POLARIZATION OF LIGHT. 161 



of the transmitted light will undergo no further diminution. 

 For the light becoming wholly polarized in the plane per- 

 pendicular to the plane of incidence, no portion of it will be 

 reflected by any of the succeeding plates ; it is therefore trans- 

 smitted without diminution through them, whatever be their 

 number. The case is different, when the light is incident 

 on the pile at any other than the polarizing angle ; and it 

 follows therefore that the intensity of the light transmitted 

 through a thick pile is greatest, when it is incident at the 

 polarizing angle. 



(175) There are certain crystals which, like the pile of 

 transparent plates, possess the property of polarizing the 

 transmitted light. This property depends upon a peculiarity 

 in the absorbing powers of double-refracting crystals, 

 namely, that the absorption of a polarized ray varies with the 

 position of its plane of polarization with respect to the crystal. 

 Thus, tourmaline absorbs a polarized ray more rapidly when 

 the plane of polarization is parallel to the axis, than when it 

 is perpendicular to it. Now a ray of common light, which 

 falls upon a plate of this crystal cut parallel to the axis, may 

 be resolved into two, one polarized in a plane passing 

 through the axis, and the other in a plane perpendicular 

 to it ; and the former of these being absorbed more rapidly 

 than the latter, the transmitted light will be partially polar- 

 ized in the plane perpendicular to the axis of the crystal. When 

 the plate is sufficiently thick, the latter portion alone will be 

 sensible, and the ray emerges wholly polarized in the per- 

 pendicular plane. 



The tourmaline, accordingly, is of much use in experi- 

 ments on polarized light, not only in affording a ready test 

 of polarization, but also in producing a polarized beam. It 

 has the disadvantages, however, that the polarization of the 



