OB^ ARTS AND SCIENCES. 79 



uncertainty thus arising does not, however, amount to more than a few 

 minutes. We give the results obtained with the naost perfect crystals 

 we could find, on which we were able to measure all the angles of the 

 principal dome between the two basal planes. 



Crystalline Form of Antimonious Iodide. 



Hexagonal Variety. 



Forms {111}, {100}, {011} 



Figs. 4 and 5, Plate I. 



Angles measured between normals. 



Ill on 100 72* 28' 



100 „ OTT 49* 22' 



OTT „ ni 58* 8' 



179° 58' 



These measurements correspond to the dimensions of a modified 

 rhombohedron in which the axes of Miller's system make with each 

 other the angle of 54* 40', or the vertical axis of Naumanu's system 

 has the value 0= 2.769. The crystals cleave readily parallel to the 

 basal plane. 



The crystals are optically uniaxial with very strong negative double 

 refraction, and the broad plates obtained by sublimation furnish excel- 

 lent objects for the polariscope ; but such preparations are not durable 

 if exposed to the atmosphere, for a reason which will appear further 

 on. The rings of the interference figure, as seen by common light, 

 are nearly black, but with strong-colored fringes, — red on the inside, 

 and greenish yellow on the outside. This is a natural result of the 

 selective absorption of this highly colored medium ; but the effect is, 

 nevertheless, very striking. 



