556 



Prof. W. H. Miller on the forms 

 Fig. 2. 



[Dec. 



Observed combinations : 1 1 0, 1 1 ; 1 0, 1 I 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 ri and 0*055 millimetre respectively; and of two adjacent laces 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. 



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 100; the latter 

 were, however, so small that the observation could not be made with much 

 accuracy. It appeared that for the ordinary ray polarized in a plane 

 parallel to the line 001, the indices of refraction of red light, of the 

 brightest part of the spectrum, and of violet light were about 2' 1 15, 2- 159, 

 and 2"314 respectively, and that, for the extraordinary ray polarized in 

 the plane 1, the indices of refraction of red light, the brightest part of 

 the spectrum, and of violet light were about 1-890, 1'973, and 2-143 re- 

 spectively. 



