180 



MINERALOGY 



to absorb any light that may fall on them after being reflected to 

 the sides of the calcite. A ray of ordinary white light on entering 

 a nicol, as at R, is divided into an ordinary and an extraordinary 

 ray, having different indices of refraction and traveling different 

 paths through the calcite; their angle of 

 total reflection will therefore differ. The 

 ordinary ray, with an index of refraction of 

 1.658 between air and calcite, is refracted 

 more than the extraordinary ray with an 

 index of refraction of 1.486; this ray will 

 meet the film of Canada balsam at an 

 angle greater than 69, which is the ap- 

 proximate critical angle of the ordinary 

 ray between Canada balsam and calcite, 

 since <o = 1.6583 divided by the index of 

 refraction of Canada balsam, 1.548, = 

 1.0712, the index of refraction of the ordi- 

 nary ray as between Canada balsam and 

 calcite. As between these two media, sin 



1 



(of the critical angle) = 



.critical an- 



1.0712' 



gle = 68 59'. All ordinary rays meet- 

 ing the film of balsam at an angle greater 

 than 68 59' will be totally reflected in the 

 direction as indicated, and absorbed by 

 the blackened walls of the cork mounting. 

 The index of refraction of the extraordi- 

 nary ray varies with the direction through 

 the crystal, but in this particular direction 

 it is but little different from that of the 

 balsam, 1.548; its path on entering the 

 calcite is deviated much less than the 

 ordinary ray, and on meeting the film of 

 balsam is but little effected, passing through 

 with little or no refraction, and emerging 

 at the opposite end of the nicol as plane 



polarized light, with a vibration plane parallel to the short di- 

 agonal cc of the rhombic section, and polarized in the plane 

 parallel to the long diagonal PP'. Other styles of polarizing 

 prisms have been devised, either to economize space or calcite, 

 as suitable calcite is very scarce and expensive, since the Iceland 



