90 REPORT ON PHYSICAL OPTICS. 



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 con- 

 clusion, that when a ray of light is transmitted 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 rela- 

 tion to the two theories of light. 



Newton proved that the fundamental laws of reflexion and 

 refraction could be derived from the operation of attractive and 

 repulsive forces exerted by the molecules of body on those of light.. 

 The phenomena of polarization, however, show that these forces 

 are exerted in very different degrees, according to the position of 

 the sides of the ray with respect to the plane of reflexion or refrac- 

 tion; and we are now to consider the additional hypotheses which 

 become necessary in the theory of emission in order to render an 

 account of these new facts. 



It has been already mentioned that, in the theory of M. Biot, 

 a polarized ray was one in which certain axes (called the axes of 

 polarization) of all the molecules were turned in the same direction. 

 This effect is ascribed to the operation of certain forces emanating 

 from the molecules of the body. These forces M. Biot denominates 

 polarizing forces ; and he considers them as distinct from the re- 

 flecting and refracting forces, although intimately connected with 

 them. The effect of a polarizing force is to give a rotation to the 

 axes of the molecules ; and that which impresses the property of 

 polarization upon the reflected ray is assumed to act in the plane 

 of reflexion. This being supposed, since a ray of common light 

 is polarized by reflexion when incident at a certain angle, we are 

 obliged to admit that, at this angle, the polarizing force turns the 

 axes of polarization of all the molecules, and brings them into the- 

 plane of reflexion ; and, since this takes place for all the molecules* 



Iol4. 



"On the Polarization of Light by oblique Transmission," ic.-PM. 



4 



