It may be desirable to know how such small differences as those betwixt the various thin glass covers can be measured. For this purpose there are two or three sorts of apparatus, all, however, depending upon the same principle. The description of one, therefore, will be sufficient. Upon a small stand is a short metal _lever_ (as it may be termed) which returns by a spring to one certain position, where it is in contact with a fixed piece of metal. At the other end this lever is connected with a “finger,” which moves round a dial like that of a watch, whereupon are figures at fixed distances. When the lever is separated from the metal which is stationary, the other end being connected with the “finger” of the dial, that “finger” is moved in proportion to the distance of the separation. The thin glass is, therefore, thrust betwixt the end of the lever and fixed metal, and each piece is measured by the figures on the dial in stated and accurate degrees. This kind of apparatus, however, is expensive, and when not at our command, the thin glass may be placed edgewise in the stage forceps, and measured very accurately with the micrometer. Cleanliness with the thin glass is, perhaps, more necessary than with the slides, especially when covering objects which are to be used with a high power; but it is far more difficult to attain, on account of the liability to breakage. The usual method of cleaning these covers is as follows:--Two discs of wood, about two inches in diameter, are procured, one side of each being perfectly flat and covered with clean wash-leather. To the other side of these a small knob is firmly affixed as a handle, or where practicable the whole may be made out of a solid piece. In cleaning thin glass, it should be placed betwixt the covered sides of the discs, and may then be safely rubbed with a sufficient pressure, and so cleaned on both sides by the leather. If, however, the glass be greasy, as is sometimes the case, it must be first washed with a strong solution of potash, infusion of nut-galls, or any of the commonly used grease-removing liquids; and with _some_ impurities water, with the addition of a few drops of strong acid, will be found very useful, but this last is not often required. The above method of cleaning thin glass should always be used by beginners; but after some experience the hand becomes so sensitive that the above apparatus is often dispensed with, and the glasses, however thin, may be safely cleaned betwixt the fingers and thumb with a cambric handkerchief, having first slightly damped the ends of the fingers employed to obtain firm hold. When the dirt is very obstinate, breathing upon the glass greatly facilitates its removal, and the sense of touch becomes so delicate that the breakage is inconsiderable; but this method cannot be recommended to novices, as nothing but time spent in delicate manipulation can give the sensitiveness required.