206 POLARIZATION BY REFLECTION. [SECT. xxi. 



light, and their planes of polarization are at right angles to 

 one another: hence, a pile of panes of glass will give a 

 polarized beam by refraction. For, if a ray of common light 

 pass through them, part of it will be polarized by the first 

 plate, the second plate will polarize a part of what passes 

 through it, and the rest will do the same in succession, till 

 the whole beam is polarized, except what is lost by reflection 

 at the different surfaces, or by absorption. This beam is 

 polarized in a plane at right angles to the plane of reflec- 

 tion, that is, at right angles to the plane passing through 

 the incident and reflected ray (K 203). 



By far the most convenient way of polarizing light is by 

 reflection. A plane of plate-glass laid upon a piece of black 

 cloth, on a table at an open window, will appear of a uniform 

 brightness from the reflection of the sky or clouds. But if it 

 be viewed through a plate of tourmaline, having its axis 

 vertical, instead of being illuminated as before, it will be 

 obscured by a large cloudy spot, having its centre quite dark, 

 which will readily be found by elevating or depressing the 

 eye, and will only be visible when the angle of incidence is 

 57, that is, when the line from the eye to the centre of the 

 black spot makes an angle of 33 with the surface of the re- 

 flector (N. 204). When the tourmaline is turned round in 

 its own plane, the dark cloud will diminish, and entirely 

 vanish when the axis of the tourmaline is horizontal, and 

 then every part of the surface of the glass will be equally 

 illuminated. As the tourmaline revolves, the cloudy spot 

 will appear and vanish alternately at every quarter revolu- 

 tion. Thus, when a ray of light is incident on a pane of 

 plate-glass at an angle of 57, the reflected ray is rendered 

 incapable of penetrating a plate of tourmaline, whose axis 

 is in the plane of incidence. Consequently it has acquired 

 the same character as if it had been polarized by trans- 

 mission through a plate of tourmaline, with its axis at right 

 angles to the plane of reflection. It is found by experience 

 that this polarized ray is incapable of a second reflection at 



