154 



Royal Society. 



Sulphocyanide of Thallium. 



Pyramidal, 01, 10 1=38° 20'*3. 



Observed forms :— 1 0, 1 1 0, 1 1. 



Fig. 2. 



1 0, 1 90 



1 0, 1 1 45 



1 0, 1 1 90 



10 0, 101 51 397 



1 1 0, 1 1 63 59 



10 1, lOl 76 40-6 



101,011 52 2 



Observed combinations :— 1 1 0, 1 1 ; 1 0, 1 1 0, 1 1. 



The crystals are remarkable for the very unequal extension of the 

 faces of the same simple form, and at first sight look as if they belonged 

 to the oblique system. The breadth and thickness of one of the 

 largest crystals were 1*1 and 0*055 millimetre respectively; and of 

 two adjacent faces of the form 1 1, one was about eleven times the 

 breadth of the other. The distribution of the large and small faces 

 did not appear to be subject to any law ; so that these crystals cannot 

 be regarded as combinations of large and small hemihedral forms. 



Twins. Twin face 101. 



Fig. 3. 



o / _ 



10M0J 

 110,011 

 I 1.0, I 1 



1 1, 110 



01 1,110 

 101, 101 



80 





52 4 





52 4 







75 56 i^^^^^^^ 





75 56 l^fflKifi^K 





26 38-8 





No cleavage observable. 



An attempt was made to determine the optical constants of the 

 crystal by observing the minimum deviation of light refracted through 

 a face of the form 110 and one of the opposite faces of the form 

 10 0; the latter were, however, so small that the observation could 

 not be made with much accuracy. It appeared that for the ordi- 

 nary ray polarized in a plane parallel to the line 1, the indices 

 of refraction of red light, of the brightest part of the spectrum, 

 and of violet light were about 2*15, 2*159, and 2*314 respectively, 

 and that, for the extraordinary ray polarized in the plane 01, 



