REFLEXION AND REFRACTION OF POLARIZED LIGHT. 89 



Neither of the two pencils, into which it was divided by a rhomb 

 of Iceland spar, ever wholly vanished ; but they varied in inten- 

 sity between certain limits, these limits being closer the more 

 remote the incidence from the angle of complete polarization. 

 Prom this he naturally concluded that, in these circumstances, 

 a portion only of the reflected pencil had received the modifi- 

 cation to which he had given the name of polarization, that 

 portion increasing as the incidence approached the polarizing 

 angle ; and that the remaining portion was unmodified, or in 

 the state of common light. In this supposition Malus has been 

 followed by most subsequent philosophers. A different view of 

 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 phenomenon which 

 he seems to have been the first to observe, namely, that com- 

 mon light may be polarized by a sufficient number of reflexions 

 &i any angle, the number of reflexions required to produce the 

 effect being greater the more remote the incidence is from the 

 polarizing angle.* 



Examining the transmitted pencil, Malus found that it was par- 

 tially polarized ; and that its plane of polarization was not,, like 

 that of the reflected pencil, coincident with the plane of reflexion, 

 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 pro- 

 duce 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 proportionate. 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 under- 

 goes a new subdivision; and so continually, whatever be the 



* Phil. Trans. 1815. 

 f Mtm. Inst. 1810. 



