24 ELEMENTARY CHEMICAL MICROSCOPY 



strument by a different arrangement of prisms. This latter 

 type has again been revived by the Bausch and Lomb Optical 

 Company in 1912, and by E. Leitz in 1914. 



A somewhat similar comparing device, consisting of two 

 totally reflecting prisms, was proposed by Ewell 1 and employed 

 by him as a colorimeter. The Van Heurck comparison eyepiece, 

 Fig. 5, as constructed by Bausch and Lomb consists of a rec- 

 tangular cell provided on the lower side with two orifices and 



FIG. 5. The Bausch and Lomb Comparison Eyepiece. 



with tubes T 1 and T 2 of the same diameter as ordinary oculars, 

 and at such a distance apart as to permit their simultaneous 

 insertion into the tubes of two microscopes placed side by side, 

 Midway between these tubes on the top of the cell is an opening 

 with a tube into which slides a Ramsden eyepiece O. Above 

 the tubes T 1 , T 2 are placed totally reflecting prisms P 1 , P 2 , 



1 Ewell, J. Roy. Micros. Soc., 1910, 14. 



