CH. /] 



MICROSCOPE AND ACCESSORIES 



13 



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 



I 18. Equivalent Focus. — In America, England, and now also on the 

 Continent, 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 V 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 principal 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 converging 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 objective, the greater is the size of the 

 real image, as the tube-length remains constant and the image in all cases is 

 formed at 160 or 250 mm. from the objective. 



I 19. Numbering or Lettering Objectives. — Instead of designating 

 objectives by their equivalent focus, many Continental opticians use letters or 

 figures for this purpose ; in most cases, however, the equivalent focus is also 



Fig. 27. Section of a dry objective 

 showing working distance and lighting by 

 reflected light. 



Axis. The principal optic axis of the 

 objective. 



B C. Back Combination, composed of 

 a plano-concave lens of flint glass {F), and 

 a double couvex lens of crown glass (c). 



F C. Front Combination. 



C, 0, si. The cover-glass, object and 

 slide. 



Mirror. The mirror is represented as 

 above the stage, and as reflecting parallel 

 rays from its plane face upon the object. 



Stage. Section of the stage of the mi- 

 croscope. • 



IV. The Working Distance, that is the distance from the front of the 

 objective tojhe object when the objective is in focus. 



given. With this method the smaller the number, or the earlier in the alpha- 

 bet the letter, the lower is the power of the objective. , (See further in Ch. IV, 

 for the power or magnification of objectives.) This method is entirely arbi- 



