202 POLARIZATION BY REFRACTION. [SECT xxi. 



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 ex- 

 perience 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 perpendicular 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, as fluids, gases, glass, &c., and a few 

 regularly crystallized minerals, it is refracted into a single 

 pencil of light by the laws of ordinary refraction, according 

 to which the ray, passing through the refracting surface 

 from the object to the eye, never quits a plane perpendicular 

 to that surface. Almost all other bodies, such as the greater 

 number of crystallized minerals, animal and vegetable sub- 

 stances, gums, resins, jellies, and all solid bodies having un- 

 equal tensions, whether from unequal temperature 'or pres- 

 sure, possess the property of doubling the image or appear- 

 ance of an object seen through them in certain directions. 

 Because a ray of natural light falling upon them is refracted 

 into two pencils which move with different velocities, and 

 are more or less separated, according to the nature of the 

 body and the direction of the incident ray. Whenever a 

 ray of natural light is thus divided into two pencils in its 

 passage through a substance, both of the transmitted rays 

 are polarized. Iceland spar, a carbonate of lime, which by 

 its natural cleavage may be split into the form of a rhom- 

 bohedron, possesses the property of double refraction in an 



