POLARIZATION OF LIGHT. 



163 



plano-convex as the first lens, and to cement it with Canada 

 balsam to the rhomb. 



(177) A more usual method is to stop one of the pencils 

 by total reflexion. A long rhomb of Iceland spar is taken, 

 and cut obliquely, in such manner that the refracting edges 

 of the two resulting prisms shall be parallel to the axis of 

 the crystal. The cut faces are then polished, and reunited 

 in their original positions by Canada balsam. Now the index 

 of refraction of Canada balsam is less than that of the 

 ordinary ray of Iceland spar, while it is greater than that of 

 the extraordinary ray. Hence, when the incidence is suffi- 

 ciently oblique, the ordinary ray will be totally reflected, 

 while the extraordinary ray is transmitted. The transmitted 

 light is therefore polarized. 



The angle of incidence at which total reflexion takes 

 place is easily calculated. The refractive index of the ordinary 

 ray in Iceland spar = 1*654 ; and that of Canada balsam 

 = 1*549. Hence the ratio of the sines of incidence and re- 

 fraction from the crystal into the balsam, for that ray, 



1*549 



0'937. Accordingly, when the refracted ray just 



grazes the surface, the sine of incidence = 0-937 ; and the 

 corresponding angle = 69 30'. The ordinary ray therefore will 

 be wholly reflected, when it is incident on the separating sur- 

 face at this or any greater angle, or when that surface is 

 inclined to the axis of the prism at any angle less than 20 30 X . 

 This ingenious contrivance is denominated, after the in- 

 ventor, Nicol's prism. 



The foregoing arrangement has been much improved 

 by M. Foucault, by dispensing with the Canada balsam 

 altogether, and shortening the rhomb. The sines of the 

 angles of total reflexion in passing from the crystal into 



M2 



