SECT, xxr.] POLARIZATION BY EEFEACTION. 201 



transmitted by them in others. Light, so modified as to be 

 incapable of reflection and transmission in certain directions, 

 is said to be polarized. This name was originally adopted 

 from an imaginary analogy in the arrangement of the 

 particles of light on the corpuscular doctrine to the poles of 

 a magnet, and is still retained in the undulatory theory. 



Light may be polarized by reflection from any polished 

 surface, and the same property is also imparted by refrac- 

 tion. It is proposed to explain these methods of polarizing 

 light, to give a short account of its most remarkable pro- 

 perties, and to endeavour to describe a few of the splendid 

 phenomena it exhibits. 



If a brown tourmaline, which is a mineral generally crystal- 

 lized in the form of a long prism, be cut longitudinally, that 

 is, parallel to the axis of the prism, into plates about the 

 thirtieth of an inch in thickness, and the surfaces polished, 

 luminous objects may be seen through them, asthrough plates 

 of coloured glass. The axis of each plate is in its longitudinal 

 section parallel to the axis of the prism whence it was cut (N. 

 199). If one of these plates be held perpendicularly between 

 the eye and a candle, and turned slowly round in its own 

 plane, no change will take place in the image of the candle. 

 But if the plate be held in a fixed position, with its axis or 

 longitudinal section vertical, when a second plate of tourma- 

 line is interposed between it and the eye, parallel to the first, 

 and turned slowly round in its own plane, a remarkable change 

 will be found to have taken place in the nature of the light. 

 For the image of the candle will vanish and appear alternately 

 at every quarter revolution of the plate, varying through all 

 degrees of brightness down to total, or almost total evanes- 

 cence, and then increasing again by the same degrees as it had 

 before decreased. These changes depend upon the relative 

 positions of the plates. When the longitudinal sections of 

 the two plates are parallel, the brightness of the image is at 

 its maximum ; and, when the axes of the sections cross at right 

 angles, the image of the candle vanishes. Thus the light, in 



