( 379 ) 



I5~Ö 



Fig. 3. 



crystals with a great axis-angle the obliciue, witli those with a small 

 axis-angle the straight extinction will predominate, when we have 

 to do with arbitrary sections, as in a rock slice. Howexer in 

 the two cases we as rarely lind an absolutely straight extinction. With 

 hexagonal and tetragonal crystals, however, exclusively straight 

 extinction with regard to the optic axis occurs, as for ]"=0 the 

 equation (4) 



cot 2'/ =1 



sin' V 



sln^ Vsin'^ a 

 sin 2a 



1 1 



tiln} V . cos'' a 

 tiin X 



always becomes x . 



In fig. 4 the maximum extinction as a function of .c is represejited 

 for one globeoclant ; MA^ refers to talc, MA.^ to olivine, MA^ to a 

 uiincral wilh an axis-angle 2 r=: JGO", The values (f'A^, ^>M, 



