DOUBLE REFRACTION. 197 



single rule all the possible cases of double refraction, 

 were thus misled, for they all admitted, as a fact of 

 which no one could doubt, that for half the light, for the 

 rays called "ordinary rays," the deviation ought to be 

 the same at the same incidence in whatever direction 

 the plane of incidence cut the crystal. The true law of 

 these complicated phenomena the law which includes, 

 as particular cases, the laws of Descartes and of Huy- 

 ghens is due to Fresnel. This discovery required in 

 an eminently high degree the union of a talent for exper- 

 iment with the genius of invention. 



I freely admit that the phenomena of double refraction 

 recently analyzed by Fresnel, and the laws which con- 

 nect them, are not exempt from a certain complexity. 

 This is indeed a subject of regret almost, I might say, 

 of lamentation among some idle minds, who would wish 

 to reduce every science to those superficial notions of 

 which they might make themselves masters by a few 

 hours' work. But does not every one see that with such 

 ideas the sciences would not make any progress ; that to 

 neglect such phenomena because one feeble intellect may 

 experience some trouble in grasping them, would be to 

 be false to our vocation, and that thus we should often 

 allow the most important discoveries to pass by us. 



Thus astronomy, while limited to a knowledge of the 

 constellations, and to some insignificant remarks on the 

 risings and settings of the stars, was within the capacity 

 of minds of any class : but could we then call it a science ? 

 From that time till after the most colossal labour which 

 one man ever went through, Kepler had substituted 

 elliptic motions not uniform, for the circular and regular 

 motions which, according to the ancients, prevailed in the 

 planets, his contemporaries might with equal right have 



