DOUBLE REFRACTION. 219 



again by the second. The ordinary ray remains an ordi- 

 nary ray, and the extraordinary undergoes solely an 

 extraordinary refraction. Thus, in traversing the first 

 crystal, the luminous rays have changed their nature ; 

 they have lost one of their former characteristics, that of 

 constantly undergoing double refraction in traversing Ice- 

 land crystal.* 



It is necessary that we should fully bear in mind what 

 rays of light are, and then, perhaps, we shall admit that 

 an experiment, by the aid of which they change their 

 original properties in so manifest a way, deserves to be 

 known even by those to whom science is merely an object 

 of curiosity. 



The idea which in the first instance presents itself to 

 the mind, when we wish to explain this singular result of 

 which I have just given an account, consists in supposing 

 that in every ray there might exist two distinct species of 

 molecules : that the one species must always undergo the 

 ordinary refraction ; the other, the extraordinary alone. 

 But a very simple experiment upsets this hypothesis en- 

 tirely. In fact, when the principal section of the second 

 crystal, instead of being directed north and south as above 

 supposed, is pointed east and west, the ray which was the 

 ordinary ray in the first crystal, becomes the extraordi- 

 nary in the second ; and reciprocally. 



What, then, is there different in reality between the 

 two experiments which give results so dissimilar ? There 

 is one circumstance, very simple, and full of import at 

 first sight : it is, that at first the principal section of the 

 second crystal cuts the rays coming from the first through 

 their north and south sides, and in the second case, through 

 their east and west sides. 



* For illustration of this subject, see note to the Life of Malus. 



