merely optical purposes, but when it is required to measure the
magnified image, we use the eye-piece invented by Mr. Ramsden, and
called, from its purpose, the micrometer eye-piece. When it is stated
that we sometimes require to measure portions of animal or vegetable
matter a hundred times smaller than any divisions that can be
artificially made on any measuring instrument, the advantage of
applying the scale to the magnified image will be obvious, as compared
with the application of engraved or mechanical micrometers to the
stage of the instrument.
The arrangement is shown in Fig. 21, where E E and F F are the eye and
field glass, the latter having now its plane face towards the object.
The rays from the object are here made to converge at A A, immediately
in front of the field-glass, and here also is placed a plane glass on
which are engraved divisions of a hundredth of an inch or less. The
markings of these divisions come into focus therefore at the same time
as the image of the object, and both are distinctly seen together.
Thus the measure of the magnified image is given by mere inspection,
and the value of such measures in reference to the real object may be
obtained thus, which, when once obtained, is constant for the same
object-glass. Place on the stage of the instrument a divided scale the
value of which is known, and viewing this scale as the microscopic
object, observe how many of the divisions on the scale attached to the
eye-piece correspond with one of those in the magnified image. If, for
instance, ten of those in the eye-piece correspond with one of those in
the image, and if the divisions are known to be equal, then the image
is ten times larger than the object, and the dimensions of the object
are ten times less than indicated by the micrometer. If the divisions
on the micrometer and on the magnified scale were not equal, it
becomes a mere rule-of-three sum, but in general this trouble is taken
by the maker of the instrument, who furnishes a table showing the
value of each division of the micrometer for every object-glass with
which it may be used.
[Illustration: Fig. 21.]
While on the subject of measuring it may be well to explain the mode
of ascertaining the magnifying power of the compound microscope, which
is generally taken on the assumption before mentioned, that the naked
eye sees most distinctly at the distance of ten inches.
Place on the stage of the instrument, as before, a known divided
scale, and when it is distinctly seen, hold a rule at ten inches
distance from the disengaged eye, so that it may be seen by that eye,
overlapping or lying by side of the magnified picture of the other
scale. Then move the rule till one or more of its known divisions
correspond with a number of those in the magnified scale, and a
comparison of the two gives the magnifying power.
Having now explained the optical principles of the achromatic compound
microscope, it remains only to describe the mechanical arrangements
for giving those principles their full effect. The mechanism of a
microscope is of much more importance than might be imagined by those
who have not studied the subject. In the first place, steadiness, or
freedom from vibration, and most particularly freedom from any
vibrations which are not equally communicated to the object under
examination, and to the lenses by which it is viewed, is a point of
the utmost consequence. When, for instance, the body containing the
lenses is screwed by its lower extremity to a horizontal arm, we have
one of the most vibratory forms conceivable; it is precisely the form
of the inverted pendulum, which is expressly contrived to indicate
otherwise insensible vibrations. The tremor necessarily attendant on
such an arrangement is magnified by the whole power of the instrument;
and as the object on the stage partakes of this tremor in a
comparatively insensible degree, the image is seen to oscillate so
rapidly, as in some cases to be wholly undistinguishable. Such
microscopes cannot possibly be used with high powers in ordinary
houses abutting on any paved streets through which carriages are
passing, nor indeed are they adapted to be used in houses in which the
ordinary internal sources of shaking exist.
One of the best modes of mounting a compound microscope is shown in
the annexed view (Fig. 22), which, though too minute to exhibit all
the details, will serve to explain the chief features of the
arrangement.
A massy pillar A is screwed into a solid tripod B, and is surmounted
by a strong joint at C, on which the whole instrument turns, so as to
enable it to take a perfectly horizontal or vertical position, or any
intermediate angle, such, for instance, as that shown in the
engraving.
This movable portion of the instrument consists of one solid casting D
E F G; from F to G being a thick pierced plate carrying the stage and
its appendages. The compound body H is attached to the bar D E, and
moves up and down upon it by a rack and pinion worked by either of the
milled heads K. The piece D E F G is attached to the pillar by the
joint C, which being the source of the required movement in the
instrument, is obviously its weakest part, and about which no doubt
considerable vibration takes place. But inasmuch as the piece D E F G
of necessity transmits such vibrations equally to the body of the
microscope and to the objects on the stage, they hold always the same
relative position, and no _visible_ vibration is caused, how much
soever may really exist. To the under side of the stage is attached a
circular stem L, on which slides the mirror M, plane on one side and
concave on the other, to reflect the light through the aperture in the
stage. Beneath the stage is a circular revolving plate containing
three apertures of various sizes, to limit the angle of the pencil of
light which shall be allowed to fall on the object under examination.
Besides these conveniences the stage has a double movement produced by
two racks at right angles to each other, and worked by milled heads
beneath. It has also the usual appendages of forceps to hold minute
objects, and a lens to condense the light upon them, all of which are
well understood, and if not, will be rendered more intelligible by a
few minutes’ examination of a microscope than by the most lengthened
description. One other point remains to be noticed. The movement
produced by the milled head K is not sufficiently delicate to adjust
the focus of very powerful lenses, nor indeed is any rack movement.
Only the finest screws are adapted to this purpose; and even these are
improved by means for reducing the rapidity of the screw’s movement.
For this purpose the lower end of the compound body H, which carries
the object-glass, consists of a piece of smaller tube sliding in
parallel guides in the main body, and kept constantly pressed upwards
by a spiral spring but it can be drawn downward by a lever crossing
the body, and acted on by an extremely fine screw whose milled head
is seen at N, and the fineness of which is tripled by means of the
lever through which it acts on the object-glass. The instrument is of
course roughly adjusted by the rack movement, and finished by the
screw, or by such other means as are chosen for the purpose. One very
ingenious contrivance, but applied to the stage, instead of the body
of the microscope, invented by Mr. Powell, will be found described in
the 50th volume of the Transactions of the Society of Arts.
[Illustration: Fig. 22.]
The greater part of the directions for viewing and illuminating
objects given in reference to the simple microscope are applicable to
the compound. An argand lamp placed in the focus of a large detached
lens so as to throw parallel rays upon the mirror, is the best
artificial light; and for opaque objects the light so thrown up may be
reflected by metallic specula (called, from their inventor,
Lieberkhuns) attached to the object-glasses.
It has been recently proposed by Sir David Brewster and by M. Dujardin
to render the Wollaston condenser achromatic, and they have
accordingly been made with three pairs of achromatic lenses instead of
the single lens before described, with very excellent effect. The
last-mentioned gentleman has also projected an ingenious apparatus,
called the Hyptioscope, attached to the eye-piece for the purpose of
erecting the magnified picture.
The erector commonly applied to the compound microscope consists of a
pair of lenses acting like the erecting eye-piece of the telescope.
But this, though it is convenient for the purpose of dissection, very
much impairs the optical performance of the instrument.
[Illustration: Fig. 23.]
For drawing the images presented by the microscope the best apparatus
consists of a mirror M (Fig. 23), composed of a thin piece of rather
dark-colored glass cemented on to a piece of plate-glass inclined at
an angle of 45° in front of the eye-glass E. The light escaping from
the eye-glass is assisted in its reflection upwards to the eye by the
dark glass, which effects the further useful purpose of rendering the
paper less brilliant, and thus enabling the eye better to see the
reflected image. The lens L below the reflector is to cause the light
from the paper and pencil to diverge from the same distance as that
received from the eye-glass; in other words, to cause it to reach the
eye in parallel lines.
[Illustration: Fig. 24.]
Dr. Wollaston’s camera lucida, as shown in Fig. 24, is sometimes
attached to the eye-piece of the microscope for the same purpose. In
this instrument the rays suffer two internal reflections within the
glass prism, as will be seen explained in the article “Camera Lucida.”
In this minute figure we have omitted to trace the reflected rays,
merely to avoid confusion.
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
Annexed are four engravings of microscopic objects, the true character
of which it is, however, impossible to give in wood, and is difficult
indeed to accomplish by any description of engraving.
[Illustration: Fig. 28.]
Fig. 25 shows a scale of the small insect called Podura Plumbea, the
common Skiptail, magnified about five hundred times. To define the
markings on this scale clearly is the highest test of a deep
achromatic object-glass; and this drawing is given rather to explain
what the observer should look for, than as a very correct
representation. Fig. 26 is a scale or feather of the Menelaus
Butterfly; Fig. 27 is the hair of a singular insect, the Dermestes;
and Fig. 28 is a longitudinal cutting of fir, showing the circular
glands on the vessels which distinguish coniferous woods. These latter
objects may be seen by half-inch or quarter-inch achromatic glasses.
Opaque objects are generally better exhibited by inch and two-inch
glasses, when a general view of them is required, and by higher powers
when we wish to examine their minute structure. In the latter case the
light must be obtained by condensing lenses instead of the metallic
specula.
Although the reflecting microscope is now very little used, it may be
expected that we should mention it. In this instrument, at Fig. 29,
the object O is reflected by the inclined face of the mirror M, and
the rays are again reflected and converged by the ellipsoidal
reflector R R, which effects the same purpose as the object-glass of
the compound microscope. It forms an image which is not susceptible of
the over-correction as to color before described, and which therefore
becomes colored in passing through the eye-piece. This fact, and the
loss of light by reflection, will probably always render the
reflecting microscope inferior to the achromatic refracting.
[Illustration: Fig. 29.]
The solar microscope has been so nearly superseded by the
oxy-hydrogen, that a brief description of the latter must suffice,
particularly as their optical principles are similar.
The primary object in both is to throw an intense light upon the
object, which is sometimes done by mirrors, and sometimes by lenses.
In Fig. 30, L represents the cylinder of burning lime, R R the
reflector, which concentrates the light upon the object O O; the rays
from which, passing through the two plano-convex lenses, are brought
to foci upon a screen placed at a great distance, and upon which is
formed the magnified image.
[Illustration: Fig. 30.]
Fig. 31 shows a combination of lenses to condense the light upon the
object. In either case the optical arrangements by which the image is
formed admit of the same perfection as those which have been described
for the compound microscopes. A few achromatic glasses for
oxy-hydrogen microscopes have been made, and they will ultimately
become valuable instruments for illustrating lectures on natural
history and physiology. One made by Mr. Ross was exhibited a few
months since at the Society of Arts to illustrate a lecture on the
physiology of woods. It should be observed, however, that the
oxy-hydrogen or solar microscope requires either a spherical screen,
or that the objects should be mounted between spherical glasses, in
order to bring the whole into focus at one time. This latter plan was
adopted on the occasion just mentioned with perfect success.
[Illustration: Fig. 31.]