120 HISTORY OF OPTICS. 



:>f dipolamed colors by MM. Arago and Biot. At the end of the 

 Notice just quoted, Fresnel says," " As soon as my occupations permit 

 me, I propose to employ a pile of prisms similar to that which I have 

 described, in order to study the double refraction of the rays which 

 traverse crystals of quartz in the direction of the axis." He then ven- 

 tures, without hesitation, to describe beforehand what the phenomena 

 will be. In the Bulletin des Sciences 3 for December, 1822, it is 

 stated that experiment had confirmed what he had thus announced. 



The phenomena are those which have since been spoken of as circu- 

 lar polarization ; and the term first occurs in this notice. 4 They are 

 very 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 observa- 

 tion 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 formula} expressed an 

 impossibility ; yet as algebraical formula?, 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 

 collected the law of the difference of undulation of the two rays. He 

 was thus able to predict that by two internal reflections in a rhomb, 

 or parallelepiped of glass, of a certain -form and position, a polarized 

 ray would acquire a circular undulation of its particles ; and this con- 

 stitution 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 exactly 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 philosophers. " As I cannot appreciate the mathe- 

 matical evidence for the nature of circular polarization," says Prof, 

 Airy, 6 " I shall mention the experimental evidence on which I receive 

 it." The conception has since been universally adopted. 



But Fresnel, having thus obtained circularly-polarized rays, saw 



2 Aim. de Ch'un. 1S22, torn. xx. p. ?>82. 



3 Ib. Ann. d; Cliim. 1822, torn. xx. p. 191. 4 Ib. p. 19-t. 



Bullet, des Sc. 18-23, p. :::;. 6 Cmnb. Trans, vol. iv. p. 81, 1S31 



