SECT. XXI. POLARIZATION BY REFLECTION. 185 



and obscured at every quarter revolution of the blackened 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 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 (1ST. 210). 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 11'. 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. 



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 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 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 greater or less than a polarizing angle, 

 a part only of the reflected ray will be polarized, but a part of 

 what is transmitted will be polarized by reflection at the surface 

 of the second plate, part at the third, and so on till the whole is 

 polarized. This is the best apparatus ; but one plate of glass 

 having its inferior surface blackened, or even a polished table, 

 will answer the purpose. 



