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, Malus 
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 erystal 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 
