Petrographic Microscope Work. 



79 



uniaxial minerals cut normal to the principal axis. This inter- 

 relation of the phenomena observed on plates in obliquely inci- 

 dent, plane-polarized light and those seen in interference 

 figures from the same plates in convergent-polarized light is 

 fundamental but has not always been appreciated by observers. 

 Recently H. Schneiderhohn* has elaborated this method of 

 oblique illumination and employed a stop in the eye circle of 

 the ocular for the purpose (method 7 above) ; this position is, 

 however, not the best for ordinary work, because of the com- 

 paratively small eye circle, and the small angle of emergence 

 of the transmitted rays (8° to 15°) and also for the reasons 

 cited above under methods 6 and 7. The special advantage 

 claimed by Schneiderhohn for this method, that it produces 

 narrower rims of total reflection on mineral grains than are 



Fig. 186. 



obtained by any of the substage methods, is of little practical 

 moment and is more than outweighed by its unfavorable fea- 

 tures. Schneiderhohn does not recognize, moreover, the con- 

 nection between the interference colors obtained on a plate in 

 obliquely incident, parallel, polarized light and those observed 

 in the interference figure, and is thus led to make several 

 incorrect statements. Thus the assertion that on a section per- 

 pendicular to the optic normal, the interference colors for all 

 obliquely incident waves are lower than that of the central 

 wave,f is not correct, as can be shown both by direct test in 

 convergent polarized light, and also by mathematical reason- 

 ing. Sections of all uniaxial minerals parallel with the princi- 

 pal axis and of biaxial minerals parallel with the plane of the 

 optic axes exhibit in convergent polarized light the well known 

 fall of interference colors in the direction of the acute bisec- 

 trix but rise of interference colors in the adjacent quadrants. 



*Zeitschr. Kryst., 1, 231-241, 1912. 

 f Ibid., 1, 237, 1912. 



