1863.] 



395 



Observed forms : 



010, 001,012, 102, 101, 110, 111. 



Angles. 



Calculated. 



Observed. 



55 

 66 



20 

 49 



64 18 



51 



No cleavage observable. 



The minimum deviations of the brightest part of the solar spec- 

 trum were observed through the faces 1 2, 1 2, through the faces 

 1 1, 1 Ol, and through the faces 1 1 0, 1 1 0, the crystal being 

 immersed in water contained in a vessel bounded by plates of glass 

 parallel to the plane bisecting the dihedral angle formed by the 

 refracting faces in each case. From these observations it appears 

 that for a ray in the plane 100, and polarized in that plane, the 

 index of refraction is about 1*837 ; for a ray in the plane 010, and 

 polarized in that plane, the index of refraction is between 1-545 and 

 1-546 ; and for a ray in the plane 001, and polarized in that plane, 

 the index of refraction is about 1*545. Hence the optic axes are in 



