296 Wright — Optical Character of Birefraeting Minerals, 



3. On a plate parallel to tlie plane of the optic axes the direc- 

 tion of the acute bisectrix can be located by two different 

 methods : 



a. Revolve mineral from the position of darkness through a 

 small angle and note that the direction in which the faint dark 

 hyperbolas recede from the field is that of the acute bisectrix. 

 b. In the diagonal position of the interference figure observe 

 the interference colors of corresponding points in adjacent 

 quadrants and note that the points in the direction of the acute 

 bisectrix show the lower interference colors. In both cases 

 the value of the acute bisectrix (c or a) can be determined 

 either in convergent or parallel polarized light by the usual 

 methods and thus the optical character of the mineral be ascer- 

 tained. 



U. S. Geological Survey, Washington. 



