REFLEXION AND REFKACTION OF POLARIZED LIGHT. 91 



of the reflected ray, such an arrangement of the axes is conceived 

 to be a necessary condition of reflexion at that incidence. 



Now, let such a polarized ray fall upon a second reflecting 

 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 ; and 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 re- 

 flexion, 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 mole- 

 cules 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 number of molecules 

 reflected by the second plate increases likewise. 



But here it is necessary to make another supposition. In 

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

 the first, except the perpendicular one, experience proves that a 

 portion of the light is reflected, and another portion refracted. Ac- 

 cording to this theory, then, some of the molecules obey the 

 polarizing force, and have their axes brought into the plane of re- 

 flexion, while others do not. To account for this diversity of 

 effect there must be some diversity of condition in the molecules 

 themselves. The theory of M. Biot supplies this by attributing to 

 them an oscillatory movement round their axes of polarization, 

 the molecules yielding to the polarizing force, or not, according to 

 the phase of the oscillation in which they are found at the moment 

 they reach the surface. 



The force which impresses the property of polarization upon the 

 refracted pencil is supposed by M. Biot to act also in the plane of 

 incidence, its operation however being to turn the axes of po- 

 larization of the luminous molecules in a direction p*rp*HKe*t*r to 

 that plane. Thus, when a ray of light traverses the surface of a 

 plate of glass at the polarizing angle, it is subjected to the action 

 of two forces, one tending to bring the axes of polarization of the- 



