ItEPORT ON PHYSICAL OPTICS. 359 



the phenomenon of partial polarization has been taken by Sir 

 David Brewster, to which I shall have occasion presently to 

 allude ; and he has employed his theory to explain a pheno- 

 menon which he seems to have been the first to observe, — 

 namely, that common light may be polarized by a sufficient 

 number of reflexions at any angle, the number of reflexions re- 

 quired to produce the effect being greater, the more remote tlie 

 incidence is from the polarizing angle *. 



Examining the transmitted pencil, Malus found that it was 

 partially polarized ; and that its plane of polarization was not, 

 like that of the reflected pencil, coincident with the plane of re- 

 flexion, but perpendicular to it f. The two portions of light 

 thus polarized in opposite planes he observed to be intimately 

 connected ; and in a subsequent memoir he announced the fact 

 that whenever we produce by any contrivance a ray polarized 

 in any plane, there is produced at the same time a second ray 

 polarized in the opposite plane. These two polarized rays 

 follow separate paths, and their quantities are always propor- 

 tionate. The connexion, however, is still more strict than was 

 supposed by Malus ; for the quantities of polarized light in the 

 reflected and transmitted pencils are not only proportionate, but 

 absolutely equal. This remarkable law was discovered by M. 

 Arago. 



When a ray, which is partially polarized by transmission 

 through a plate of glass, is received upon a second plate at the 

 same angle, the portion of common light which it contains 

 undergoes a new subdivision ; and so continually, whatever be 

 the number of plates. Hence when that number is sufficiently 

 great, the transmitted light will be, as to sense, completely 

 polarized; and the whole light is thus subdivided into two 

 pencils oppositely polarized, one of which is reflected from, and 

 the other transmitted through, the pile. These facts were also 

 observed by Malus. The laws of the phenomena have since been 

 investigated, in much detail, by Sir David Brewster ; and he 

 has arrived at the conclusion, that when a ray of light is trans- 

 mitted successively through any number of parallel plates, the 

 tangent of the angle at which the polarization of the refracted 

 pencil appears complete is inversely as their number %. 



I may now proceed to consider these phenomena in their re- 

 lation to the two theories of light. 



Nevpton proved that the fundamental laws of reflexion and 

 refraction could be derived from the operation of attractive and 



* Phil. Trans. 1815. f Mem. Inst. 1810. 



i " On the Polarization of Light by oblique Transmission," Src, Phil. Trans. 



