IX. ON THE PEOBABLE NATURE OF THE LIGHT TRANS- 

 MITTED BY THE DIAMOND AND BY GOLD LEAP. 



[Proceedings of the Royal Irish Academtj, VOL. I. p. 27- Read Jan. 9, 1837-] 



PROFESSOR MAC CTJLLAGH made a verbal communication on the 

 probable nature of the light transmitted by the diamond and by 

 gold leaf. He conceives that as there is a change of phase caused 

 by reflexion from these bodies, so there is also a change of phase 

 produced by refraction ; the change being different according as 

 the incident light is polarized in the plane of incidence, or in the 

 perpendicular plane. Consequently, if the incident ray be po- 

 larized in any intermediate plane, the refracted ray should be 

 elliptieally polarized ; and on examining the light transmitted 

 by gold leaf, this was found to be the case. Of course the same 

 thing is true of the light which enters the other metals, and 

 which is subsequently absorbed. The same remark explains the 

 appearance of double refraction in specimens of the diamond 

 which give only a single image ; and it is likely that other 

 precious stones will be found to possess similar properties. Mr. 

 Mac Cullagh has obtained a general formula for the difference 

 of phase between the two component portions of the refracted 

 light one polarized in the plane of incidence, and the other 

 perpendicular to it. He finds from this formula, that the dif- 

 ference of phase, which is nothing at a perpendicular incidence, 

 increases until it becomes equal to the characteristic at an inci- 

 dence of 90; and when the light emerges into air, the difference 

 of phase is doubled. The formula has not yet been submitted 

 to the test of experiment. 



