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



after revolution of the section from that position through 

 known angles about Y, and Y„. 



For the ellipsoidal planes the readings were: 



H, Ho H 3 V, V 2 



a^ plane 1S0° 90° 23G°-5 .. 0° 



ay plane' " " 326- —17-5 



For a in coincidence with the microscope axis, the readings 

 were found from the projection to be : &„ 185°, H 2 , 90°, H 3 , 

 326°, Y„ 30°, Y 2 , 22° -5. After the revolution about Y, and 

 Y 2 , the angles recorded were : H„ lS9°-5, H„, 90°, H 3 , 326°, 

 Y,, 40°, Y 9 -15°. 



From these angles 2Y was measured in projection and found 

 to be m°. 



By direct observation of the optic axis, the same angle was 

 also obtained. This method may in favorable instances give 

 reliable results, but in general it cannot be considered an accu- 

 rate method, owing to the undue influence in projection of 

 slight deviations of the extinction angle on the value of the 

 optic axial angle. 



Second Section : 





Hi 



H 2 



H 3 



Vi 



v 2 



a/3 plane 



180° 



90° 



140 c 







+ -5 



ay plane 



u 



a 



234 





— 32-5 



Optic axis A 1 



u 



a 



140 



4° 



+ -5 



The optic axis A x was determined by direct readings. After 

 proper reduction to true angles the value 2Y=63° was obtained 

 from the projection plat. 



Topaz. Section about perpendicular to the optic normal. — 

 In this instance the principal elliposoidal planes were first 

 determined and ellipsoidal axis /3 brought to coincide with the 

 microscope axis and the extinction angle measured in that posi- 

 tion. By trial that position of Fl 2 was found for which A x coin- 

 cided with the principal plane of the lower nicol, and the optic 

 axial angle thus ascertained by measuring the extinction angles 

 of the section in different positions of Y x and comparing the 

 data of observation with those obtained by graphical methods 

 from the projection plat on the assumption that A, did actually 

 coincide with the principal plane of the lower nicol. In like 

 manner, the section was revolved about Y 2 and extinction 

 angles measured until theory and observation furnished identi- 

 cal results. 



The principal ellipsoidal planes of the section were deter- 

 mined by the readings: 



H, H, H 3 V: V. 



^y plane 180° 90° 333° .. +16° 



/3a plane " " 241-5 .. - 1 



