178 POLARIZATION NY PLATES OF GLASS. SECT, XXI. 



of deep regret to all who take an interest in the higher 

 paths of scientific research. 



When a beam of common light is partly reflected at, 

 and partly transmitted through, a transparent surface, 

 the reflected and refracted pencils contain equal quanti- 

 ties of polarized 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 dif- 

 ferent surfaces, or by absorption. This beam is polar- 

 ized in a plane at right angles to the plane of reflection, 

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

 incident and reflected ray (N. 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 tour- 

 maline, having its axis vertical, instead of being illumi- 

 nated as before, it will be obscured by a large cloudy 

 spot, having its center 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 center 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 en- 

 tirely vanish when the axis of the tourmaline is horizon- 

 tal, 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 revolution. Thus, when a ray of light is inci- 

 dent on a pane of plate-glass at an angle of 57, the re- 

 flected 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 transmission through a plate 

 of tourmaline, with its axis at right angles to the plane 



