OPTICAL PHOPUM'll..- 01 UYSTALS 



FIG. 360. Crossed Dispersion. 



where there i- mi fixed direction to which any of the axes of the 

 indicntrix must conform, it is possible for all varieties of disper- 

 sion described in the other systems to take place at one and tin- 

 same time, and the interfer- 

 ence figure may be entirely 

 without symmetry. 



Methods of determining 

 the indices of refraction. 

 The index of refraction of 

 any substance is a physical 

 constant, characteristic of 

 the substance. It not only 

 serves as a means of identi- 

 fication, but also as a meas- 

 ure of purity. The value 

 of the index of refraction 

 varies with the temperature, 

 but this variation in case of 

 solids, at ordinary temper- 

 atures, is small and within 

 the limits of error, therefore negligible. The index of refraction, 

 when all precautions and when great care are taken, together with 

 an average of several observations, may be determined within .0002. 

 The value of the index of refraction for minerals will lie between 



1.3, that of ice, and 

 3.08, that of pyrar- 

 gyrite. 



I. The most ac- 

 curate method is that 

 in which the angle of 

 deviation of the re- 

 fracted ray is ac- 

 tually measured, as 

 transmitted through 

 a prism of 60, or 

 one not varying more 



than 5 from 60. As the angle of deviation will vary with the 

 inclination of the ray, the angle of least deviation is found and 

 measured as follows. 



In Fig. 361 abc is the prism, with the angle at a nearly 60. This 

 angle is accurately measured with the goniometer. The ray of 



FIG. 361. 



