4. =Eye-pieces= (Figs. 13, 14, and 15) are of variable length, but structurally they are somewhat similar. The eye-piece consists of a metal tube with a blackened inner tube. In the centre of this tube there is a small diaphragm for holding the ocular micrometer. In the lower end of the tube a lens is fastened by means of a screw. This, the field lens, is the larger lens of the ocular. The upper, smaller lens is fastened in the tube by a screw, but there is a projecting collar which rests, when in position, on the draw-tube. [Illustration: FIG. 13.] [Illustration: FIG. 14.] [Illustration: FIG. 15. Eye-Pieces.] The longer the tube the lower the magnification. For instance, a two-inch ocular magnifies less than an inch and a half, a one-inch less than a three-fourths of an inch, etc. The greater the curvature of the lenses of the ocular the higher will be the magnification and the shorter the tube-length. FORMS OF COMPOUND MICROSCOPES The following descriptions refer to three different models of compound microscopes: one which is used chiefly as a pharmacognostic microscope, one as a research microscope stand, while the third type represents a research microscope stand of highest order, which is used at the same time for taking microphotographs. [Illustration: FIG. 16.--Pharmacognostic Microscope] PHARMACOGNOSTIC MICROSCOPE The =pharmacognostic microscope= (Fig. 16) is an instrument which embodies only those parts which are most essential for the examination of powdered drugs, bacteria, and urinary sediments. This microscope is provided with a stage of the dimensions 105 × 105 mm. This factor and the distance of 80 mm. from the optical centre to the handle arm render it available for the examination of even very large objects and preparations, or preparations suspended in glass dishes. The stand is furnished with a side micrometer, a fine adjustment having knobs on both sides, thereby permitting the manipulation of the micrometer screw either by left or right hand. The illuminating apparatus consists of the Abbé condenser of numerical aperture of 1.20, to which is attached an iris diaphragm for the proper adjustment of the light. A worm screw, mounted in connection with the condenser, serves for the raising and lowering of the condenser, so that the cone of illuminating pencils can be arranged in accordance to the objective employed and to the preparation under observation. The objectives necessary are those of the achromatic type, possessing a focal length of 16.2 mm. and 3 mm. Oculars which render the best results in regard to magnification in connection with the two objectives mentioned are the Huyghenian eye-pieces II and IV so that magnifications are obtained varying from 62 to 625. It is advisable, however, to have the microscope equipped with a triple revolving nose-piece for the objectives, so that provision is made for the