S*cr. XXI. THE OPTIC AXES OP CRYSTALS. 177 



ways, and the impossibility 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 po- 

 larized rays, yet there is one direction along which it is 

 refracted in one ray only, and that according to the or- 

 dinary law. This direction is called the optic axis 

 (N. 202). 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 refraction. The extraordinary ray is some- 

 times refracted toward the optic axis, as in quartz, zir- 

 con, 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 crys- 

 tals are negative, which is the most numerous class. 

 The ordinary ray moves with uniform velocity within a 

 doubly refracting substance, but the velocity of the ex- 

 traordinary ray varies with the position of the ray rela- 

 tively to the optic axis, being a maximum when its mo- 

 tion within the crystal is at right angles to the optic axis, 

 and a minimum when parallel to it. Between these ex- 

 tremes its velocity varies according to a determinate law. 



It has 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 

 uncrystalized substances, and that it must necessarily 

 exist in carbonate of lime, quartz, and other bodies hav- 

 ing one optic axis, but that in a numerous class of sub- 

 stances 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 gen- 

 eralization is perhaps only inferior to the law of gravita- 

 tion, has enrolled the name of Fresnel among those 

 which pass not away, and makes his early loss a subject 



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