Washington — Kaersutite from Linosa and Greenland. 195 



ments was 3'137 at 25° C, while Ussing obtained 3*237 (tem- 

 perature not stated) and Lorenzen 3'04r at 18°. 



The extinction angle on m 110 was found to be c : c = — 7*8° 

 in the obtuse angle /3, measured in sodium light (an average 

 of 14 readings on different sections). In Li light the extinc- 

 tion was c /\ c = — 9*3°, which would indicate that there is some 

 slight dispersion of the bisectrices with c /\ c Na < c /s c Li . That 

 it is slight, however, is evident from the comparatively sharp 

 position of extinction in ordinary light. Ussing gives the 



"' 



extinction angle as c : c = about 10° in the obtuse angle /3, but 

 does not state whether this was measured on 010 or 110. He 

 remarks that it is somewhat greater for red than for green, 

 which is in accord with the observations of Wright. The 

 plane of the optic axes is the clinopinacoid. 



The refractive indices were determined directly by the 

 method employed in the preceding case, and were found to be 

 as follows: a = 1-676, j3 = 1-694:, 7 = 1-708, the probable 

 error being less than db 0;02. This gives for the birefrin- 

 gences : 7 - a = 0-032, 7 - /3 = 0*014, - a = 0-018. The 

 optic axial angle calculated from these indices is 2Y = 82° 6 r , 

 while measurement of a section nearly perpendicular to the 



