When an object is viewed through three lenses, the magnification is greater than when viewed through one or two lenses, but a smaller part of the object is magnified. FORMS OF SIMPLE MICROSCOPES TRIPOD MAGNIFIER The =tripod magnifier= (Fig. 1) is a simple lens mounted on a mechanical stand. The tripod is placed over the object and the focus is obtained by means of a screw which raises or lowers the lens, according to the degree it is magnified. WATCHMAKER’S LOUPE The =watchmaker’s loupe= (Fig. 2) is a one-lens magnifier mounted on an ebony or metallic tapering rim, which can be placed over the eye and held in position by frowning or contracting the eyelid. [Illustration: FIG. 1.--Tripod Magnifier] [Illustration: FIG. 2.--Watchmaker’s Loupe] FOLDING MAGNIFIER The =folding magnifier= (Fig. 3) of one or more lenses is mounted in such a way that, when not in use, the lenses fold up like the blade of a knife, and when so folded are effectively protected from abrasion by the upper and lower surfaces of the folder. [Illustration: FIG. 3.--Folding Magnifier] [Illustration: FIG. 4.--Reading Glass] READING GLASSES =Reading glasses= (Fig. 4) are large simple magnifiers, often six inches in diameter. The lens is encircled with a metal band and provided with a handle. [Illustration: FIG. 5.--Steinheil Aplanatic Lens] STEINHEIL APLANATIC LENSES =Steinheil aplanatic lenses= (Fig. 5) consist of three or four lenses cemented together. The combination is such that the field is large, flat, and achromatic. These lenses are suitable for field, dissecting, and pocket use. When such lenses are placed in simple holders, they make good dissecting microscopes. [Illustration: FIG. 6.--Dissecting Microscope] DISSECTING MICROSCOPE The =dissecting microscope= (Fig. 6) consists of a Steinheil lens and an elaborate stand, a firm base, a pillar, a rack and pinion, a glass stage, beneath which there is a groove for holding a metal plate with one black and one white surface. The nature of the object under observation determines whether a plate is used. When the plate is used and when the object is studied by reflected light it is sometimes desirable to use the black and sometimes the white surface. The mirror, which has a concave and a plain surface, is used to reflect the light on the glass stage when the object is studied by transmitted light. The dissecting microscope magnifies objects up to twenty diameters, or twenty times their real size. CHAPTER II COMPOUND MICROSCOPES