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



position of second optic axis A, is readily obtained, since the 

 angle A,a or A ,7 is by definition equal to A a a resp. A,7. 



After some practice, the exact relative positions of H„ H, 

 can be found without difficulty for which darkness remains for 

 all angles of inclination about ~V\. To insure accuracy, how- 

 ever, the fact of remaining dark should be scrutinized very 

 sharply, since the correct position is not always that of absolute 

 darkness but rather that for which the same degree of dark- 

 ness or intensity of uniform lighting obtains throughout. 



From the complete determination by this method of the 

 positions of a, /3 and 7, which should be mutually 90° apart, 

 Fedorow has shown that the average refractive index of the 

 mineral can be derived approximately, although the determina- 

 tion is not of sufficient accuracy to be of great practical value. 



By this method of determining the positions of the principal 

 sections of the ellipsoid, the distinction between uniaxial and 

 biaxial minerals is greatly facilitated and the general problem 

 solved for all possible sections. In case the position of neither 

 optic axis can be determined directly, both optic axes lying 

 outside the field of vision, the methods for measuring the optic 

 axial are based solely on the determination of extinction angles 

 along certain directions, and are of such a nature that by their 

 use only very rough approximations to the true value of 2Y can 

 be obtained, errors of d= 10° and over being easily possible 

 within the range of possibility. Fedorow has suggested one 

 principal method applicable to such cases and the writer has 

 had occasion to use several others. They are not so satisfac- 

 tory, however, as the above methods, and are not of equal 

 practical value. For the sake of completeness, they are 

 described briefly in fine type below. 



Section nearly perpendicular to the optic normal ft. 



In case the section of a mineral is so cut that it makes an angle 

 of 30° or less with the plane of the optic axes, neither optic axis 

 appearing, in consequence, within the field of vision, the above 

 method places the observer in a position to measure the optic 

 axial without even seeing either optic axis. The exact position 

 of yS can first be determined by this method, and then brought to 

 coincidence with the microscopic axis, in which case the plane 

 of the optic axis is horizontal. In this position the circles V 1 and 

 Hj are free and the section can be revolved about V x and extinc- 

 tion angles determined on H t . (Figs. 27 and 28.) 



Since the exact positions of a and y have been determined and 

 the two optic axes make equal angles with these bisectrices, it is 

 possible by trial to bring one of the optic axes A 3 to coincidence 

 with the normal to Y 1 (fig. 27), and to test the accuracy of its 

 position by means of extinction curves for different inclinations 



