S*tT. XXI. POLARIZATION BY REFLECTION. 179 



of reflection. It is found by experience that this polar- 

 ized ray is incapable of a second reflection at 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 re- 

 flected in its surface, and will be alternately illuminated 

 and obscured at every quarter revolution of the black- 

 ened pane, according as the plane of reflection is parallel 

 or perpendicular to the plane of polarization. Since 

 this happens by whatever means the light has been 

 polarized, it evinces another general property of polar- 

 ized 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 polar- 

 ized by water, unless the angle of incidence be 53 11'. 

 The angles at which different substances polarize light 

 are determined by a very simple and elegant law, dis- 

 covered 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 re- 

 fractive 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 Iceland spar, each image will alternately vanish and 

 reappear 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 sym- 

 metrical 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 may be polarized at any angle of incidence by a 

 sufficient number of reflections. For if a ray falls upon 

 the upper surface of a pile of plates of glass at an angle 



