180 PROPERTIES OF TOURMALINE. SECT. XXI. 



account of its most remarkable properties, 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, as through 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. 204). 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 tourmaline 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 teetotal or almost 

 total evanescence, and then increasing again by the same degrees 

 as it had before decreased. These changes depend upon the rela- 

 tive 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 

 passing through the first plate of tourmaline, has acquired a 

 property totally different from the direct light of the candle. 

 The direct light would have penetrated the second plate equally 

 well in all directions, whereas the refracted ray will only pass 

 through it in particular positions, and is altogether incapable of 

 penetrating it in others. The refracted ray is polarized in its 

 passage through the first tourmaline, and experience shows that 

 it never loses that property, unless when acted upon by a new 

 substance. Thus, one of the properties of polarized light is the 

 incapability of passing through a plate of tourmaline perpendi- 

 cular to it, in certain positions, and its ready transmission in other 

 positions at right angles to the former. 



Many other substances have the property of polarizing light. 

 If a ray of light falls upon a transparent medium, which has the 

 same temperature, density, and structure throughout every part, 



