CONFIRMATION OF THE UNDULATORY THEORY. 119 



In the case of this great theory, as in that of gravitation, by far the 

 most remarkable of these confirmatory researches were conducted by 

 the authors of the discovery, especially Fresnel. And in looking at 

 what he conceived and executed for this purpose, we are, it appears to 

 me, strongly reminded of Newton, by the wonderful inventiveness and 

 sagacity with which he devised experiments, and applied to them 

 mathematical reasonings. 



1. Double Refraction of Confessed Glass. One of these confir- 

 matory experiments was the production of double refraction by the 

 compression of glass. Fresnel observes, 1 that though Sir D. Brewster 

 had shown that glass under compression produced colors resembling 

 those which are given by doubly-refracting crystals, "very skilful 

 physicists had not considered those experiments as a sufficient proof 

 of the bifurcation of the light." In the hypothesis of moveable pola- 

 rization, it is added, there is no apparent connexion between thes<; 

 phenomena of coloration and double refraction; but on Young's 

 theory, that the colors arise from two rays which have traversed the 

 crystal with different velocities, it appears almost unavoidable to 

 admit also a difference of path in the two rays. 



" Though," he says, " I had long since adopted this opinion, it did 

 not appear to me so completely demonstrated, that it was right to ne- 

 glect an experimental verification of it;" and therefore, in 1819, he 

 proceeded to satisfy himself of the fact, by the phenomena of diffrac- 

 tion. The trial left no doubt on the subject ; but he still thought it 

 would be interesting actually to produce two images in glass by com- 

 pression ; and by a highly-ingenious combination, calculated to exag- 

 gerate the effect of the double refraction, which is very feeble, even 

 when the compression is most intense, he obtained two distinct 

 images. This evidence of the dependence of dipolarizing structure 

 apon a doubly-refracting state of particles, thus excogitated out of the 

 general theory, and verified by trial, may well be considered, as he 

 says, " as a new occasion of proving the infallibility of the principle of 

 interferences." 



2. Circular Polarizatior,. Fresnel then turned his attention to 

 another set of experiments, related to this indeed, but by a tie so 

 recondite, that nothing less than his clearness and acuteness of view 

 could have detected any connexion. The optical properties of quartz 

 had been perceived to be peculiar, from the period of the discovery 



Ann, de Chim. 1822, torn. xx. p. 377. 



