DOUBLE REFRACTION. 149 



himself of the ingenious method by which he found the 

 index of refraction by means of total reflexion. It ap 

 pears that in 1808 these verifications had not appeared 

 sufficient to the physicists of the Academy of Sciences, 

 since they proposed the question as the subject of a prize 

 for experimenters. However this may have been, Mains 

 translated the construction of Huyghens into analytical 

 formulas : he compared the deviation of the extraordi 

 nary rays deduced from these formulas with the numbers 

 resulting from very accurate observations, and the accord 

 ance was in all cases very perfect. Thus the geomet 

 rical conception of Huyghens was found to be completely 

 established, although originally the author was led to it 

 by theoretical views. 



A ray of light divides itself into two rays which are of 

 exactly the same intensity whatever be the position of 

 the crystal which it traverses, and in which the division 

 into two is produced. But the case is different when the 

 rays pass out of one crystal and are received into, and 

 analyzed by, a second crystal exactly similar. If this sec 

 ond crystal is situated relatively to the first in such a way 

 that the corresponding faces are respectively parallel to 

 each other, the ordinary ray in traversing it only under 

 goes the ordinary refraction, and the extraordinary ray 

 also remains exclusively an extraordinary ray. The 

 natural light then in traversing the first crystal has thus 

 changed its nature. In fact, if, in becoming double, it 

 had preserved its original properties, the ordinary ray 

 and the extraordinary would each have been divided into 

 two rays in traversing the second crystal. At emer 

 gence from the second crystal we should have had four 

 images instead of two. The first idea which occurs to 

 the mind would be that the natural light is composed of 

 parts which are susceptible, some of them undergoing the 



