_Object-glasses._--The object-glasses are the most valuable parts of the instrument. There are generally three or more of them; and, by means of an “adapter,” any object-glass can be made to fit any microscope. Great care is required in their use, especially to avoid scratching the lower surface of the glass, which is sometimes accidentally done by pressing the surface against any hard body, or allowing such a body to fall upon it. When not in use, the object-glasses should either be put away in the brass boxes or covered with a small bell-glass, to prevent their receiving any injury. The object-glasses possess various magnifying powers, according to the distance at which they require to be placed from the object for distinct vision: this is not, however, absolutely correct, yet may serve as a general expression. Thus we have a 1-inch, ½-inch, ¼-inch object-glass, &c. The object-glasses, for brevity, are often called powers. As a beginner may at first have some difficulty in distinguishing a high from a low power, it may be remarked that the size of the lower glass is larger the lower the power: but, in the case of the better object-glasses, the focal distance is engraved on the box in which the object-glass is packed when put away. As the coarse movement raises or depresses the body and object-glass through comparatively large distances, it must be used only with the lower object-glasses, _i. e._ those of low or little magnifying power, as the 2-inch, 1-inch, or ½-inch, or to bring the object-glass near the focal distance with the higher powers; whereas the fine movement serves to adjust the higher powers, as the ¼-inch, &c. If the object-glasses should become soiled on the lower face, this should be wiped very gently with an old silk handkerchief or piece of very soft wash-leather, previously shaken to displace dust. The same method will answer to cleanse the upper surface of the eye-piece. Great care must be taken that a slide which has been warmed in any experiment be not placed near the object-glass until quite cold. _Mirror._--The mirror has sometimes one silvered face only, at others two--one flat, the other concave. The flat surface is used to reflect the light upon the object when the light is too great with the concave surface. Beneath the stage, in most microscopes, is a circular moveable “diaphragm,” perforated with holes of various sizes, to allow more or less of the light reflected by the mirror to pass through, as may be required. When opake objects are viewed, the mirror should be turned aside, so as not to reflect any light through the stage. _Eye-pieces._--With all microscopes, two or more eye-pieces are supplied. These possess different magnifying powers, and are lettered or numbered accordingly; the lowest power with the earliest letters of the alphabet, or with the smallest numbers, thus: A, B, C; or 1, 2, 3, &c.