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 
