ON LIGHT. 351 



beam of unpolarized light may, conversely, always be re- 

 garded as a mixture of two equal rays oppositely polar- 

 ized, in any two planes at right angles to each other. 



(130.) If a ray reflected from any medium at the 

 polarizing angle (and therefore wholly polarized) be 

 received on a second surface of the same medium at 

 the same angle of incidence, and in a plane coincident 

 with that of the first reflexion, it undergoes partial re- 

 flexion just as an unpolarized ray would do, and both 

 the reflected and refracted portions retain their polariza- 

 tion. But if the plane of the second incidence be at 

 right angles to that of the first, no portion of the light is 

 reflected, but the whole passes into the refracted ray, re- 

 taining its polarization, just in the same manner as, 

 had it been incident on our doubly refracting prism held 

 with its edge at right angles to its plane of polarization, 

 it would have wholly passed into the extraordinary ray. 

 Vice versa, if the ray extraordinarily refracted by such 

 a prism be received on a glass plate at the polarizing 

 angle of incidence, no reflexion will take place if the 

 edge of the prism be parallel to the plate. Hence we 

 are entitled to conclude that it is the very same property 

 which is impressed on light in both cases, and that a 

 ray polarized by reflexion differs in no respect from one 

 which has received this property by passing through a 

 doubly refracting crystal. 



(131.) A ray partially polarized by reflexion at a 

 greater or less incidence than that at which it would 

 have been completely so, may be wholly polarized, or 

 nearly so, by repeated reflexions at the same angle. 



