35 ON LIGHT. 



and extends to all media, whatever their refractive 

 powers. Thus, for water, the polarizing angle is 53 n', 

 and for diamond 68 6' numbers concluded from this 

 simple rule (equivalent to the geometrical property above 



D 

 Fig. 13. 



stated) that the index of refraction is in all cases the tangent 

 of the polarizing angle. 



(129.) If a ray of light so polarized by reflexion be 

 received on such a prism as is above described, held 

 with its refracting edge (i.e. t the optic axis of the crystal) 

 at right angles to the plane of reflexion, or parallel to 

 the reflecting surface, it will entirely pass into the ordi- 

 nary, or most refracted image ; vice versa if the prism be 

 turned round 90, so as to have its edge parallel to the 

 plane of reflexion (/.<?., of polarization), wholly into the 

 extraordinary. The same interchange will of course 

 take place if the prism be held immovable, and the re- 

 flecting glass turned round, so as to change the plane of 

 reflexion, and thus we perceive that rays oppositely polar- 

 ized are distinguished by the characters of passing entirely 

 into the one and entirely into the other of the two images, 

 when so refracted. A beam of light partially polarized 

 may be regarded as a mixture of two portions, the one 

 wholly polarized, the other wholly unpolarized ; and a 



