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



by the factors on which the method itself is dependent and 

 their relative exactness under the conditions of observation. 



Measurements loith Axial Angle Apparatus. 



The optic axial angles obtained in sodium light on the 

 Wulfing axial angle apparatus varied slightly and the average 

 of five determinations of each angle is given below : 



Topaz, Willard Co., Utah. 



2E = 126° 13'. 2V = 66° 42'. 

 Aragonite, Bilin, Bohemia. 



2E = 31° 09'. 2V = 18° 22'. 



Muscovite (a) 



2E = 71° 40' 

 Muscovite (b) 



2E = 59° 42'. 



Measurements with the Becke drawing -table. — To economize 

 space, the results are given below in their reduced form ready 

 for plotting directly in projection, the angle 6 denoting the 

 longitude from the horizontal E-W line of the projection and 

 p the polar distance ; A 1 as usual denotes the visible axial point 

 and P, any point on the dark axial bar. 



Topaz. 



(a) <p p 



A l 0° 5°-0 



P x +65 20-5 



In projecting these angles and performing the requisite 

 mechanical operations, the optic axial angle thus determined on 

 this section was 2 V = 62°'5. For a second section the values 

 were : 



<t> p 



A l — 8° 3°-8 2V = 10° 



P x —72 19 -6 



For a third section : 



4> P 



A, -.. +27° 8°-8 2V = 63° 



Pj .- —58 23 



The average of these three values is 65° # 2. 



Aragonite. — In aragonite the optic axial angle is so small 

 that both axial bars A l and A 2 are visible and the direct deter- 

 mination of 2Y AlA2 should in all cases be accurate within one 

 degree. The birefringence is so strong, however, that the meas- 

 urements involving a point P 2 or P a on the dark axial bar and 

 consequent introduction of the refractive index /3 for that 

 point, may be decidedly incorrect. The use of the refractive 



