Notes on some Points in the Theory of Light. 217 



rejected, because I saw that it would lead to the result above 

 mentioned. 



Another result of M. Cauchy's, which he has given twice in 

 the Oomptes Rendus* requires to be noticed. When a polarized 

 ray is reflected by a metal, the phase of its vibration is altered ; 

 and if the incidence be oblique, the change of phase is different 

 according as the light is polarized in the plane of incidence, or 

 in the perpendicular plane. But when the ray is reflected at a 

 perpendicular incidence, it is manifest that the change is a con- 

 stant quantity, whatever be the plane of polarization. In fact, 

 the distinction between the plane of incidence and the perpen- 

 dicular plane no longer exists, and the phenomena must be the 

 same in all planes passing through the ray. Yet M. Cauchy, 

 in the two places above quoted, asserts it to be a consequence of 

 his theory, that in this case the alterations of phase are different 

 for two planes of polarization at right angles to each other, and 

 that the difference of the alterations amounts to half an undula- 

 tion. The same singular hypothesis had been previously made 

 by M. Neumann,f whom M. Cauchy appears to have followed ; 

 but M. Neumann has since admitted it to be erroneous. $ 



* Tom. ii. p. 428, and torn. viii. p. 965. 

 t Poggendorff s Annals, Vol. xxvi. p. 90. 

 J Ibid. VOL. XL. p. 513. 



