120 



THE MICROSCOPE. 



halving the cones of rays above the eye-pieces, where, 

 by simply turning the caps with the diaphragms, 

 orthoscopic or pseudoscopic effects can be instanta- 

 neously produced. This arrangement is particularly 

 suitable for the cheaper forms of microscopes, and for 

 those of foreign manufacture, which are usually shorter 

 in the body than English -made instruments. 



FIG. 71. Professor Abbe's Stereoscopic Eye-pieces. 



The most important improvement effected by Wen- 

 ham consists in splitting up or dividing the pencil of 

 rays proceeding from the objective by the interposition 

 of a prism of the form shown in fig. 72. This is placed 

 in the body or tube of the microscope (fig. 72&, a) so as 

 to interrupt only one-half (a. c) of the pencil, the other 

 half (a &) going on continuously to the field-glass, eye- 

 piece, of the principal body. The interrupted half of 

 the pencil, on its entrance into the prism, is subjected 

 to very slight refraction, since its axial ray is perpen- 



