160 POLARIZATION OF LIGHT. 



produce this result increasing as the incidence is more re- 

 moved from the polarizing angle. 



(173) It remains to describe the modification which light 

 undergoes in refraction. 



When common light is suffered to fall upon a plate of 

 glass, a portion of it in all cases enters the plate, and is re- 

 fracted ; and this refracted portion is found to be partially 

 polarized. The quantity of polarized light in the refracted 

 pencil increases with the incidence, being nothing at a per- 

 pendicular incidence, and greatest when the incidence is most 

 oblique. The plane of polarization is not (as in the case of 

 reflected light) coincident with the plane of incidence, but 

 perpendicular to it. 



The connexion between the light thus polarized, and that 

 polarized by reflexion, is very intimate, the two portions being 

 always of equal intensity. This remarkable law was dis- 

 covered by Arago, and may be thus enunciated " When 

 an unpolarized ray is partly reflected at, and partly transmitted 

 through a transparent surface, the reflected and transmitted por- 

 tions contain equal quantities of polarized light ; and their planes 

 of polarization are at right angles to each other. 9 ' 



(174) ,If the transmitted light be received upon a second 

 plate parallel to the first, the portion of common light which 

 it contains undergoes a new subdivision ; and so continually, 

 whatever be the number of plates. Hence, when that num- 

 ber is sufficiently great, the transmitted light will be, as to 

 sense, completely polarized, the plane of polarization being 

 perpendicular to the plane of incidence. These facts were 

 discovered by Malus. 



It is a remarkable consequence of these principles, that 

 when a ray is incident upon a pile of parallel plates at the 

 polarizing angle, after passing a certain number the intensity 



