346 



OPTICAL PROJECTION 



a dark field. But all practical purposes have lately been secured 

 by a polariser described by Delezenne, and which gives a field 

 of any size at a very moderate expense. 



196. Direct Reflecting Polariscopes. The principle is 

 simply that of bringing the reflected beam from an elbow 

 polariscope, by another reflection, back into the original 

 direction of the rays from the lantern. It has been applied 

 in practice in two different ways. Mr. Ahrens has constructed 

 a combination shown in fig. 192, where T K is a totally re- 

 flecting prism of glass, so cut that its end faces are normal 



FIG. 192 



FIG. 193 



to the horizontal incident, and the emergent rays, which are 

 totally reflected midway at the polarising angle ; then these 

 rays falling on the polariser p of black glass, or two or three 

 thin plates with a black glass at the bottom, also at the 

 polarising angle, are reflected in a horizontal direction into 

 the instrument. This polariser is efficient, but two objections 

 to it are the needless expense of so large a glass prism, and 

 that the absorption in so large a mass is very perceptible. 



A cheaper and upon the whole better arrangement was 

 adopted by the Kev. P. E. Sleeman. In this the parallel beam 

 is first reflected by the silvered mirror B (fig. 193), and then 



