WOLFF AND MELCZER. — HARDYSTONITE AND SCHEFFERITE. 117 



As regards the differences between the boundary angles of the individual 

 plates, these are too large to be ascribed to indefiniteness of the boundary 

 lines, for as already mentioned these were sharp, especially in (3) and 

 (4). In order to be quite sure I repeated the measurements on three 

 plates and found : 



Prism. 



(1) t 64° 273-' and 64° 23}/ 



(2) 64° 21' ... 



(3) 64° 21' ... 



There the above supposition was correct. 



Since the refractive index of the glass of the Abbe hemisphere, accord- 

 ing to one of my previous determinations, is 1.8903 for Na light, and 

 that of the glass prism used 1.6724, there follows from the boundary 

 angles given above for the individual plates : 



Tna /^na ''''na 



(1) and (2) 1.7060 ± 0.0002 1.6840 ± 0.0002 1.6766 ± 0.0004 



(3) 1.7050 ± 0.0005 1.6834 ± 0.0001 1.6757 ± 0.0001 



(4) 1.7045 ± 0.0001 1.6827 ± 0.0001 1.6752 ± 0.0002 



and as a mean : y — ^ = 0.0218 

 /3- a = 0.0075 



y - a = 0.0293 



These variations of the indices in the individual plates could perhaps be 

 referred to local variations in the chemical composition of the zinc 

 schefferite, or to the uneven distribution of pigment. The latter is not 

 noticeably different, and yet it is known that for certain minerals at 

 least very small variations produce such differences in the indices. 

 If from the above values we take as the mean of the indices 



y = 1.705 

 )8= 1.683 

 a= 1.676 



then by calculation 2 Va for sodium = 59° 29|:'. 



For the direct measurement of the axial angle the plate (1) could not 

 be used, for although one optic axis and the middle of the figure could 

 bo seen, and therefore, Hj might have been measured, yet the geometric 

 orientation of the surface could not be determined ; the thin section (2) 

 gave both hyperbolas, but somewhat indistinctly, so that they could only 



