66 ELEMENTARY CHEMICAL MICROSCOPY 
stmment 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 EwelE and employed 
by him as a colorimeter. The Van Heurck comparison eyepiece. 
Fig. 27, as constructed by Bausch and Lomb consists of a rec¬ 
tangular cell provided on the lower side with two orifices and 
1'' '' 
1 
i P 
-3 
V 
k 
tC— 
/ 
i' I 
A 
V 3! 
with tubes T^ and T^ 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^, T^ are placed totally reflecting prisms P^, 
* Ewell, J. Roy. Micros. Soc., 1910, 14. 
