103 



devices for regulating the light, the generally accepted 

 forms are those devised by Prof. Abbe. One of them 

 with a numerical aperture of 1.20 consists of a com- 

 bination of two lenses, and the other with an aper- 

 ture of. 1.42, of three lenses. These condensers are 

 applied to microscopes by each maker in a different 

 way, each seeking in his own manner to provide 

 means for controlling the volume or angle of light as 

 well as to obtain any degree of obliquity. To bring 

 the condenser to the minimum of cost, the Bausch & 

 Lo'mb Optical Co. quote both condensers in plain 

 adapters, giving the entire condensing capacity which 

 may be varied only by varying their distance from 

 the object. In this form they are incomplete, how- 

 ever, as their effectiveness depends upon the nicety 

 with which the light can be controlled. The most 



Fig. 23. 



simple device for this purpose consists of the mount- 

 ing with a slide in which various diaphragms wilh cen- 

 tral stops and openings may be dropped into it, and 



