PETROGRAPHY AND METALLOGRAPHY. i6l 



similar to those characteristic of hexagonal crystals, and 

 shows no extinction angle. 



When certain sections of anisotropic crystals are exam- 

 ined with convergent polarized hght obtained by a con- 

 verging lens fitted over the polarizer, interference figures 

 are produced, consisting of dark or colored rings and 

 crosses. It is impossible to enter here into the optical 

 principles which condition these phenomena; but the 

 difference between the circular figures produced by uni- 

 axial crystals, and the elliptical appearances characteristic 



Fig. 58. — Intekfeeence Figukes. (After Luquer.) 



of biaxial crystals (Fig. 58), are easily apparent. With 

 some sections which show only an indistinct bar instead 

 of a clear figure, if the stage be rotated, the bar follows 

 it and remains straight, in the case of uniaxial crystals, 

 while with biaxial crystals it moves in the opposite direc- 

 tion and becomes curved. Quartz and gypsum will again 

 serve for comparison, the former being uniaxial, the latter 

 biaxial. 

 Two more characters remain to be noted, absorption 



