70 Mr. A. E. Tutton. A Com j>" rut ire Ory&afloffraphical 



stant 1-5067 is diminished by 0*0060; owing to apprecial.lv increased 

 dispersion in the y direction, the y indices are not so accurately repro- 

 duced, but are approximately given if the constant is iiuTcasud by 

 0-0109. 



.Ill, ft> /inn <>f l;<frn<-tiou In/ Rise of Tempemture. Determinations at 

 60 showed that the indices are reduced by 0*0018 by 45 rise of tem- 

 perature (1560). 



Axes of the Optical Ell^soid. Following are the values of the axial 

 ratios of the two optical ellipsoids 



Axes of optical indicatrix : 



a : p : y = 0'9961 : 1 : 1'0072. 

 Axes of optical velocity ellipsoid : 



a : b : t = 1'0039 : 1 : 0-9929. 



Molecular Optical Constants. The calculated values of these constants 

 are as follows : 



Optic Axial Angle. The following measurements were obtained with 

 the aid of three pairs of section-plates, ground perpendicular to the 

 first and second median lines, according to the indications of their 

 orientation afforded by the extinction angles : 



Apparent Angle in Air of Rubidium Zinc Selenate. 



