X, which is at the top of the pillar, D. This arm may be moved backwards
and forwards in the socket X, and sidewise by the pin, D; so that the
magnifier, which is screwed into the ring at the end, E, of this bar,
may be easily made to traverse over any part of the object lying on the
stage or plate B. F is a polished silver speculum, with a magnifying
lens placed at the center thereof, which is perforated for this
purpose. The silver speculum screws into the arm E, as at F. G, another
speculum of a different concavity from the former, with its lens. H, the
brass semicircle which supports the mirror, I; the pin, R, affixed to
the semicircle, H, passes through the hole which is towards the bottom
of the pillar, A. B, the stage or the plane on which the objects are to
be placed; it fits into a small dove-tailed arm which is at the upper
end of the pillar, A. C, a plane glass, with a small piece of black silk
stuck on it; this glass is fitted to a groove made in the stage, B. M, a
deep concave glass, to be laid occasionally on the stage instead of the
plane glass, C. L, a pair of nippers; these are fixed to the hole of the
stage, _a_, by the pin K; the steel wire of these nippers slides
backwards and forwards in the socket, and this socket is moveable
upwards and downwards by means of the joint, so that the position of the
object may be varied at pleasure. The object may be fixed in the
nippers, stuck on the point, or affixed by a little gum-water, &c. to
the ivory cylinder, N. O, a small pair of brass forceps to take up
minute objects by. P, a brush to clean the glasses.
To use this microscope; begin by screwing the pillar, A, to the cover
thereof; pass the pin, R, of the semicircle which carries the mirror
through the hole that is near the bottom of the pillar, A; push the
stage into the dove-tail at B; slide the pin into the pillar, then pass
the bar, E, through the socket, X, which is at the top of the pin D, and
screw one of the magnifying lenses into the ring at F.
Now place the object either on the stage, or in the nippers L, and in
such a manner, that it may be as nearly as possible over the center of
the stage; bring the speculum, F, over the part you mean to observe;
then get as much light on the speculum as you can, by means of the
mirror, I; the light received on the speculum is reflected by it on the
object. The distance of the lens, F, from the object is regulated by
moving the pin, D, up and down, until a distinct view of it is obtained.
The rule usually observed is, to place the lens beyond its focal
distance from the object, and then gradually slide it down, till the
object appears sharp and well defined. The adjustment of the lenses to
their foci, and the distribution of the light on the object, are what
require the most attention.
These microscopes are sometimes fitted up with a rack and pinion to the
pillar A, and pin D, for the more ready adjustment of the glasses to
their proper foci.
DESCRIPTION OF LYONET’S ANATOMICAL MICROSCOPE. Plate VI. Fig. 3.
Fig. 3 represents the instrument with which M. Lyonet made his
microscopical and wonderful dissection of the chenille de saule or
caterpillar of the goat moth,[36] of which a specimen is given in Plate
XII. Fig. 1, &c. of this work. This portable instrument needs no further
recommendation. By it, other observers may be enabled to dissect insects
in general with the same accuracy as M. Lyonet, and thus advance the
knowledge of comparative anatomy, by which alone the characteristic,
nature, and rank of animals, can be truly ascertained.
[36] Phalæna cossus. Linn. 63.
A B is the anatomical table, which is supported by the pillar O N; this
is screwed on the mahogany foot, D C. The table A B, is prevented from
turning round by means of two steady pins; in this table or board there
is a hole, G, which is exactly over the center of the mirror, F E, that
is to reflect the light on the object; the hole, G, is, designed to
receive a flat or a concave glass, on which the objects are to be placed
that you design to examine or dissect. R X Z is an arm formed of several
balls and sockets, by which means it may be moved in every possible
position; it is fixed to the board by means of the screw, H; the last
arm, I Z, has a female screw, into which a magnifier may be screwed, as
at Z. By means of the screw, H, a small motion may be occasionally given
to the arm I Z, for adjusting the lens with accuracy to its focal
distance from the object. Another chain of balls is sometimes used,
carrying a lens to throw light upon the object; the mirror is also so
mounted, as to be taken from its place at K, and fitted on a clamp, by
which it may be fixed to any part of the table, A B.
TO USE THE DISSECTING TABLE. Let the operator sit with his left side
near a light window; the instrument being placed on a firm table, the
side, D L, towards his breast, the observations should be made with the
left eye: this position is well adapted for observing, drawing, or
writing. In dissecting, the two elbows are to be supported by the table
on which the instrument rests, the hands resting against the board, A B,
in order to give it greater stability, as a small shake, though
imperceptible to the naked eye, is very visible in the microscope; the
dissecting instruments are to be held one in each hand, between the
thumb and two fore-fingers. Farther directions are given on the mode of
dissecting small objects in the following chapter.
DR. WITHERING’S BOTANICAL MICROSCOPE. Plate VI. Fig. 1.
This small instrument consists of three brass parallel plates, A, B, C;
two wires, D and E, are rivetted into the upper and lower plate; the
middle plate or stage is moveable on the aforesaid wires, by two little
sockets which are fixed to it. The two upper plates each contain a
magnifying lens, but of different powers; one of these confines and
keeps in their places the fine point F, the forceps G, and the small
knife H.
To use this instrument, unscrew the upper lens, and take out the point,
the knife, and the forceps; then screw the lens on again, place the
object on the stage, and then move it up or down till you have gained a
distinct view of the object, as one lens is made of a shorter focus than
the other; and spare lenses of a still deeper focus are sometimes added.
The principal merit of this microscope is its simplicity.
THE POCKET BOTANICAL AND UNIVERSAL MICROSCOPE.
This pocket instrument is represented at Plate VI. Fig. 2. It is by most
naturalists deemed preferable to Dr. Withering’s, being equally simple,
more extensive in its application, and the stage unincumbered; though
that of M. Lyonet seems better adapted than either to the purposes of
dissection only.
A B, a small arm, carrying three magnifiers, two fixed to the upper
part, as at B, the other to the lower part of the arm, at C; these may
be used separately or combined together, by which you have seven powers.
The arm, A B, is supported by the square pillar I K, the lower end of
which fits into the socket, E, of the foot, F G; the stage, D L, is made
to slide up and down the square pillar. H, a mirror for reflecting light
on the object.
To use this microscope, place the object on the stage, L, reflect the
light on it from the mirror H, and regulate it to the focus, by moving
the stage nearer to or further from the lenses at B C. The ivory sliders
pass under the stage, L; other objects may be fixed in the nippers, M N,
and then brought under the magnifiers; or they may be laid on one of the
glasses fitted to the stage. The apparatus to this instrument consists
of three ivory sliders, a pair of nippers, a pair of forceps, a flat
glass, and a concave ditto, all fitted to the stage, L. By taking out
the pin, M, the pillar, I K, may be turned half round, and the foot, F
G, made to answer as an handle.[37]
[37] An adjusting screw, Fig. 13*, to move the stage, with other
additions, are made by Messrs. Jones; and which then, in my opinion,
constitute the most complete pocket microscope hitherto made; for the
particulars of which, I refer the reader to their printed description.
Fig. 14, represents the common flower or insect microscope. There are
two lenses, _a_ and _b_, that are used separately or conjointly. EDIT.
BOTANICAL MAGNIFIERS.
Since botany has been cultivated with so much ardor, it has been found
necessary to contrive some very portable instrument, by which the
botanist might investigate the object of his pursuits as it rises before
him. Plate VIII. Fig. 7 and 8, represent two of the most convenient
sort.
In the tortoiseshell case, Fig. 7, three lenses are contained, _d_, _e_,
_f_, of different foci, which are all made to turn into the case, and
may be used combined or separately. The three lenses in themselves
afford three different magnifying powers; by combining two and two, we
make three more; and the three together make, a seventh magnifying
power. When the three lenses are used together, it is best to turn them
into the case, and look through the hole, for more distinctness, and the
exclusion of superfluous light. In the case, Fig. 8, are also three
lenses, _g_, _h_, _i_, of different magnifying powers, that all turn up,
and shut into the case; but these are not capable of combination.
DESCRIPTION OF A PORTABLE MICROSCOPE AND TELESCOPE. Plate VIII. Fig. 1,
to 6.
The telescope is one of those which are composed of several sliding
drawers or tubes, for the convenience of being put into the pocket; the
sliding tubes are made of thin brass, the outside tube of mahogany. The
sliding tubes are contrived to stop, when drawn out to a proper length,
so that by applying one hand to the outside tube, and the other hand to
the end of the smallest tube, the telescope may at one pull be drawn out
to its full length; then any of the tubes (that next the eye is most
generally used) may be pushed in gradually, while you are looking
through it, till the object is rendered distinct to the eye. To make the
tubes slide properly, they all pass through short springs or tubes;
these springs may be unscrewed from the ends of the sliding tubes, by
means of the milled edges which project above the tubes, and the tubes
taken from each other if required, and the springs set closer if at any
time they be too weak.
Fig. 5 represents the exterior tube of the telescope, which is to be
unscrewed from the rest, at _m l_, as it does not make any part of the
microscope; the cover, _k_, which protects the object-glass, serves also
as a box to contain two ivory wheels, Fig. 1 and 2, with the objects,
and a small mirror, Fig. 6.
Fig. 4 is a view of this cover when taken off: unscrew the top part of
it, and the mirror, Fig. 6, may be taken out; unscrew the cover of the
lower part, and you will find therein the two circular object-wheels
above mentioned.
Fig. 3 represents the three internal tubes of the telescope, which
constitute the microscopic part thereof. Draw the tubes out in the
manner as shewn in the figure; then at the inside, but at the lower end
of the exterior tube, a, you will find a short tube, which serves as a
stage to hold the object and support the mirror; pull this tube partly
out, and turn it, so that a circular hole which is pierced in it may
coincide with a similar hole in the exterior tube. This tube is
represented as drawn out at Fig. 3, the mirror, Fig. 6, placed therein
at _b c_, and the transparent object-wheel fixed at a.
Fig. 1 represents the slider with transparent objects.
Fig. 2, that with the opake. They are made of ivory, and turn on a pin
at the center; the slit end of this pin fits on the edge of the tube,
which is then to be pushed up, so that the lower end of the exterior
tube may bear lightly on the upper side of the slider, agreeable to the
view which is given at a, Fig. 3. Now push down the second tube till the
milled part falls on the milled edge of the extreme tube, being careful
of the circular hole in the exterior one. Nothing now remains to be done
but to adjust for the focus, which is effected by pushing in the tube R,
and moving only the first, _n_.
The instrument may be used in two ways for transparent objects: first,
in a vertical position, when the light is to be thrown on the object by
the mirror, _b c_; or it may be examined by looking up directly at the
light; in the latter case the mirror must be taken away. In viewing
opake objects the mirror is not used; as much light as possible must be
admitted on them through the circular holes of the tubes. Any object may
be viewed by first pushing in the tube, R, and then bringing the tube,
_n_, to its focal distance from the object. The telescope, when shut up,
is about eight inches in length, and when drawn out, is about twenty
inches. It is of the achromatic construction.
DESCRIPTION OF AN INSTRUMENT FOR CUTTING THIN TRANSVERSE SECTIONS OF
WOOD, Plate IX. Fig. 1.
It consists of a wooden base, which supports four brass pillars; on the
top of the pillars is placed a flat piece of brass, near the middle of
which there is a triangular hole. A sharp knife which moves in a
diagonal direction, is fixed on the upper side of the afore-mentioned
plate, and in such a manner, that the edge always coincides with the
surface thereof. The knife is moved backwards and forwards by means of
the handle, _a_. The piece of wood is placed in the triangular trough,
which is under the brass plate, and is to be kept steady therein by a
milled screw which is fitted to the trough; the wood is to be pressed
forward for cutting, by the micrometer screw, _b_. The pieces of wood
should be applied to this instrument immediately on being taken out of
the ground, or else they should be soaked for some time in water, to
soften them, so that they may not hurt the edge of the knife. When the
edge of the knife is brought in contact with the piece of wood, a small
quantity of spirit of wine should be poured on the surface of the wood,
to prevent its curling up; it will also make it adhere to the knife,
from which it may be removed by pressing a piece of blotting paper on
it.
Fig. 2, is an appendage to the cutting engine, which may be used instead
of the micrometer screw, being by some practitioners preferred to it. It
is placed over the triangular hole, and kept flat down upon the surface
of the brass plate, while the piece of wood is pressed against a
circular piece of brass which is on the under side of it. This circular
piece of brass is fixed to a screw, by which its distance from the flat
plate on which the knife moves may be regulated.[38]
[38] Many other kinds of cutting engines have been constructed, but
the specimens from them have not yet appeared with that perfection
which is requisite to this sort of objects; whether it lies in the
preparation of the woods, or engine, I do not take upon me to
determine. Mr. Custance has certainly produced the most exquisite.
EDIT.
CHAP. IV.
GENERAL INSTRUCTIONS FOR USING THE MICROSCOPE AND PREPARING THE OBJECTS.
As the advantages which are obtained from any instrument are
considerably increased, if it be used by a person who is master of its
properties, attentive to its adjustments, and habituated by practice to
the minutiæ of management, it is the design of this chapter to point out
those circumstances which more peculiarly require the attention of the
observer, and to give such plain directions, as may enable him to
examine any object with ease; to shew how he may place it in the best
point of view, and if necessary, prepare it for observation.
A small degree of diligence will render the observer master of every
necessary rule, and a little practice will make them familiar and
habitual: the pains he takes to acquire these habits will be rewarded by
an increasing attachment to his instrument, and the wonders it displays.
Let him only persevere till he has overcome the natural indolence that
opposes the advancement of every kind of knowledge, and he will most
assuredly find himself very amply recompensed, by the gratification
arising from the acquisition of a science that has the unlimited
treasures of INFINITE WISDOM for the object of its researches: and his
mind being strengthened by the victory it has gained, will be more keen
in perceiving, and more patient in the investigation of truth.
It has long been a complaint,[39] that many of those who purchase
microscopes are so little acquainted with their general and extensive
usefulness, and so much at a loss for objects to examine by them, that
after diverting themselves and their friends some few times with what
they find in the sliders, which generally accompany the instrument, or
perhaps two or three common objects, the microscope is laid aside as of
little further value: whereas no instrument has yet appeared in the
world capable of affording so constant, various, and satisfactory an
entertainment to the mind. This complaint will, I hope, be obviated by
these Essays, in which I have endeavoured to make the use of the
microscope easy, point out an immense variety of objects, and direct the
observer how to prepare them for examination.
[39] Baker’s Microscope made Easy, p. 51.
The subject treated of in this chapter naturally divides itself into
three heads: the first describes the necessary preparation and
adjustment of the microscope; the second treats of the proper quantity
of the light, and the best method of adapting it to the objects under
examination; and the third shews how to prepare and preserve the various
objects, that their nature, organization, and texture, may be properly
understood.
OF THE NECESSARY PREPARATION OF THE MICROSCOPE FOR OBSERVATION.
We have in the last chapter explained those particulars that constitute
the difference of one microscope from another, and shewn the manner of
using each instrument, and how the several parts are to be applied to
it. We shall now proceed to give some general directions applicable to
every microscope. The observer is therefore supposed to have made
himself master of his instrument, and to know how to adapt the different
parts of the apparatus to their proper places.
The first circumstance necessary to be examined into, is, whether the
different glasses belonging to the microscope are perfectly clean or
not; if they be not clean, they must be taken out and wiped with a piece
of wash leather, taking care at the same time not to soil the surface of
the glass with the fingers: in replacing the glasses, you must also be
careful not to lay them in an oblique situation, to place the convex
sides as before, and if one glass be taken out, wiped, and replaced
before the next, it may prevent the misplacing of them by an unskilful
hand.
The object should be brought as near the center of the field of view as
possible, for there only will it be exhibited in the greatest
perfection.
The eye should be moved up and down from the eye-glass of a compound
microscope, till you find that situation where the largest field, and
most distinct view of the object is obtained; and as the sight differs
very much in different persons, and even in the same person, we
frequently find each eye to have a different sight from the other,
particularly in those called myopes, or short-sighted, every one ought
to adjust the microscope to his own eye, and not depend upon the
situation in which it was placed by another.
Care must be taken not to let the breath fall upon the eye-glass, nor to
hold that part of the body of the microscope where the glasses are
placed with a warm hand, because the damp that is expelled from the
metal by the heat will be attracted and condensed by the glasses, and
obstruct the sight of the object.
The observer should always begin with a small magnifying power; with
this he will gain an accurate idea of the situation and connection of
the whole, and will therefore be less liable to form any erroneous
opinion, when the parts are viewed separately by a deeper lens. By a
shallow magnifier he will also discover those parts which merit a
further investigation. Objects that are transparent will bear a much
greater magnifying power than those that are opake.
Every object should, if possible, be examined first in that position
which is most natural to it: if this circumstance be neglected, very
inadequate ideas of the structure of the whole, as well as of the
connection and use of the parts, will be formed. If it be a living
animal, care must be taken not to squeeze, hurt, or discompose it.
There is a great difference between merely viewing an object by the
microscope, and investigating its nature: in the first, we only consider
the magnified representation thereof; in the second, we endeavour to
analyse and discover its nature and relation to other objects. In the
first case, we receive the impression of an image formed by the action
of the glasses; in the second, we form our judgment by investigating
this image. It is easy to view the image which is offered to the eye,
but not so easy to form a judgment of the things that are seen; an
extensive knowledge of the subject, great patience, and many
experiments, will be found necessary for this purpose: for there are
many circumstances where the images seen may be very similar, though
originating from substances totally different; it is here the
penetration of the observer will be exercised, to discover the
difference, and avoid error.[40]
[40] Fontana sur les Poisons, vol. ii, p. 245.
Hence Mr. Baker cautions us against forming too suddenly an opinion of
any microscopic object, and not to draw our inferences till after
repeated experiments and examinations of the objects, in all lights and
various positions; to pass no judgment upon things extended by force, or
contracted by dryness, or in any manner out of a natural state, without
making suitable allowances.
The true colour of objects cannot be properly determined when viewed
through the deepest magnifiers; for, as the pores and interstices of an
object are enlarged, according to the magnifying power of the glasses
made use of, the component parts of its substance will appear separated
many thousand times farther asunder than they do to the naked eye; it
is, therefore, very probable, that the reflection of the light from
these particles will be very different, and exhibit different colours.
Some consideration is also necessary in forming a judgment of the motion
of living creatures, or even of fluids, when seen through the
microscope; for as the moving body, and the space wherein it moves, are
magnified, the motion will also be increased.
If an object be so opake as not to suffer any light to pass through it,
as much as possible must be thrown on its upper surface, by that part of
the apparatus which is peculiarly adapted to opake objects. As the
apertures of deep magnifiers are but small, and consequently admit but
little light, they are not proper for the examination of opake objects:
this, however, naturally leads us to our second head.
OF THE MANAGEMENT OF THE LIGHT.
The pleasure arising from a just view of a microscopic object, the
distinctness of vision, &c. depend on a due management of the light, and
adapting the quantity of it to the nature of the object, and the focus
of the magnifier; therefore, an object should always be viewed in
various degrees of light. It is difficult to distinguish in some objects
between a prominency and a depression, between a shadow and a black
stain; and in colour, between a reflection and a whiteness; a truth
which the reader will find fully exemplified in the examination of the
eye of the libellula, and other flies, which will be found to appear
exceedingly different in one position of the light from what they do in
another.
The brightness of an object depends on the quantity of light; the
distinctness of vision, on regulating the quantity to the object; for
some will be lost and drowned, as it were, in a quantity of light that
is scarce sufficient to render another visible, as a different portion
of light under the same apparatus will often exhibit in perfection, or
totally conceal an object in the substance to be examined. This is more
particularly the case with the animalculæ infusoriæ, whose thin and
transparent form blend as it were with the water in which they swim; the
degree of light must therefore be suited to the object, which, if dark,
will be seen best in a strong and full light, but if very transparent,
it should be examined in a fainter.