POLARIZATION OF LIGHT. 159 



The intensity is therefore constant, and independent of the 

 position of the plane of reflexion with respect to the ray ; and 

 this, we have seen, is the distinctive character of common, or 

 unpolarized light. 



(172) We now proceed to consider the effects which take 

 place, when the light is incident upon the reflecting surface 

 at an angle different from the polarizing angle. 



Malus observed, that when the angle of incidence was 

 either greater or less than the polarizing angle, the proper- 

 ties already described were only in part developed in the re- 

 flected light ; that neither of the two pencils into which it 

 was divided by a rhomb of Iceland spar ever wholly vanished ; 

 but that they varied in intensity between certain limits, these 

 limits being closer the more remote the incidence from the 

 polarizing angle. From this he naturally concluded that, in 

 these circumstances, a portion only of the reflected light had 

 received the modification to which he had given the name of 

 polarization, that portion increasing as the incidence ap- 

 proached the polarizing angle ; and that the remaining portion 

 was unmodified, or in the state of common light. Partially 

 polarized light, then, according to Malus, is composed of two 

 portions, one of which is wholly polarized, while the other 

 is in the state of ordinary or unpolarized light. . In this 

 supposition Malus has been followed by most subsequent 

 philosophers. 



If this partially polarized light be reflected at a second 

 surface in the same plane, and at the same angle, the reflected 

 pencil is found to contain a greater portion of polarized light ; 

 and by increasing the number of successive reflexions, the light 

 may become, as to sense at least, wholly polarized. This fact 

 was first observed by Sir David Brewster ; and it was found 

 that light may be polarized at any incidence, by a sufficient 

 number of reflexions, the number of reflexions necessary to 



