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 Roerford. 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. Besides, 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. 



