CONFIRMATION OF THE UNDULATORY THEORY. 481 



and sagacity with which he devised experiments, 

 and applied to them mathematical reasonings. 



1. Double Refraction of Compressed Glass. One 

 of these confirmatory experiments was the produc- 

 tion of double refraction by the compression of glass. 

 Fresnel observes 1 , that though Sir D. Brewster had 

 shown that glass under compression produced co- 

 lours 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 polarization, it is added, there is no 

 apparent connexion between these phenomena of 

 coloration and double refraction ; but on Young's 

 theory, that the colours arise from two rays which 

 have traversed the crystal with different velocities, 

 it appears almost unavoidable to admit also a dif- 

 ference 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 neglect an experi- 

 mental verification of it;" and therefore, in 1819, 

 he proceeded to satisfy himself of the fact, by the 

 phenomena of diffraction. The trial left no doubt 

 on the subject; but he still thought it would be 

 interesting actually to produce two images in glass 

 by compression; and by a highly-ingenious com- 

 bination, calculated to exaggerate the effect of the 

 double refraction, which is very feeble, even when 



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

 VOL. II. 1 1 



