62 Laws of Reflexion from Metals. 



neighbourhood of the polarizing angle. It will pass through 

 90, when mm' = 1 ; after which the change will be very rapid, 

 and the value of & will soon rise to nearly 180. This is exactly 

 the phenomenon which Mr. Airy observed in the diamond. 



Another set of phenomena to which the author has applied 

 his formulae are those of the coloured rings formed between 

 a glass lens and a metallic reflector ; and he has thus been 

 enabled to account for the singular appearances described by 

 M. Arago in the Memoires d'Arctteil, torn, iii., particularly the 

 succession of changes which are observed when common light 

 is incident, the intrusion of a new ring, &c. But there is one 

 curious appearance which he does not find described by any 

 former author. It is this. Through the last twenty or thirty 

 degrees of incidence the first dark ring, surrounding the central 

 spot, which is comparatively bright, remains constantly of the 

 same magnitude ; although the other rings, like Newton's rings 

 formed between two glass lenses, dilate greatly with the obli- 

 quity of incidence. This appearance was observed at the same 

 time by Professor Lloyd. The explanation is easy. It depends 

 simply on this circumstance (which is evident from the Table), 

 that the angle 180 - S', at these oblique incidences, is nearly 

 proportional to'cos i. 



As to the index of refraction in metals, the author conjec- 



M 



tures that it is equal to 



cos 



