2Q± F. E. Wright — Transmission of Light through 



which was also the interval of the vernier of the lower nicol ; 

 with this arrangement both nicols were situated outside the 

 optical system, and during each set of observations neither the 

 optical system nor the crystal plate were touched. Experience 

 showed that if the upper nicol remained in the tube, as is 

 ordinarily the case, and then rotated, a shift of the optical cen- 

 ter resulted, and although almost negligible, was still distinctly 

 noticeable. The cross grating ocular 1 served to locate accu- 

 rately the different points in the field. The axial bars of the 

 interference figure were plotted directly on cross section paper 

 as they appeared for different angles of rotation of the crossed 



Fig. 15. 



nicols. The interference figures were sharp and the dark, 

 axial bars clearly defined, although not perfectly dark, and 

 readings could be made to one half of one division (about 1° 

 in angular coordinates ) of the coordinate scale of the ocular. 



Muscovite. — Several fresh cleavage flakes of this mineral 

 were observed in convergent polarized light and the positions 

 of the axial bars determined for different angles of rotation of 

 the crossed nicols. The results are plotted in the stereographic 

 projections ( figs, lia, b ), in which the axial bars are drawn 

 for the two positions of the extinguishing plane of the upper 

 nicol ( —10° and +15° ) as indicated by the dotted lines. The 

 1 This Journal (4), xxix, 423, 1910. 



