267 



of incidence, was incident upon a thin plate bounded by 

 media of unequal refractive powers, a remarkable change in 

 the reflected Hght should take place, when the angle of inci- 

 dence was intermediate to the polarizing angles of the two 

 surfaces of the plate. This theoretical anticipation was fully 

 verified by experiment. When a lens of low refracting power 

 was laid upon a plate of high refracting power, the rings 

 which were formed appeared with a black centre, when the 

 angle of incidence was less than the polarizing angle of the low 

 refracting substance, or greater than the polarizing angle of the 

 high refracting substance ; while, when the incidence was in- 

 termediate to these two angles, the rings were white-cenlred, 

 and the whole system was complementary to what it had been 

 before. At the polarizing angle itself the rings disappeared, 

 there being no light reflected from one of the surfaces of the 

 plate, and therefore no interference. 



The examination of this subject has since been re- 

 sumed by Sir David Brewster ; and he has repeated the ex- 

 periments of Mr. Airy in a more general form, the incident 

 light being jwlarized in anyplaJie. He has thus been led to 

 many new results. The rings are found to disappear under 

 circumstances in which light is reflected from both surfaces 

 of the plate; and there are many remarkable peculiarities in 

 the transition of the rings into the complementary system.* 



It was to the theoretical explanation of these phenomena 

 that Professor Lloyd now invited the attention of the Aca- 

 demy. In the conduct of the investigation he has generalized 

 the methods followed by M. Poisson and Mr. Airy on the 

 same subject. The incident vibration being resolved into 

 two, one in the plane of incidence, and the other in the per- 

 pendicular plane, each portion will give rise to an infinite 

 series of reflected vibrations, into which it is subdivided at 

 the bounding surfiices of the plate. The expression of the 

 resultant intensity, for each portion, being then deduced, the 



* The researches of Sir David Btetviter arc now published in the Fhilosophiral 

 Transactions foi 1841. 



