CONFIRMATION OF THE UNDULATORY THEORY. 483 



remarkable, both by their resemblances to, and their 

 differences from, the phenomena of plane-polarized 

 light. And the manner in which Fresnel was led to 

 this anticipation of the facts is still more remarkable 

 than the facts themselves. Having ascertained by 

 observation that two differently -polarized rays, 

 totally reflected at the internal surface of glass, 

 suffer different retardations of their undulations, he 

 applied the formulae which he had obtained for the 

 polarizing effect of reflection to this case. But in 

 this case the formulae expressed an impossibility; 

 yet as algebraical formulae, even in such cases, have 

 often some meaning, "I interpreted," he says 5 , "in 

 the manner which appeared to me most natural and 

 most probable, what the analysis indicated by this 

 imaginary form ;" and by such an interpretation he 

 hence collected the law of the difference of undula- 

 tion of the two rays. lie was thus able to predict 

 that by two internal reflections in a rhomb, or 

 parallelepiped of glass, of a certain form and posi- 

 tion, a polarized ray would acquire a circular undu- 

 lation of its particles ; and this constitution of the 

 ray, it appeared, by reasoning further, would show 

 itself by its possessing peculiar properties, partly 

 the same as those of polarized light, and partly 

 different. This extraordinary anticipation was ex- 

 actly confirmed ; and thus the apparently bold and 

 strange guess of the author was fully justified, or at 

 least assented to, even by the most cautious philo- 



4 Bullet, des Sc. 1823, p. 33. 



112 



