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



angle p and longitude angle 0, of any point in the projection, — 

 and to plot the same in stereographic or orthographic or gno- 

 nionic projection and thus to measure the angular distance 

 between any two points, as those between optic axes occurring 

 in the field of vision. 



Fig. 7. In this figure, C is the camera hicida ; D, revolving disk of 

 drawing table graduated into degrees and supported by the arm E, which in 

 turn fits into the collar F clamped to the stand of the microscope at the 

 proper distance from C ; R, axial angle reflector; B, Bertrand-Amici lens ; S, 

 sliding stop diaphragm ; L, upper lens of condenser system fitted in brass 

 ring in horizontal circle H 3 of universal stage (page 343) ; N, brass cylinder 

 extension to hold lower portion M of condenser lens system (page 342). 



In a recent article,* Professor Becke has described an 

 ingenious method by which any section, in which only one 

 optic axis appears in the field, can be used for the measurement 



*F. Becke, Tschermak's Min. petr. Mitth., xxiv, 35-44, 1905. 



