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

 16 17 



18 



19 



x T-- : --r v 4v 



Figs. 16. 17, 18, 19. In figs. 16-19 the axial curves (zero isogyres) are 

 constructed under the assumption that the mean refractive index of both 

 the mineral and the medium in which its interference phenomena are 

 observed, are identical ; in short, an orthographic projection of the phenom- 

 ena as they appear within the crystal is given. In fig. 16, the section is nor- 

 mal to an optic axis ; in fig. 17, the section makes angles, / = 5°, fi = 6° 

 (small circle coordinates) with the optic binomial ; in fig. 18, the angles are 

 % = 12°, fi = 13°, while in fig. 19, the section makes angles A = 20°, ft = 20° 

 with the optic axis ; the angle of revolution of the stage and consequent new 

 position of the trace of principal plane of lower nicol in each case is indi- 

 cated by the arrows. The area included by the inner circle of fig. 16 indicates 

 the relative field of vision of ordinary microscopes. 



