CHAPTER IX. 



POLARIZATION OF LIGHT. 



(164) IN the various phenomena which take place when 

 a ray of light encounters the surface of a new medium, it has 

 been supposed that the direction and intensity of the several 

 portions into which it is subdivided will continue the same, 

 on whatever side of the ray the surface is presented, provided 

 that the angle and the plane of incidence continue unchanged. 

 In other words, it was taken for granted that a ray of light 

 had no relation to space, with the exception of that dependent 

 on its direction ; that, around that direction, its properties 

 were on all sides alike ; and that, if the ray be made to revolve 

 round that line as an axis, the resulting phenomena would 

 be unaltered. 



Huygens was the first to prove that this was not always 

 the case. In the course of his researches on the law of double 

 refraction, he found that when a ray of solar light is received 

 upon a rhomb of Iceland crystal, in any but one direction, it 

 is always subdivided into two of equal intensity. But, on 

 transmitting these rays through a second rhomb, he observed 

 that the two portions into which each of them was subdivided, 

 were no longer equally intense ; that their relative brightness 

 depended on the position of the second rhomb with regard to 

 the first ; and that there were two positions in which one of 

 the rays vanished altogether. 



On analyzing the phenomena, it is found that they de- 

 pend on the relative positions of the planes, passing through ^S 

 the axes of the crystals, and perpendicular to the refracting 



