EE —— ee ae Te 
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 erystal, 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. 
