484 HISTORY OF OPTICS. 



sophers. "As I cannot appreciate the mathematical 

 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 (NA). 



But Fresnel, having thus obtained circularly- 

 polarized rays, saw that he could account for the 

 phenomena of quartz, already observed by M.Arago, 

 as we have noticed in Chap, ix., by supposing two 

 circularly-polarized rays to pass, with different velo- 

 cities, along the axis. The curious succession of 

 colours, following each other in right-handed or 

 left-handed circular order, of which we have already 

 spoken, might thus be hypothetically explained. 



But was this hypothesis of two circularly-polar- 

 ized rays, travelling along the axis of such crystals, 

 to be received, merely because it accounted for the 

 phenomena? Fresnel's ingenuity again enabled him 

 to avoid such a defect in theorizing. If there were 

 two such rays, they might be visibly separated 7 by 

 the same artifice, of a pile of prisms properly 

 achromatized, which he had used for compressed 

 glass. The result was, that he did obtain a visible 

 separation of the rays; and this result has since 

 been confirmed by others, for instance, Professor 

 Airy 8 . The rays were found to be in all respects 

 identical with the circularly-polarized rays produced 

 by the internal reflections in Fresnel's rhomb. This 



6 Caml. Trans, vol. iv. p. 81, 1831. 



7 Bull, des Sc, 1822, p. 193. * Camb. Tr. iv. p. 80. 



