Examination of Objects--Principles of Illumination--Mirror
and its Action--Substage Condenser--Use of Bull’s-eye--Opaque
Objects--Photography of Microscopic Objects.
So much depends upon a right method of employing the microscope, as
regards both comfort and accuracy, that we propose to devote a little
space to the consideration of the subject.
Let us first warn the intending observer against the use of powers
higher than are required to bring out the details of the object. Mere
magnification is of very little use: it increases the difficulties both
of illumination and of manipulation, and, as already said, interferes
with that grasp of the object which it is most desirable to obtain.
Rather let the beginner lay himself out to get the very most he can out
of his lowest powers, and he will find that, by so doing, he will be
able far better to avail himself of the higher ones when their use is
indispensable.
The essential means to this end is a mastery of the principles of
illumination, which we now proceed to describe.
We suppose the microscope to be inclined at an angle of about 70° to
the horizontal, with a low-power objective attached to it, a one-inch
by preference. Opposite to the microscope, and about a foot away from
it, is a lamp with the edge of the flame presented to the microscope,
the concave mirror of which is so arranged as to receive the rays from
the flame and direct them up the tube of the microscope. Upon the stage
is placed a piece of ground-glass, and the mirror-arm is now to be
moved up or down upon its support until the ground-glass receives the
maximum of illumination, which it will do when the lamp-flame is at one
conjugate focus of the mirror and the ground-glass at the other. The
focus will not be an image of the flame, but a bar of light.
If an object be now placed upon the stage, instead of the ground-glass,
and the objective focused upon it, it will, if the mirror be properly
adjusted, be brilliantly illuminated.
It will be understood that every concave mirror has a focus, and
converges the rays which fall upon it to this focus, behaving exactly
like a convex lens. The principal focus of a concave mirror is its
radius of curvature, and this is not difficult to determine. Place side
by side a deep cardboard box and the lamp, so that the concave mirror
may send the rays back, along a path only slightly inclined to that by
which they reached it, to the bottom of the box. The lamp and box being
equidistant from the mirror, it is evident that when the mirror forms
an image of the former upon the latter equal to the flame in size, we
have the equivalent of the equal conjugate foci shown in Fig. 2. Now
move the box to the distance from the mirror which corresponds to the
distance of the stage of the microscope from the mirror when the latter
is in position upon the microscope, and then move the lamp to or fro
until the mirror casts a sharp image of the flame upon the bottom of
the box, which is not to be moved. The lamp distance so found will be
the correct one for working with the concave mirror. The writer is led
to lay special stress upon this matter, from the fact that he almost
invariably finds that the mirror is arranged to be used for parallel
rays, _i.e._ for daylight, and is therefore fixed far too close to the
stage to be available for correct or advantageous working with the
lamp, unless, indeed, the bull’s-eye condenser be used, as hereinafter
described, to parallelise the rays from the lamp.
Work done with the concave mirror can, however, under the most
favourable conditions, only be looked upon as a _pis aller_. The
advantages gained by the use of some substage condenser, even the most
simple, in conjunction with the plane mirror, or even without any
mirror at all, are so manifold that the beginner is strongly urged
to provide himself with some form or other of it, and we now proceed
to describe the way in which this should be used to produce the best
effect.
To reduce the problem to its most simple elements, turn the mirror
altogether out of the way, and place the microscope upon a block at
such a height as shall be convenient for observation, and shall allow
the rays from the lamp, placed in a line with it on the table, to shine
directly into the tube of the microscope. Ascertain that this is so
by removing both objective and eye-piece and looking down the tube,
when the flame should be seen in the centre, edgewise. Now replace
the eye-piece, and screw on to the tube the one-inch combination or
objective. Place upon the stage an object, preferably a round diatom or
an echinus-spine, and focus it as sharply as possible. Now place the
substage condenser in its jacket, and slide it up and down until the
image of the object is bisected by the image of the flame.
The centre of the object will now be brilliantly illuminated by rays
travelling in the proper direction for yielding the best results.
The object is situated at the common focus of the microscope and the
condenser, and, whatever means of illumination be adopted, this is the
result which should always be aimed at.
Satisfactory as this critical arrangement is, however, from a
scientific point of view, it has its drawbacks from an artistic and
æsthetic one. It is not pleasant, for most purposes, to have merely the
centre of an object lighted up, and we have now to consider how the
image of the edge of the flame may be so expanded as to fill the field
without sacrificing more than a very small fraction of the accuracy of
the arrangement just attained.
Referring to Fig. 1, we see that if we place the lamp at the principal
focus of a lens, it will emit a bundle of parallel rays equal in
diameter to the diameter of the lens. This is the key of the position.
We cannot place the lamp at an infinite distance from the substage
condenser, but we can supply the latter with rays approximately
parallel, so that it shall bring them to a focus upon the object
at very nearly its own principal focus. This we do by means of the
bull’s-eye condenser. Place the latter, with its flat side toward the
edge of the flame, and at its principal focal distance (the method of
determining which has already been described) from the latter, so that
the bundle of parallel rays which issue from it may pass up to the
substage condenser. On examining the object again, it will be found
that, after slight adjustments of the position of the bull’s-eye have
been made, the object lies in the centre of an evenly and brilliantly
lighted field.
It may be necessary to place the bull’s-eye a little farther from or
nearer to the lamp, or to move it a little to one side or the other,
but when it is at the correct distance, and on the central line between
the lamp and the substage condenser, at right angles to this line,
the effects will be as described. It may help in securing this result
if we mention that when the bull’s-eye is too far from the lamp, the
image of the flame is a spindle-shaped one; whilst, when the distance
between the two is too short, _i.e._ less than the principal focal
length of the lens, the field is crossed by a bar or light, the ends of
which are joined by a ring, whilst on either side of the bar there is a
semi-circular dark space.
We have hitherto supposed the objects viewed to be transparent, but
there are many, of great interest, which are opaque, and call for other
means of illumination. Of these there are several. The simplest and, in
many ways, the best is to use the bull’s-eye condenser to bring to a
focus upon the object the rays of light from some source placed above
the stage of the microscope. If light can be obtained from the sun
itself, no lens will be needed to concentrate it; and indeed, if this
were done, there would be considerable risk of burning the object. The
light from a white cloud, however, with the help of the bull’s-eye,
answers admirably. At night-time an artificial source of light, the
more intense and the more distant the better, is required. For most
cases, and with powers not higher than one inch, a good paraffin lamp,
placed about two feet away from the stage, and on one side of it, so as
to be about a foot above the level of the object, will give all that is
needed. Such a lamp is shown in Fig. 14. Low magnifications are, as a
rule, all that is called for in this method.
Lieberkuhn’s condensers are useful aids, but are somewhat expensive.
They are concave mirrors, which are so adjusted to the objective that
the latter and the reflector come into focus together, the light being
sent in from below, or from one side.
One other method of illumination must be mentioned before leaving the
topic, and this is the illumination of objects upon a “dark field.”
With suitable subjects, and when carefully managed, there is no method
which gives more beautiful effects, and it has the great advantage of
allowing the object to be brilliantly lighted, without the strain to
the eyes which is involved in such lighting by the usual method of
direct illumination.
[Illustration: Fig. 14.]
It consists essentially in allowing the light to fall upon the object
from below, at such an angle that none of it can enter the objective
directly. Thus the concave mirror, turned as far as possible to one
side, and reflecting on to the object the rays from the lamp placed
upon the opposite side, will give very fair results with low powers;
this plan, however, is capable of but very limited application. Again,
a disc of black paper may be stuck on to the middle of the bull’s-eye,
and the latter be placed below the stage between it and the mirror. In
this case everything depends upon the size of the disc, which, if too
small, will not give a black ground, and if too large will cut off all
light from the object.
The best and only really satisfactory plan is to arrange the
illumination with the substage condenser, as previously described, and
then to place below the lens of the latter a central stop of a suitable
size, which can only be determined by trial. When this has been done
the object will be seen brilliantly illuminated upon a field of velvety
blackness. Such stops are supplied with the condenser.
We have devoted a considerable portion of space to this question, since
it is, of all others, the most important to a successful, satisfactory,
and reliable manipulation of the microscope; but even now, only the
main points of the subject have been touched upon, and the worker
will find it necessary to supplement the information given by actual
experiment. A few failures, rightly considered, will afford a great
amount of information, but those who desire to go thoroughly into the
matter are recommended to consult the present writer’s _Guide to the
Science of Photomicrography_, where it is treated at much greater
length, as an essential part of the subject-matter of the book.
It may be added here, that no method of reproducing the images of
objects is on the whole so satisfactory as the photographic one; and
whilst a lengthened reference to the topic would be out of place in
a work of the character of the present one, the one just mentioned
will be found to contain all that is necessary to enable the beginner
to produce results which, for faithfulness and beauty, far excel any
drawing, whilst they have the additional advantage that they can, if
required, be exhibited to hundreds simultaneously.