SECT. XXI. POLARIZATION BY BEFB ACTION. 173 



as to render it one of the most delightful branches of 

 experimental inquiry, and so fertile in the views it lays 

 open of the constitution of natural bodies, and the 

 minuter mechanism of the universe, as to place it in the 

 very first rank of the physico-mathematical sciences, 

 which it maintains by the rigorous application of geome- 

 trical reasoning its nature admits and requires. 



Light is said to be polarized, which, by being once 

 reflected or refracted, is rendered incapable of being 

 again reflected or refracted at certain angles. In gene- 

 ral, when a ray of light is reflected from a pane of plate- 

 glass, or any other substance, it may be reflected a 

 second time from another surface, and it will also pass 

 freely through transparent bodies. But if a ray of light 

 be reflected from a pane of plate-glass at an angle of 

 57, it is rendered totally incapable of reflection at the 

 surface of another pane of glass in certain definite po- 

 sitions, but it will be completely reflected by the second 

 pane in other positions. It likewise loses the property 

 of penetrating transparent bodies in particular positions, 

 while it is freely transmitted by them in others. Light 

 so modified as to be incapable of reflection and trans- 

 mission 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 re- 

 fraction. It is proposed to explain these methods of 

 polarizing light, to give a short account of its most re- 

 markable properties, and to endeavor to describe a few 

 of the splendid phenomena it exhibits. 



If a brown tourmaline, which is a mineral generaDy 

 crystalized in the form of a long prism, be cut longitu- 

 dinally, 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, as through plates of colored 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 

 pne of these plates be held perpendicularly between 



