192 -FRESNEL. 
occupied the attention of the greatest geometers of the 
seventeenth century. 
The question had arrived at this point, when a travel- 
ler, returning from Iceland, brought to Copenhagen some 
beautiful crystals from the Bay of Roérford. Their 
great thickness and remarkable transparency rendered 
them particularly proper for experiments on refraction. 
Bartholinus (1669), to whom they were sent, took care 
to subject them to different trials ; but what was his 
astonishment when he perceived that the light divided 
itself into two distinct beams of precisely equal intensi- 
ties,—when he recognized, in one word, that seen through 
the Iceland spar (which has been since found in many. 
other localities, being nothing but carbonate of lime) all 
objects appear double! The theory of refraction, so 
many times recast, had now need of a new examination. 
At all events it was incomplete, for it spoke only of one 
ray, and-two were here seen. Jesides, the direction and 
the magnitude of the deviation of the two rays changed, 
apparently in the most capricious manner, when we passed 
from one face of a crystal to another, or when on one face 
the direction of the incident ray varied.* 
Huyghens surmounted all these difficulties ; a general 
law was found to comprehend in its announcement all the 
lesser details of the phenomena; but this law, in spite of 
its simplicity and elegance, was misconstrued. Hypoth- 
eses had been for so many ages useless or faithless guides ; 
they had been so long considered as constituting the whole © 
of physics, that, at the epoch of which I speak, experi- 
menters had on this point arrived at a sort of reaction ; 
and in such reactions, even in science, it is rare to be 
* See above, note, p. 160. 
