3U0 FOURTH REPORT — 1834. 



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 refraction ; and we are now to consider the additional 

 hypotheses whicli 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 AewommvLiiis polarizing forces ; and he considers them 

 as distinct from the reflecting 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 sup- 

 posed, since a ray of common light is polarized by reflexion 

 when incident at a certain angle, Ave 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 of the reflected 

 ray, such an arrangement of the axes is conceived to be a ne- 

 cessarjr condition of reflexion at that incidence. 



Now let such a polarized ray fall upon a second i-eflecting 

 surface at the polarizing angle, and let the plane of the second 

 reflexion be perpendicular to that of the first. Then the axes of 

 polarization of the molecules, in their incidence on the second 

 plate, are perpendicular to the plane of reflexion ; consequently 

 the polarizing force acting in that plane affects equally the two 

 halves of the axis, and cannot therefore turn it into the plane 

 of reflexion, — a condition which is assumed to be necessary to 

 reflexion at that angle. No light therefore is reflected. But 

 when the plane of the second reflexion is inclined to that of the 

 first at any angle less than 90°, the polarizing force of the 

 second plate no longer acts symmetrically on the two halves of 

 the axes of the molecules : it may therefore turn these axes so 

 as to make them coincide with the plane of reflexion, and thus 

 subject the molecules to the action of the reflecting force. The 

 effect of the polarizing force increases as the inclination of the 

 two planes of reflexion diminishes ; and consequently the num- 

 ber of molecules reflected by the second plate increases likewise. 



But here it is necessary to make another supposition. In 

 finy position of the plane of the second reflexion with respect to 



