SECT, xxi.] POLAETZATION BY EELFECTION. 207 



certain angles and in certain positions of the incident plane. 

 For if another pane of plate-glass, having one surface 

 blackened, be so placed as to make an angle of 33 with 

 the reflected ray, the image of the first pane will be reflected 

 in its surface, and will be alternately illuminated and ob- 

 scured at every quarter revolution of the blackened pane, 

 according as the plane of reflection is parallel or perpen- 

 dicular to the plane of polarization. Since this happens 

 by whatever means the light has been polarized, it evinces 

 another general property of polarized light, which is, that 

 it is incapable of reflection in a plane at right angles to 

 the plane of polarization. 



All reflecting surfaces are capable of polarizing light, but 

 the angle of incidence at which it is completely polarized is 

 different in each substance (N. 205). It appears that the 

 angle for plate-glass is 57 ; in crown-glass it is 56 55', 

 and no ray will be completely polarized by water unless 

 the angle of incidence be 53 II 7 . The angles at which 

 different substances polarize light are determined by a very 

 simple and elegant law, discovered by Sir David Brewster, 

 " That the tangent of the polarizing angle for any medium 

 is equal to the sine of the angle of incidence divided by the 

 sine of the angle of refraction of that medium." Whence 

 also the refractive power even of an opaque body is known 

 when its polarizing angle has been determined. 



Metallic substances, and such as are of high refractive 

 powers, like the diamond, polarize imperfectly. 



If a ray, polarized by refraction or by reflection from any 

 substance not metallic, be viewed through a piece of Ice- 

 land spar, each image will alternately vanish and re-appear 

 at every quarter revolution of the spar, whether it revolves 

 from right to left, or from left to right ; which shows that 

 the properties of the polarized ray are symmetrical on each 

 side of the plane of polarization. 



Although there be only one angle in each substance at 

 which light is completely polarized by one reflection, yet it 



