Cover glasses are either square or round. Of each there are four different thicknesses and two different sizes. The standard thicknesses are: The small size is designated three-fourths and the large size seven-eighths. [Illustration: FIG. 41.--Round Cover Glass] [Illustration: FIG. 42.--Square Cover Glass] [Illustration: FIG. 43.--Rectangular Cover Glass] =Cover glasses= are circular (Fig. 41), square (Fig. 42), or rectangular (Fig. 43) pieces of transparent glass used in covering the specimens mounted on glass slides. A few years ago much difficulty was experienced in obtaining uniformly thick and transparent cover glasses, but no such difficulty is experienced to-day. The type of cover glass used depends largely upon the character of the specimen to be mounted. The square and rectangular glasses are selected when a series of specimens are to be mounted, but in mounting powdered drugs and histological specimens the round cover glasses are preferable because they are more sightly and more readily cleaned and rinsed. GLASS SLIDES [Illustration: FIG. 44.--Glass Slide] =Glass slides= (Fig. 44) are rectangular pieces of transparent glass used as a mounting surface for microscopic objects. The slides are usually three inches long by one inch wide, and they should be composed of white glass, and they should have ground and beveled edges. Slides should be of uniform thickness, and they should not become cloudy upon standing. SLIDE AND COVER-GLASS FORCEPS Slides and cover glasses should be grasped by their edges. To the beginner this is not easy. In order to facilitate holding slides and cover glasses during the mounting process, one may use a slide and a cover-glass =forceps=. The slide forceps consists of wire bent and twisted in such a way that it holds a slide firmly when attached to its two edges. [Illustration: FIG. 45.--Histological Forceps] [Illustration: FIG. 46.--Forceps] [Illustration: FIG. 47.--Sliding-pin Forceps] There are various forms of cover-glass holders, but only two types as far as the method of securing the cover glass is concerned. First, there are the bacteriological and the histological forceps (Fig. 45), which are self-closing. The two blades of such forceps must be forced apart by pressure in securing the cover glass. The second type of forceps is that in which the two blades are normally separated (Fig. 46), it being necessary to press the blades to either side of the cover glass in order to secure and hold it. There is a modification of this type of forceps which enables one to lock the blades by means of a sliding pin (Fig. 47), after the cover glass has been secured. It is well to accustom oneself to one type, for by so doing one may become dexterous in its use. NEEDLES [Illustration: FIG. 48.--Dissecting Needle]