MICROSCOPE AND ACCESSORIES 



[CM. I 



Axis. The principal optic axis of the microscope and of the eye. 



Cr. Cornea of the eye. L. 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. 



F. F. 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). 



15- 



NOMENCLATURE OR TERMINOLOGY OF OBJECTIVES 

 Equivalent Focus. — In America, England, and sometimes 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 \, 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 fa inch or 2 millimeters (Fig. II). 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 accord- 

 ance 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 sizeof the real image, as the tube-length remains con- 

 stant and the image in all cases is found at about 160 or 250 mm. from the objective. 

 ''1, 16. 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, the lower is the power of the objective. ( See further in Ch. IV, for 

 the power or magnification of objectives). This method is entirely arbitrarv and 

 does not, like the one above, give direct information concerning the objective. 



\ 17. Air or Dry Objectives. — These are objectives in which the space be- 

 tween the front of the objective and the object or cover-glass is filled with air 

 (Fig. 22). Most objectives of low and medium power (z. e,, \ in. or 3 mm. and 

 lower powers) are dry. 



Fig. 22. Sec/ion of a dry objective showing 

 working distance and lighting by refected 

 light. 



Axis. The principal optic axis of the ob- 

 jeclive. 



B C. Back Combination, composed of a 

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

 double convex lens of crown glass (c). 

 F C. Front Combination. 

 C, O, si. The cover-glass, object and slide. 

 Mirror. The minor 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 microscope. 



