352 ON LIGHT. 



This, which might be concluded a priori hy considering 

 that the unpolarized portion differs in no respect from 

 ordinary light, and is therefore susceptible of so receiv- 

 ing partial polarization, while the polarized portion re- 

 tains its polarization unchanged by reflexion, is verified 

 by experiment. 



(132.) If a ray partially polarized in any plane be 

 received on the doubly refracting prism already men- 

 tioned, with its edge perpendicular to the plane of 

 polarization, the polarized portion will pass wholly into 

 the ordinary image, while the unpolarized will be equally 

 divided between the two. Thus the two images will be 

 unequally bright. By turning round such a prism, then, 

 till a position is found at which the contrast between 

 the two images is most striking, this plane will be dis- 

 covered, and the difference of their illuminations is the 

 measure of the quantity of polarized light in the beam. 



(133.) PolarizatioJi of light by refi'action. When light 

 is incident on glass or any uncrystallized transparent 

 body at the polarizing angle, the reflected portion (a 

 small per-centage, not more than one-twelfth of the whole 

 light) is wholly polarized in the plane of incidence, as 

 already stated. The refracted beam (by far the larger 

 portion), when examined in the mode just described, is 

 found to be partially polaj'ized in a plane at light angles 

 to that of incidence, and the amount of polarized light 

 which it contains to be precisely equal to that in the 

 reflected beam. Thus we see that when light falls upon 

 .such a surface, the greater portion passes unchanged, 

 while the other is divided into two equal portions oppo- 



