Kozu and Washington — Augite from Stromboli. 465 



The optic axial angle computed from these indices is 

 2V = 57° 58-7'. 



If we compare these refractive indices with those 6 of 

 the augite crystals from the Auvergne and Renfrew, 

 determined by Michel-Levy 7 and Lacroix s respectively, it 

 will be seen that the values for the Stromboli augite are 

 much lower than these. It will also be seen that the bire- 

 fringences (7 — «) and (7-/3) of the Stromboli augite are 

 higher than those of the others, while the value of (/3—a) 

 is practically equal to them. Hence the optic axial angle 

 of the Stromboli augite must be smaller than those of 

 the others. 



In order to prove this, the measurement of the angle 

 was made by the more direct method, using Wulfmg's 

 axial angle apparatus. 9 The mineral section was cut 

 nearly perpendicular to the acute bisectrix and was 

 immersed in a liquid whose refractive index for sodium 

 light was 1-6220. The result obtained is 2H = 61° 24-5' 

 for sodium light. 2V computed from H, fi, and the 

 refractive index of the liquid is 2V = 58° 20-5'. Compar- 

 ing this value with that obtained by computation from 

 the refractive indices, the difference between them is 

 21-8', which is so small as to be negligible in this case. 10 



°o 8 y y — a 



Auvergne 1-706 1-712 1-728 0022 Michel-Levy 



Auvergne 1-712 1-717 1-733 0021 and Laeroix 



Eenfrew 1-6975 1-7039 1-7227 00252 Wtilfing 



7 Michel-Levy and Laeroix, Les Mineraux des Bodies, 1888, p. 265. 



8 E. A. Wiilfing, Min. petr. Mitt., 15, p. 45, 1896. 



* Thanks are offered to Prof. B. Koto for his kind permission to make rise 

 of the instrument in his laboratory at the Geological Institute, Science Col- 

 lege, Tokyo, Imperial University. 



10 The effect on the calculated optic axial angle of errors in the ref ractive 

 indices is given by 



„ T 9V , 9V , a 9V , 



dV = -—da 4- -^— d)8 + -=— dy . 



da dp c'y 



Assuming that the refractive indices are each affected by 0-001, we have 



9V 9V 9V 



~da= -2°2'b",-~a/3= +2°5V'58", C ^-dy= -0°35'48" 

 c'a dp dy 



Hence we see that any one of the three values given above is greater than 

 the difference (21-8') between the optic axial angles obtained by the dif- 

 ferent methods. Supposing that dy = 0-003 and da = — 0-001 when d/3 is 

 the only one which is zero, then dV = 14-7', which is less than the differ- 

 ence above. In such a case, however, the birefringence (7 — a) must differ 

 to a great extent from that obtained by the total refiectometer. To test 

 this, the compensation method was used as described below and it was 

 proved that the value of (7 — a) must not differ much from 0027. 



