Procure the flat lid of a cylindrical tin box, or the lid of a glass or porcelain pomade pot, such lid to be about two inches in diameter and about half an inch in depth. Cement the flat side of this lid to a small slab of hard wood, or to a square piece of sheet lead, by means of acetic glue--ordinary glue or gelatine dissolved in glacial acetic acid--to give it the necessary steadiness during the dissection. When the cement is quite hard, pour into the lid some melted paraffin (paraffin wax) which has been blackened by the admixture of a small quantity of lamp-black in the form of a fine powder. The paraffin should be melted by putting it into a beaker or wide-mouthed bottle, and standing it in hot water, and the lamp-black should be added, with stirring, as soon as it is entirely liquefied. The quantity of the mixture used must be sufficient to half fill the lid, thus leaving a space to contain water to the depth of about a quarter of an inch. The blackened wax provides a good background on which to work, and provides a hold for pins when these are necessary in order to fix the object under examination. [Illustration: FIG. 46.--A SMALL DISSECTING TROUGH] The complete trough is represented in fig. 46; and will be found to answer its purpose admirably, except that it occasionally displays one fault, but one that is easily remedied. The wax contracts on cooling, and may, therefore, detach itself from the trough; and, being lighter than water, will float instead of remaining submerged. This may be prevented by securing the disc of wax in its place by means of a ring of brass wire, or by weighting the wax with two or three small pieces of lead pushed down into it while it is yet soft. With such a dissecting microscope and trough as we have described one may do a great deal of exceedingly useful work, both hands being quite free to manipulate the object under examination. The dissection may be conducted with the aid of a small scalpel or other very sharp knife, the parts being arranged or adjusted by means of a needle, mounted in a handle, and held in the left hand. Sometimes, however, the object to be dissected is so minute that even a small scalpel is too large for the purpose, and in such cases nothing is better than little dissecting instruments made by mounting large sewing needles in suitable handles, and then grinding down the points of the needles on two opposite sides, on a hone, so as to produce little pointed, two-edged blades. Bent needles are often useful, too, and these may be prepared by heating the points to redness in a gas-flame, bending them as desired while hot, and then hardening them by suddenly thrusting them, at a red heat, into cold water.