In making microscopic measurements it is only necessary to find how many lines of the ocular scale are covered by the object. The number of lines multiplied by the equivalent of each line will be the size of the object in microns, or _micromillimeters_. [Illustration: FIG. 24.--Micrometer Eye-Piece] MICROMETER EYE-PIECES =Micrometer eye-pieces= (Figs. 23 and 24) may be used in making measurements. These eye-pieces with micrometer combinations are preferred by some workers, but the ocular micrometer will meet the needs of the average worker. MECHANICAL STAGES Moving objects by hand is tiresome and unsatisfactory, first, because of the possibility of losing sight of the object under observation, and secondly, because the field cannot be covered so systematically as when a mechanical stage is used for moving slides. The =mechanical stage= (Fig. 25) is fastened to the stage by a screw. The slide is held by two clamps. There is a rack and pinion for moving the slide to left or right, and another rack and pinion for moving the slide forward and backward. [Illustration: FIG. 25. Mechanical Stage] CAMERA LUCIDA The =camera lucida= is an optical mechanical device for aiding the worker in making drawings of microscopic objects. The instrument is particularly necessary in research work where it is desirable to reproduce an object in all its details. In fact, all reproductions illustrating original work should be made by means of the camera lucida or by microphotography. [Illustration: FIG. 26.--Camera Lucida] [Illustration: FIG. 27.--Camera Lucida] A great many different types of camera lucidas or drawing apparatus are obtainable, varying from simple-inexpensive to complex-expensive forms. Figs. 26, 27, and 28 show simple and complex forms. [Illustration: FIG. 28.--Drawing Apparatus] MICROPHOTOGRAPHIC APPARATUS The =microphotographic apparatus= (Fig. 29), as the name implies, is an apparatus constructed in such a manner that it may be attached to a microscope when we desire to photograph microscopic objects. It consists of a metal base and a polished metal pillar for holding the bellows, slide holder, ground-glass observation plate, and eye-piece. In making photographs, the small end of the bellows is attached to the ocular of the microscope, the focus adjusted, and the object or objects photographed. More uniform results are obtained in making such photographs if an artificial light of an unvarying candle-power is used. [Illustration: FIG. 29.--Microphotographic Apparatus]