SECT. XXL POLARIZED LIGHT. 183 



grating or wires of a cage, if presented edgeways, and the impos- 

 sibility of its passing in a transverse direction. 



Although it generally happens that a ray of light, in passing 

 through Iceland spar, is separated into two polarized rays, yet 

 there is one direction along which it is refracted in one ray only, 

 and that according to the ordinary law. This direction is called 

 the optic axis (N. 207). Many crystals and other substances 

 have two optic axes, inclined to each other, along which a ray of 

 light is transmitted in one pencil by the law of ordinary refrac- 

 tion. The extraordinary ray is sometimes refracted towards the 

 optic axis, as in quartz, zircon, ice, &c., which are therefore said 

 to be positive crystals ; but when it is bent from the optic axis, 

 as in Iceland spar, tourmaline, emerald, beryl, &c., the crystals 

 are negative, which is the most numerous class. The ordinary 

 ray moves with uniform velocity within a doubly refracting sub- 

 stance, but the velocity of the extraordinary ray varies with the 

 position of the ray relatively to the optic axis, being a maximum 

 when its motion within the crystal is at right angles to the 

 optic axis, and a minimum when parallel to it. Between these 

 extremes its velocity varies according to a determinate law. 



It had been inferred, from the action of Iceland spar on light, 

 that in all doubly refracting substances one only of two rays is 

 turned aside from the plane of ordinary refraction, while the 

 other follows the ordinary law ; and the great difficulty of 

 observing the phenomena tended to confirm that opinion. M. 

 Fresnel, however, proved by a most profound mathematical 

 inquiry, a priori, that the extraordinary ray must be wanting in 

 glass and other uncrystallized substances, and that it must 

 necessarily exist in carbonate of lime, quartz, and other bodies 

 having one optic axis, but that in a numerous class of substances, 

 which possess two optie axes, both rays must undergo extraor- 

 dinary refraction, and consequently that both must deviate from 

 their original plane ; and these results have been perfectly con- 

 firmed by subsequent experiments. This theory of refraction, 

 which for generalization is perhaps only inferior to the law of 

 gravitation, has enrolled the name of Fresnel among those which 

 pass not away, and makes his early loss a subject of deep regret 

 to all. who take an interest in the higher paths of scientific 

 research. 



When a beam of common light is partly reflected at, and 



