36S F. E. Wright — Measurement of the Optic Axial Angle 



(h) In parallel polarized light, the methods involving the 

 Fedorow-Fuess universal stage are used and furnish satisfactory 

 results, provided the position of one optic axis can be deter- 

 mined directly. If both optic axes are outside of the Held of 

 vision, the results obtained are usually unsatisfactory and inac- 

 curate. Theoretically, it is possible to measure the optic axial 

 angle of an}' biaxial transparent mineral on any section by 

 means of the universal stage. If both optic axes appear within 

 the field of vision, the error of determination should not exceed 

 1°, and if only one of the optic axes be visible, the accuracy 

 may decrease to ± 5°. The exact location of a visible optic 

 axis is assisted somewhat by use of the method of optical 

 curves. Having once fixed the location of one optic axis, that 

 of the second is determined by the method of extinction curves. 

 If both optic axes lie entirely outside of the field, special 

 methods must be resorted to, but in general without marked 

 success, owing to the great difference in the value of 2V caused 

 by a very slight deviation in the measured extinction angle. 



The range of the field of vision of the universal stage is 

 greater than that of any possible interference figure; the Fed- 

 orow universal stage methods are, therefore, applicable to a 

 greater number of sections than the methods with convergent 

 polarized light and ma} 7 furnish results on sections otherwise 

 useless for ordinary methods. Both experience and theory 

 show that for all these methods the accuracy of the determina- 

 tion varies considerably with the section and mineral in ques- 

 tion. The most accurate results can be obtained on sections for 

 which both optic axes appear within the field of vision ; less 

 accurate but still satisfactory measurements can be made when 

 only one optic axis appears, particularly when it is situated 

 about midway from the center to the margin of the field. 



For convergent polarized light, the general extension of the 

 Mallard method by means of the new double screw micrometer 

 ocular is the most satisfactory and accurate method available, 

 but good results can be had by use of the Becke drawing table. 



The readings in both methods require to be reduced to equiv- 

 alent crystal angles and plotted in stereographic projection. 

 The optical axial angle is then measured upon the plot by 

 graphical methods. 



(2) For the purpose of plotting and measuring observed and 

 calculated angles, the stereographic projection is without doubt 

 best adapted to optical work in general. The stereographic 

 plot of Plate I is a photographic reproduction of the accurate 

 drawing by Professor Wulff published in the Zeitschrift fur 

 Krystallographie. 



(3) Since the interference figures are roughly orthographic 

 projections of the phenomena in space, an accurate ortho- 



