from Bradley ( to FresneL 109 



ordinary ray given by Huygens had lain neglected for a century ; 

 and the degree of accuracy with which it represented the 

 observations was unknown. At Young's suggestion Wollaston* 

 investigated the matter experimentally, and showed that the 

 agreement between his own measurements and Huygens' rule 

 was remarkably close. " I think," he wrote, " the result must be 

 admitted to be highly favourable to the Huygeniaii theory ; 

 and, although the existence of two refractions at the same time, 

 in the same substance, be not well accounted for, and still less 

 their interchange with each other, when a ray of light is made 

 to pass through a second piece of spar situated transversely to 

 the first, yet the oblique refraction, when considered alone, seems 

 nearly as well explained as any other optical phenomenon." 



Meanwhile the advocates of the corpuscular theory were not 

 idle ; and in the next few years a succession of discoveries on 

 their part, both theoretical and experimental, seemed likely to 

 imperil the good position to which Young had advanced the 

 rival hypothesis. 



The first of these was a dynamical explanation of the 

 refraction of the extraordinary ray in crystals, which was 

 published in 1808 by Laplace.f His method is an extension of 

 that by which Maupertuis had accounted for the refraction of 

 the ordinary ray, and which since Maupertuis' day had been so 

 developed that it was now possible to apply it to problems of 

 all degrees of complexity. Laplace assumes that the crystalline 

 medium acts on the light-corpuscles of the extraordinary ray so 

 as to modify their velocity, in a ratio which depends on the 

 inclination of the extraordinary ray to the axis of the crystal : 

 so that, in fact, the difference of the squares of the velocities of 

 the ordinary and extraordinary rays is proportional to the 

 square of the sine of the angle which the latter ray makes with 

 the axis. The principle of least action then leads to a law of 

 refraction identical with that found by Huygens' construction 



* Phil. Trans., 1802, p. 381. 



tMem. de PInst., 1809, p. 300: Journal de Physique, Jan., 1809; Mem. de 

 la Soc. d'Arcueil, ii. 



