174 POLARIZATION BY REFRACTION. SECT. XXI. 



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 to total, or almost total 

 evanescence, and then increasing again by the same de- 

 grees as it had before decreased. These changes de- 

 pend 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 pass- 

 ing 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 po- 

 sitions, 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 po- 

 larized 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 polar- 

 izing 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 crystalized minerals, it is re- 

 fracted into a single pencil of light by the laws of ordi- 

 nary 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 



