We have already described the simpler forms of magnifying instruments,
together with the best method of using them. We now purpose to describe
the more complicated instrument called the compound microscope, and
hints will be given as to the best method of making preparations for it.
The great distinction between the simple and compound microscope is,
that whereas the former instrument magnifies the object, the latter
magnifies the magnified image of the object. In the least elaborate
form of this instrument there are two glasses, one at each end of
a tube, the small glass magnifying the object, and being therefore
called the “object-glass,” while the other, which magnifies the image
of the object, is placed next to the eye, and is therefore termed the
“eye-glass.” In practice, however, this arrangement is found to be so
extremely defective, that the instrument was quite useless except as
an experimental toy; for the two enemies of the optician, chromatic
and spherical aberration, prevailed so exceedingly, that every object
appeared as if surrounded with prismatic colors, and every line was
blurred and indistinct.
In this uncertain state the compound microscope remained for many
years, its superb capabilities being scarcely recognized. The chief
fault was thought to be in the material of which the object-glass was
made, and for a long series of years all experiments were conducted
with a view to an improvement in this respect. When, however, the
diamond had been employed as an object-glass, and had failed equally
with those of less costly material, attention was directed to the right
point--namely, the arrangement of the different glasses,--and at length
opticians succeeded in obtaining a pitch of excellence which can be
almost termed perfection. It would be impossible to describe the method
which is employed for this purpose, and it must suffice to say that the
principle is that of playing off one defect against another, and so
making them mutually correct their errors.
The magnifying powers of the compound microscope can be very great,
and it is therefore necessary that extreme care should be taken in its
manipulation. It will be possible for a clumsy person to do more damage
to a good instrument in three minutes than can be repaired in as many
weeks.
Before proceeding to the management of the microscope and the
construction of the “slides,” we will briefly describe one or two chief
forms of the compound microscope.
The simplest form of the compound microscope, as at present made,
consists of a stand and a sliding tube, in which are set the glasses
which magnify the object and its image. At the top is the tube, which
is capable of being slid up and down in the shoulder of the stand, so
as to obtain the proper focus. Above is seen the eye-glass; and the
object-glass is shown at the bottom of the tube. Below the object-glass
is the “stage” on which the object to be magnified is laid; and lowest
of all is a mirror, which serves to reflect the light upwards through
the object, and which can be turned by means of the knobs at the
sides. The object-glass is composed of two pieces, which can readily
be separated. If both are used, sufficient magnifying power is gained
to show the scales on a butterfly’s wing and similar minute objects;
while, if one is removed, the object is not magnified to so great
an extent, but a larger portion can be seen, and the definition is
clearer. The cost of this instrument, together with a few accessories,
is about $2.50.
The proper light is our next point, and upon it rests the chief
beauty of the effect. The light which will suit one object will not
suit another, and even the same object should be examined under every
variety of light. Some objects are best shown when the light is
thrown _upon_ them from above, and others when it is thrown _through_
them from below. Again, the direction of light is of vast importance;
for it will easily be seen that an oblique light will exhibit minute
projections by throwing a shadow on one side and brilliancy on the
other, while a vertical illumination would fail to show them. On the
same principle, one object will be shown better with the light in
front, and another when it is on one side.
One of the most effective means of attaining this object is by using
the “bull’s-eye condenser,” which is sometimes fixed to the stage, but
is usually detached. As the upright stem is telescopic, the glass can
be raised to a considerable height, while the joint and sliding-rod
permit the lens to be applied at any angle which promises the most
brilliant light.
As for the kind of light that is employed, there is nothing which
equals that of a white cloud; but such clouds are rare, and are at
the best extremely transient, and can only be seen by day, various
artificial methods of illumination have been invented. Novices
generally think that when the sky is bright and blue they will be very
successful in their illumination, and feel grievously disappointed
at finding that they obtained much more light from the clouds, whose
disappearance they had anxiously been watching. Finding that the blue
sky gives scarcely any light at all, they rush to the other extreme,
turn the mirror towards the sun, and pour such a blaze of light upon
the object, that the eye is blinded by the scintillating refulgence,
and the object is often injured because the mirror is capable of
reflecting heat as well as light.
In the daytime there is nothing better than the “white-cloud
illuminator,” which is made easily enough by means of plaster of
Paris. A sheet of thin white paper fastened against a window-pane is
also useful; and the simple plan of dabbing the glass with putty will
have a beneficial effect in softening the light, when the window has
a southern aspect. In default of these conveniences, it will be often
sufficient to fix a piece of white letter paper over the mirror, or
even to dull its surface with wax. At all events, he who aspires to
be a true microscopist must be ready with expedients, and if he finds
himself in a difficulty, he must summarily invent a method of obviating
it.
At night a lamp is necessary; candles are useless, because they have
two faults--they flicker, and they become lower as they burn. The
latter defect can be cured by using a candle-lamp, but no arrangement
will cure the flame of flickering; it is peculiarly trying to the
eyes, and destructive of accurate definition. An ordinary moderator
lamp answers pretty well, and a small one is even better for the
microscopist than one of large dimensions. The chief drawback to the
moderator lamp is that the flame cannot be elevated or lowered, so
that the only way to procure a light at a higher elevation is to stand
the lamp on a block of wood or a book. Small lamps are, however, made
expressly for the microscope, and, if possible, should be procured, and
used for no other purpose, and intrusted to no other hands.
If you want a really brilliant, clear, white light, you must trim
the lamp yourself. A small piece of pale blue or neutral-tint glass
interposed between the lamp and the microscope has a wonderful effect
in diminishing the yellow hue which belongs more or less to all
artificial lights which are produced by the combustion of oil or fat.
We have no doubt but that in a few years we shall be rid of the clumsy
and dirty machines that we call lamps, and have substituted for them
the pure brilliancy of the electric light.
Whatever lamp you use, a shade is absolutely necessary, in order to
defend the eyes. Let me here warn my young readers that they cannot
be too careful of their eyes. In the exuberance of youthful strength
and health we are too apt to treat our eyes as unceremoniously as our
digestion, and in later years we awake to unavailing repentance.
Another point which calls for extreme attention is the perfect
cleanliness of the glasses. It is astonishing how a tiny dust-mote,
or the least condensation of damp, will diminish the powers of the
microscope, and how often the instrument is blamed for indistinctness
when the real fault lies in the carelessness of the operator. Even
when the greatest care is taken, dust is sure to settle on the
glasses, especially on the eye-piece, and before using the microscope
the glasses ought to be carefully examined. Never wipe them with an
ordinary handkerchief, but get a piece of new wash-leather; beat it
well until no dust issues from it, and then put it into a box with a
tightly-fitting cover. Use this, and nothing else, for cleaning the
glasses, and you will avoid those horrid scratches with which the
eye-glass and object-glass of careless operators are always disfigured.
Moisture is very apt to condense on the glasses and to ruin their
clearness. If the microscope be brought from a cold into a warm room,
the glasses will be instantly covered with moisture, just as the
outside of a tumbler of cold water is always covered with fine dew
when brought into a warm room. The microscope should therefore be
kept at least an hour in the room wherein it is to be used, so that
the instrument and the atmosphere may be of the same temperature.
You should make the microscope a trifle warmer than the surrounding
atmosphere, and so avoid all danger of condensation. When changing
the object glass or eye-piece, always keep the hand as far away from
the glass as possible, and manipulate with the tip of the forefinger
and thumb. The human skin always gives out so much exhalation, that
even when the hand is cold the glasses will be dimmed; and it is a
peculiarity of such moisture, that it adheres to the glasses with great
pertinacity, and does not evaporate like the dew which is condensed
from the atmosphere.
In order to insure perfect success in this important particular, the
young microscopist will do well to get the optician from whom he
purchased his instrument to explain its construction, and to give him a
lesson or two in the art of taking it to pieces and putting it together
again; for unless each glass can be separately cleaned, no one can be
quite sure that the instrument will perform as it ought to do. The best
method of ascertaining whether it is quite clean is to throw the light
upwards by means of the mirror, and then to turn the eye-piece slowly
round. If any dust or moisture has collected either upon the eye-glass
or the “field-glass,” which forms the second lens of the eye-piece,
it will be immediately detected. Turning the object-glass will in a
similar manner detect impurities upon its surface.