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 



Fig. 57. — ^Diagram of a Nicol 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 



