. The microscope and microscopical methods. Microscopes; 1896. CH. /.] MICROSCOPE AND ACCESSORIES. Axis. The principal optic axis of the microscope and of the eye. Cr. Cornea of the eye. Crystalline lens of the eye. R. Single, ideal, re- fracting surface at which all the refractions of the eye may be assumed to take place. The principal focus of the positive ocular and of the objective. Mirror. The mirror reflecting parallel rays to the object. The light is central. See Ch. II. Pos. Ocular. An ocular in which the real image is formed outside the ocular. Compare the positive ocular wit
. The microscope and microscopical methods. Microscopes; 1896. CH. /.] MICROSCOPE AND ACCESSORIES. Axis. The principal optic axis of the microscope and of the eye. Cr. Cornea of the eye. Crystalline lens of the eye. R. Single, ideal, re- fracting surface at which all the refractions of the eye may be assumed to take place. The principal focus of the positive ocular and of the objective. Mirror. The mirror reflecting parallel rays to the object. The light is central. See Ch. II. Pos. Ocular. An ocular in which the real image is formed outside the ocular. Compare the positive ocular with the simple microscope {Fig. 16). NOMENCLATURE OR TERMINOLOGY OF OBJECTIVES. § l3 Equivalent Focus.—In America, England, and sometimes also on the Con- tinent, objectives are designated by their equivalent focal length. This length is given either in inches (usually contracted to in.) or in millimeters (mm). Thus : An objective designated T\ in. or 2 mm., indicates that the objective produces a real image of the same size as is produced by a simple converging lens whose prin- cipal focal distance is ^ inch or 2 millimeters (Fig. 11). An objective marked 3 in. or 75 mm., produces approximately the same sized real image as a simple con- verging lens of 3 inches or 75 millimeters focal length. And in accordance with the law that the relative size of object and image vary directly as their distance from the center of the lens (Figs 14. 15, see Ch. IV,) it follows that the less the focal distance of the simple lens or of the equivalent focal distance of the objec- tive, the greater is the size of the real image, as the tube-length remains constant and the image in all cases is found at about 160 or 250 mm. from the objective. \ 14 Numbering or Lettering Objectives.—Instead of designating objectives by their equivalent focus, many Continental opticians use letters or figures for this purpose. With this method the smaller the number, or the earlier in the alpha- bet the letter,
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