CH. /.] 



MICROSCOPE AND ACCESSORIES. 



II 



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.E. 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. Aft ocular in which the real image is formed outside the ocular. 

 Compare the positive ocular ivith the simple microscope {Fig. 16). 



NOMENCLATURE OR TERMINOLOGY OF OBJECTIVES. 



§ 13 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 J j 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 T x s 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, the lower is the power of the objective. (See further in Ch. IV, for 

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

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



\ 15. Dry Objectives. — These are objectives in which the space between the front 

 of the objective and the object or cover-glass is filled with air (Fig. 22). Most ob- 

 jectives of low and medium power (i. e., ^th in. or 3 mm. and lower powers) are 

 dry. 



Fig. 22. Section of a dry objective showing 

 working distance and lighting by reflected 

 light. 



Axis. The principal optic axis of the ob- 

 jective. 



B C. Back Combination, composed of a 

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

 double convex lens of crown glass {c). 

 E 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 microscope. 



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

 the object when the objective is hi focus. 



