SECT, xxi.] POLARIZATION BY PLATES OF GLASS. 205 



by the law of ordinary refraction. The extraordinary ray is 

 sometimes refracted towards the optic axis, as in quartz, 

 zircon, ice, &c., which are therefore said to be positive crys- 

 tals ; but when it is bent from the optic axis, as in Iceland 

 spar, tourmaline, emerald, beryl, &c., the crystals are nega- 

 tive, which is the most numerous class. The ordinary ray 

 moves with uniform velocity within a doubly refracting 

 substance, 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 refrac- 

 tion, 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, d priori, that the extraordinary ray 

 must be wanting in glass and other uncrystallized sub- 

 stances, 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 

 optic axes, both rays must undergo extraordinary refraction, 

 and consequently that both must deviate from their original 

 plane ; and these results have been perfectly confirmed 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 

 partly transmitted through a transparent surface, the reflected 

 and refracted pencils contain equal quantities of polarized 



