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.