318 F. E. Wright — Measurement of the Optic Axial Angle 



angles of minerals in thin section are to be described briefly, 

 together with several novel methods which have been found 

 serviceable, and a summary given of the results of comparative 

 tests of all these methods on mineral sections of known and 

 measured optic axial angles. As many of these methods have 

 not been described in English, it has seemed desirable to intro- 

 duce the general discussion by a mention of their underlying 

 principles and an outline of their mode of application to differ- 

 ent sections. 



Several of the methods are in part graphical methods and 

 involve the use of stereographic, orthographic or gnomonic 

 projection plats. (Plate I and fig. 1.)* A few paragraphs, 



* Plate I is a photolithographic production of the stereographic projection 

 plat by Professor G. Wulff (Zeitschr. f. Kryst., xxxvi, 14-15, 1902). Both 

 great circle and small circle arcs are located two degrees apart in the pro- 

 jection plat, the great circle arcs having a radius Ri for a given angle p, R: = 



v 



- , r being the radius of the projection plat : the small circle arcs, a 



sin p 



diameter, E 2 = r cot p. In the projection plat, these arcs have been con- 



