156 ELEMENTS OF APPLIED MICROSCOPY. 
images of an object viewed through them (double re- . 
fraction). 
The polariscope is an instrument for detecting the 
polarizing power or double refraction of bodies, and con- 
sists, in essence, of a pair of Nicol prisms, each of which 
is made by cutting a rhomb of calcite diagonally and 
cementing the halves together again, with a layer of 
Fic. 57.—D1acRaAmM OF A Nicot Prism. (After Clark.) 
Canada balsam between them. Through such a prism 
(Fig. 57) one of the rays of plane polarized light (EFGH), 
known as the extraordinary ray, passes unchanged, while 
the other (EFKL), the ordinary ray, is so strongly 
refracted in the first half of the prism that it meets 
the layer of balsam at such an angle as to be totally 
reflected from the balsam surface, and is thus re- 
moved. A Nicol prism is, then, a device for removing 
