IM;M|'I.I;TIKS OF CRYSTALS 



IfiO 



FIG. 313. 



tionary, :uid this is what would be expected if the crystal were an 

 ix .tropic substance, as there is no refraction when the ray falls 

 normal to the surfaces. This ray follows the ordinary law and is 

 there.fore termed the ordinary ray. Its index of refraction is 

 writ t en o>. In the case of calcite the index measured with monochro- 

 matic sodium light (yellow) is written, y = 1.658. The second 

 ray follows another law which is entirely different from that of the 

 ordinary ray, and its velocity and 

 therefore its index of refraction 

 (written ) will vary with the di- 

 reetion; this ray is known as the 

 extraordinary ray. The index of 

 refraction taken at its maximum 

 difference from that of the or- 

 dinary ray and for sodium light 

 is written y = 1.486. 



When the index of refraction of 

 the extraordinary ray is smaller 

 than that of the ordinary ray, or 

 the extraordinary ray is the fast 

 ray, o, the crystal is said to be optically negative, written ( ) 

 as in calcite. 



In quartz, where a> y = 1.544 and y = 1.553, >o>, it is optically 

 (+), and the extraordinary ray is the slow ray. 



All crystals of the tetragonal and hexagonal systems have two 

 indices of refraction ; one, that for the ordinary ray, is constant for 

 all directions in the crystal, as in isotropic substances ; the other, 

 that for the extraordinary ray, varies with the direction in the crys- 

 tal, from the value of the index of refraction for the ordinary ray as 

 one limiting value, to a maximum or minimum as the other limit, 

 according to the ( ) or (+) character of the crystal. 



Wave surfaces in hexagonal and tetragonal crystals. In Fig. 

 314, if any point within a hexagonal or tetragonal crystal, as o, 

 be illuminated, and act as the source of light for the smallest frac- 

 tion of a second, that portion illuminated will be bounded by the 

 wave front. Its distance from the source of light o, in any direc- 

 tion, will depend upon the velocity with which the ray travels 

 through the crystal in any given direction. At the end of any short 

 period of illumination the ordinary ray co has traveled the distance 

 ox ; as the ray travels with the same velocity in any and all direc- 

 tions, the circle with o as a center and a radius ox will represent the 



