206 



MINERALOGY 



and the angle in air would be 180. It is then necessary to immerse 

 the section in a strongly refracting liquid, which decreases the 

 apparent angle. 2 H is the term used when measured in a strongly 

 refracting liquid, as oil or any of those liquids given under the 



determination of the index of re- 

 fraction, page 215. Then sin V = 



sin H, where n is the index of re- 



P 



fraction of the liquid in which the 



section is immersed. 



Dispersion of the optic axes. 

 When light of different wave lengths 

 is used in the measurement of the 

 axial angle, the value will differ 

 with different colors and change 

 progressively from one end of the 

 spectrum to the other. . This change 



of the axial angle for light of different wave lengths is termed the 

 dispersion of the optic axes, Fig. 357. Whether the angle is 

 greater for red light, p, than for violet light, u, or the reverse, 

 will depend upon the relative values of the three indices of re- 

 fraction for these individual 

 wave lengths. When the 

 angle is greater for violet 

 light than for red, it is ex- 

 pressed u>p, and the reverse, 



FIG. 356. 



This can usually be deter- 

 mined by a close inspection 

 of the interference figure 

 yielded by white light. If 

 the angles for all colors were 

 the same, i.e. no dispersion 

 of the optic axes, the hyper- 

 bolas for each color would lie 

 in the same position and 

 those for all colors would be 



superimposed; but when they differ for different wave lengths, 

 as when that for red is greater than that for violet, the hy- 

 perbola for red is farther away from the acute bisectrix than 

 that for violet ; and when white light is used, the red wave will be 



FIG. 357. Dispersion of the Optic Axes. 



