163 



vol. i. p. a), two sii|)positions relative to the value of the 

 refractive index presented themselves. Putting m for the 

 modtilus, and x for the charaeteristic, I had to choose be- 



M 



tween the values Mcos v and . The latter value is that 



^ cos X 



which I adopted ; the former, which is M. Cauchy's, was re- 

 jected 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 Comptes Rendus (torn. ii. p. 4^8, and torn. viii. p. 9G5) 

 requires to be noticed. When a polarized ray is reflected 

 by a metal, the phase of its vibration is altered, and if the in- 

 cidence be oblique, the change of phase is different, accord- 

 ing 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 

 constant quantity, whatever be the plane of polarization. In 

 fact, the distinction between the plane of incidence and the 

 perpendicular 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 altera- 

 tions of phase are different for two planes of polarization at 

 right angles to each other, and that the difference of the al- 

 terations amounts to half an undulation. The same singular 

 hypothesis had been previously made by M. Neumann {Pog- 

 gendorff's Annals, vol. xxvi. p. 90), whom M. Cauchy appears 

 to have followed ; but M. Neumann has since admitted it to 

 be erroneous {Ibid. vol. xl. p. 513). 



Mr. J. Huband Smith communicated to the Academy some 

 particulars connected with the recent discovery of a cairn 

 containing cinerary urns, which appears to have been accom- 

 panied by some circumstances not unworthy of notice. 



It took place at Loughanmorc, in the county of Antrim, 



