of Minerals in the Thin Section. 



339 



The curves of figs. 13, 14 and 15 are constructed for min- 

 erals with a refractive index /? = 1,600, while in those of figs. 

 16-19 orthographic projections of the actual positions of the 

 directions of 0° extinction within the crystal are given (refrac- 

 tive index of mineral and medium in which the phenomena 

 are observed, being considered equal). It is evident from the 

 figures that the differences in curvature of the axial bars for 

 the different values of 2 V are sufficient to warrant their use in 

 measuring optic axial angles approximately. The accuracy of 

 the method depends on the accuracy with which the points A 1 

 and P (fig. 12) can be determined. The positions most favor - 



Fig. 15. This figure differs in construction from figs. 12 and 13 only in 

 the fact that the section is considered cut at an angle "k = 20°, }i = 20° with 

 one of the optic binormals. 



able for these points are located one-half to two-thirds the 

 distance from center of field to its margin. Xear the center 

 of the field, the errors of construction increase rapidly, while 

 near the margin, errors due to imperfections in the lenses and 

 to elliptical polarization tend to modify the interference fig- 

 ures, and decrease the accuracy to be attained. 



The actual diameter of the field covered by the micrometer 

 screw movements of the writer's microscope measures about 

 600 micrometer screw divisions. The distance covered by the 

 extremes of the curves for 0° and 90° is less than 200 divisions, 

 or on an average, about 2 divisions for one degree. Taking 

 into consideration the indistinctness and width of the axial 

 bars, it is easily possible to make an error of three or four divi- 

 sions of the micrometer scale in these readings, so that an 



