PETROGRAPHY AND METALLOGRAPHY. 161 
similar to those characteristic of hexagonal crystals, and 
shows no extinction angle. 
When certain sections of anisotropic crystals are exam- 
ined with convergent polarized light 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 
\, 
ih 
Fic. 58.—INTERFERENCE Ficures. (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 gypsuin will again 
serve for comparison, the former being uniaxial, the latter 
biaxial. 
Two more characters remain to be noted, absorption 
