184 POLARIZATION BY REFLECTION. SECT. XXI. 



partly transmitted through a transparent surface, the reflected 

 and refracted pencils contain equal quantities 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 different surfaces, or by 

 absorption. This beam is polarized in a plane at right angles to 

 the plane of reflection, that is, at right angles to the plane pass- 

 ing through the incident and reflected ray (N. 208). 



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 reflector (N. 209). When 

 the tourmaline is turned round in its own plane, the dark cloud 

 will diminish, and entirely vanish when the axis of the tourma- 

 line 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 

 revolution. 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 transmission 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 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 



