacy of
movement before he proceeds any further.

All microscopes made in this country and in England are
now constructed so that the body may be inclined to any angle,
thus giving the power of using the microscope in any position
vertical, inclined or horizontal. The importance of this is
easily seen when we consider that on the one hand, when liquids
are to be examined, it is sometimes necessary, or at least desira-
ble to use the microscope in a vertical position, though this is
a very tiresome and inconvenient position, and one that is not
calculated to enable the observer to obtain the best possible re-
sults; and on the other, it is equally necessary that the body of
the microscope should be capable of assuming the horizon-
tal position when the camera lucida is to be employed for
making drawings, as will be hereafter explained. And yet
Frey actually gives the preference to microscopes that do not
incline, and which must always be used in a vertical position !
This, of course, necessitates the complicated and expensive ar-
rangement which lie describes for adapting the camera lucida to
the vertical instrument, a singular instance of prejudice against
an obvious and successful improvement.

The Stage. In every case, a large, roomy stage is of the
utmost importance. One great objection to most French in-



t*ss

struments is that the stages are too small. It should also be
firm and substantial, so that its position in regard to the other
parts of the stand cannot be varied by slight pressure.* The
most important points connected with the stage are the means
provided for holding and moving the object, and the facilities
afforded for attaching accessory apparatus.

In the most complete stands, the object is held between
sliding clips, which form a sort of clamp that is capable of
being moved in two directions, at right angles to each other,
by mechanical means, which generally consist of a screw for
one direction and a rack and pinion for the other. This form,
which is known as the mechanical stage, enables even a com-
paratively unskilled person to bring any part of the object into
the desired position in the field of view, and this with the
utmost precision. These mechanical stages may be said to be
characteristic of the higher classes of English microscopes, and
as they are expensive, they are not generally used. Neither
are they absolutely necessary for ordinary work with low or
medium powers, for with any objective lower than one- twelfth
of an inch focus, the object can be moved by hand quite as
readily as by the screws, and we hold it to be a well established
rule in all manipulations connected with scientific work, that
whenever any operation can be performed satisfactorily by
means of the hands alone, all special contrivances should be
dispensed with. For low and moderate powers, therefore, we
prefer the plain stage, on which the object is moved by means
of the hands alone. But when very high powers are used, and
especially when delicate micrometrical or goniometrical meas-
urements are to be made, a well-made mechanical stage becomes
a necessity. For while it is easy enough to bring an object
very near to a given point by means of the fingers alone, it is
almost impossible to secure perfect accuracy. In the effort to
attain this the mechanical stage is a great assistance, and
therefore when Frey utters a wholesale condemnation of the

*At the same time, however, it must be borne in mind that no stage
ever was made so firm that even a slight pressure would not afiect it. If,
therefore, the reader is determined not to rest content with anything
short of a perfectly rigid stage, he will reject all the best microscopes in
market.



OF Tim MICROSCOPE.



109



English microscopes, and asserts that they are unpleasantly
loaded with what he is pleased to call "screws and unessential
appurtenances," it seems to us that he commits a great error.
These costly and complex instruments are intended for the
highest class of work, and the most powerful objectives; per-
fection of the work to be done, and not simplicity in the means
by which it is to be done, is the end sought, and this can be
attained only by the complex means employed.

We have never found any of the so-called lever stages that
fulfilled the requirements of the highest class of work, and,
therefore, if a mechanical stage is to be chosen at all, the best
form should be procured.

A microscope fitted with a good mechanical stage leaves
nothing to be desired, but when other forms are used, it is evi-
dent that the chief points to be attained are these: 1. The
object should be held steadily, but at the same time perfect
freedom of motion should be allowed. 2. It should be possi-
ble to remove instantly from the surface of the stage, every-
thing in the shape of clips and holders, so that a clear field
should be left for the adjustment of very large slides, plates,
etc., or for the rotation of the object in relation to the light.
3. Even the simplest forms of the stage should be so constructed
that it may be possible to pass every part of the object under
the field of view, and this, without any risk of omitting even
the smallest portion. This point is of special importance to
physicians and naturalists. Thus, it not unfrequently happens
that it is desirable to know whether or not certain forms are
present in a given drop of liquid; unless we can subject every
part of that drop to microscopical examination, we cannot be
sure that the forms we are looking for are absent. There is
always a risk of omitting some portion of the slide, and conse-
quently doubt must always hang over the exhaustiveness of all
our examinations. The only certain means of avoiding all risk
of missing any portion of a given slide is to pass it across the
field of view in successive parallel bands, just as a plowman
plows a field. The process is clearly shown in the diagrams
on the following page, Fig. 22, showing the mode in which the
entire surface is completely covered with a series of parallel rib-
bons, the breadth of each of winch is the diameter of the field



110 SELECTION AJfl) tfsfc

of view, while Fig. 23 shows the hap-hazard way in which ex-
aminations are usually made, abundant room being left (as
shown by the small crosses) for the escape of important fea-
tures, Now, with ordinary clips, it is difficult to effect this,





Fig. 22. Fig. 23.

although it may be done by the aid of a plate of metal or vul-
canite, a little thicker than the glass object-slide, and having a
straight edge. The forward movement is made by pushing the
vulcanite plate, while by sliding the object along the edge of
this plate, we are enabled to examine a narrow strip, the width
of the field of view, as shown in Fig. 22. A somewhat similar
device is shown on the stage of Zentmayer's Histological Micro-
scope, Plate VI. When slightly modified, this device also
serves as a guide for the Maltwood finder.

In the simpler forms of the stage, the object is held in place
by spring clips, which press it down, and under which it is
moved. These clips are frequently screwed to the stage, which
is a great mistake, as we are thus prevented from slipping them
off, so as to leave the stage entirely clear. They should always
be held in such a way that they may be instantly removed, and
they should also be very thin and springy, otherwise it is im-
possible to move the object with sufficient delicacy.

The so-called glass stage, or Zentmayer stage, has come into
extensive use, and is very much liked by some. As made by
Zentmayer, it consists of a plate of glass, held against a brass
support by means of a spring, which /is pointed with ivory.
The friction of the glass plate upon its metal support is thus
easily adjusted, and may be either so reduced that the plate will
respond to the slightest touch, or it may be so firmly clamped
as to be practically immovable.



or THE XiicBoscorfi. lii

In some forms the glass plate is stationary, and the object-
carrier moves over it, the points of contact being very small.
Such a stage is seen in the microscope figured in Plate II.

Revolving Stage. It is often desirable to rotate an
object in the optic axis of the microscope, either for the pur-
pose of measuring angles or changing the direction of the
illumination in regard to the object. Means for effecting this
with perfect accuracy have been applied both to the mechanical
stage and the glass stage, though the latter is generally so con-
structed that rotation is impossible. In the large Boss stand (PL
I), the stage rotates, and is graduated for measuring angles.

M. Nachet has devised a special form of the glass stage, in
which provision is made for rotating it. In this form of the

stage the object-carrier, F, is
held to the glass plate, E, by
means of two springs with ivory
points, the springs being at-
tached to the frame in which
the glass plate, E, is set. Both
the plate, E, and the object-
carrier rotate in a light brass
frame. In all these cases an
attempt is made to secure coin-
Fig- 24 - cidence of the centre of rota-
tion with the optic axis of the

instrument. It will, however, be found that it is an impossi-
bility to attain this object with any great degree of accuracy
unless provision is made for occasionally adjusting the centre
of rotation of the stage. No stage that ever was made will re-
main for any length of time so true that angles can be correctly
measured by it, and therefore several devices have been sug-
gested for securing accuracy without adjusting the stage, since
when well-made an adjustable stage is expensive, and when
poorly made it is worthless.

The best known of these devices is what is called the center-
ing nose-piece. This is a kind of adapter which is screwed on
the body and receives the objective. It is provided with center-
ing arrangements, and the objective having been screwed into




112

it, the latter inay now be adjusted exactly over the centre of
rotation of the stage. With common objectives this answers
very well, but first-class objectives are apt to show the effect
of being thrown out of centre.

Where rotation in regard to the illumination alone is required,
several plans may be employed. One of the oldest is that
found in the large microscopes of Hartnack and others, and
recently adopted by Mr. Browning. This device consists in
forming the stage in two pieces, the lower part being firmly
connected Avith the foot, and the means of illumination, while
the upper part, which rotates on the lower, is rigidly attached to
the arm which carries the body. In this way the body, the arm,
the upper part of the stage and the object may all be simultane-
ously rotated in relation to the illumination, and for ordinary
purposes this answers very well.

When polarized light is used, however, it is obvious that it
is impossible to rotate the object in relation to the polarized
ray, without also rotating the analyzer, and, as every one that
has worked much in this direction knows, it is often of great
importance that both polarizer and analyzer should be kept
stationary, while the object itself rotates between them.

The little diatom stage devised by the author, and shown in
Figure 26, page 114, also affords very simple and convenient
means for rotating objects either above or below the stage.
And it would be a very easy matter to so construct it, that its
centre of rotation might be accurately adjusted to the optic
axis of the instrument.

Hot Stage. It is frequently desirable to keep certain pre-
parations at a given temperature or to raise them considerably
above the usual temperature of the atmosphere, so as to observe
the action of reagents upon them and the effect of heat upon
their vitality, where living organisms are under observation.
Various devices have been worked out for this purpose. A
stage consisting of two plates, each with a central aperture,
and so united as to form a tight box through which a heated
fluid is made to circulate, is probably the best. For tempera-
tures under 212 F. water is the best liquid; for higher tem-
peratures oil or saline solutions may be used.



: 111 \i M'f



113



For heating o'ojects under the microscope, we have long
Used a thick copper wire coiled in the flat so as to lie on the
stage and with a projecting end which may be heated by a
lamp. It is not difficult to adapt a small thermometer so as to
indicate very nearly the temperature to which the object is
subjected. To regulate the degree of heat employed we pass
the end of the wire through a copper tube which is made to act
as a chimney to the lamp, and by moving this tube in or out,
the coil on which the object lies may be made more or less hot.

Stages for Special Purposes. It may be safely asserted
that there has never yet been constructed a stage which would
suit the requirements of every worker with the microscope.
Indeed, each investigator seems to require special modifications
of his own. Thus, it will be found that the ordinary stage,
with all its appurtenances, is too thick to admit the use of that
very oblique illumination which is required by the worker on
diatoms, while if the stage be made thin enough it loses the
necessary rigidity. Some makers have sought to obviate this
by supplying two stages a stout one for common work, and a
thin one for diatoms. A microscope now in our possession is
furnished with an extra thin stage, which, by a very simple
and ingenious device, can be instantly substituted for the heavy
one. The microscope is said to have been made by Spencer or
Tolles, and must have been made about the year 1856, or even
earlier. Thin stages, on the same principle, called Diatom Stages,
have been recently introduced by several makers, thus affording
another illustration of the aphorism that history repeats itself.

The same object is also attained by means of the secondary
stage, invented by Mr. Lewis Eutherfurd. This is simply a
skeleton stage, which is placed on the ordinary stage, and is
raised so far above it that the illumination may be applied
between them. Rays of great obliquity may thus be passed
through the object. Rutherfurd's skeleton stage forms also an
admirable safety stage, since the object, being held against the
under side of the skeleton stage, yields to the slightest pressure
of the objective. Mr. Spencer has also taken advantange of this
principle, and so formed the under side of the stage in some of
his stands, that the object may be pressed against it by the



AND



clips, which for this purpose are pushed through from below
upwards. In focussing, the objective is passed through the
stage if necessary. Great obliquity, and perfect safety against
breakage of the object by the objective, are secured. When the
microscopist is using valuable slides, costing from ten to one
hundred dollars, the latter feature is one of great importance.

In many microscopes, however notably those of English
manufacture the under side of the stage is not flat and even,
so that a slide cannot be laid against it. The following simple
device obviates this difficulty : A rectangular plate of metal is

pierced with a hole of the same
size as the interior diameter of
the sub-stage ring of the micro-
scope, and in this hole is fastened
a tube, which just fits this sub-
stage ring. The plate is provided
with two light spring clips, which
hold the object against its under
side, and it is easy to see that this
simple contrivance, which is shown
in Figure 26, serves three very im-
portant functions: 1. It affords
means for obtaining light of great
obliquity, since in reality it forms
a stage which has no thickness at
all. 2. It serves as a perfect safety
stage, thus enabling us to avoid all

risks not only to valuable slides, but to still more valuabe ob-
jectives. 3. It enables us to rotate the object very nearly in
the optic axis of the instrument.

The centricity of this rotation is not sufficiently accurate to
enable us to measure angles with any degree of precision, but
it gives us the opportunity of placing lined diatoms and other
objects in any position in regard to the illuminating ray.

Sub-Stage. The sub-stage is used chiefly for holding and
adjusting illuminating apparatus beneath the stage, and may
justly be regarded as one of the most important parts of the
stand.




Fig. 26.



OF THE MICliOoCOPE. 115

It forms a prominent feature in the New American Model, as
well as in all first-class microscopes.

In its simplest form it consists simply of a short tube or ring,
which is attached to the under side of the stage, and is fitted to
receive polarizers, paraboloids, condensers, etc. It should be
made removable, so that it may offer no obstacle to the employ-
ment of the most oblique illumination.

In the New American Model it is attached to a swinging arm,
so that the illuminating apparatus, which it carries, may be
placed at any angle with the optic axis of the microscope.

When a mere ring is used for receiving the accessory pieces
of apparatus, the latter are adjusted by simply pushing them
up or down, but a much better plan is to have the ring itself
moveable, as is the case in the instruments shown in Plates III,
IV, V, and VI. The accessory is then placed in the ring, and
the latter may be slid up or down the carrying bar, so as to
allow of proper adjustment. A still better plan is that shown
in Plate I, where the distance of the sub-stage from the stage
may be adjusted by means of a rack and pinion.

In any case the sub-stage itself, or the apparatus which it
carries, should have facilities for accurately centering the
various pieces of illuminating apparatus.

The Mirror. The mirrors employed for illuminating
microscopic objects are either plane or concave, and in the
better class of instruments both kinds are provided, while the
cheaper forms of the microscope have only the concave mirror.
The plane mirror reflects the light just as it falls on it that is
to say, divergent rays (as from a lamp) remain divergent after
reflection, and parallel rays (as sunlight or rays from the bull's-
eye condenser) remain parallel. The concave mirror, on the
other hand, causes parallel rays to converge and meet at a point,
while other rays are rendered either less divergent, parallel or
convergent, as the case may be. It is very important that the sur-
face of the mirror should be accurately formed, and therefore in
all good microscopes they are made of glass, which has been
accurately ground and polished. Blown or cast glass will not
answer. And as the quality and quantity of the light is greatly
effected by the reflecting surface, the best mirrors are silvered



116

with pure silver, instead of with the amalgam of tin and
mercury, ordinarily used. The mirror should be so hung that
it may throw a beam of great obliquity through the object. In
many cases this is absolutely necessary, and even for ordinary
work it is of great advantage, since it not only enables us to
resolve lined objects, but to secure important changes in the
illumination of common objects. A. very fair dark ground
illumination may be secured if the light is so oblique that none
of it can enter the object-glass directly.

The mirror should not only be hung so as to swing to any
angle, but it should be movable on the mirror-bar so that the
rays which it reflects may be brought exactly to a focus on the
object. This is done by sliding the mirror out or in, according
as the rays are more or less divergent.

The concave mirror should be large, so that it may collect
plenty of light. The plane mirror may be small without much
loss. The concave mirror is frequently used for the illumina-
tion of opaque objects, as when large it concentrates the light
very strongly. For this purpose it is either mounted on a,
separate stand, or the mirror-bar is so hung that it may be
turned up over the stage, so as to reflect the light down upon
the object.

The Body. The only points connected with the body of
the microscope which require consideration are its diameter and
its length, and these must of necessity vary so much according
to the purposes to which the microscope is to be applied, that
no rule can be laid down. Pocket microscopes are of necessity
small ; microscopes intended for use with objectives of low
power and large angles, must have a large diameter. And since
the distance of the eye-piece from the objective affects the cor-
rection of the latter, it has been found necessary to adopt a
standard length of body. This has been fixed at ten inches iu
this country and in England. On the continent of Europe,
eight inches is the length that has been adopted, and most of
the continental objectives are corrected for this length of body.
Provided it is large enough to take the new broad-gauge screw,
the diameter is not of very great importance in bodies of
moderate length; but, Pionlo tells us that in his long tubes, in,-



; j E



117



tended to produce great magnifying power, a diameter of two
to two-and-a-half inches was found to be absolutely necessary
to prevent the glare arising from internal reflection. An inch
and a quarter is a good size for ordinary instruments. Since a
very long body is inconvenient when the microscope is used in
a vertical position, the best instruments are furnished with a

Draw-Tube, whereby the length of the body may be
varied at pleasure. As explained in a former paragraph (page
20), when the distance between the eye-piece and the object-
glass is increased, the magnifying power is increased also.
The draw-tube, therefore, gives us the means of varying and
adjusting the magnifying power of the microscope, and this is
sometimes of great use. Thus, suppose it were required to
draw an object to a scale magnified exactly one hundred diame-
ters ; it might be impossible to procure an eye-piece and an
objective that, with a fixed length of body, would give exactly
this amplification, but when we are able to vary the magnify-
ing power by changing the length of the body, it is easy to get
at it exactly. This, however, is but one of many advantages
afforded by the draw-tube. If the objective be good, and the
eye-piece not very high, an easy and very satisfactory way to
increase the magnifying power of the microscope is to lengthen
the body by means of an additional tube, which may even be made
of smooth paper. But it must be borne in mind that any change
in the distance between the eye-piece and the objective affects
the corrections of the latter, just as a change in the thickness
of the covering glass would do. Consequently, unless the ob-
jective has a considerable range of correction, it may be impos-
sible to get good results when a very long draw-tube is used.
On the other hand, the draw-tube may be used to good ad-
vantage as a means of correcting for covering glass when non-
adjusting objectives are used. We have a one-fifth objective
now before us, with which we can see clearly the lines on the
P. angulalum on the balsam Probe Platte when the draw-tube
is out, but when the tube is pushed in, the view is foggy and
indistinct. This is due to a disturbance of the corrections.

The insides of all draw-tubes and bodies should be well
Slackened When bright or white the glare greatly injures the



118 SELECTION ANT) T'SE


1 2 3 4 5 6 7 8 10 12 13 14 15 16 17 18 19 20 21 22

Online LibraryJohn PhinHow to use the microscope; being practical hints on the selection and use of that instrument, intended for beginners → online text (page 10 of 22)