Study of the Double Selcnates of the Series R 3 M(Se0 4 ) 3 ,6H 3 0. 75 



The a indices are reproduced with precisely equal accuracy if the 

 constant 1-5187 is diminished by 0'0036; owing to slightly increased 

 dispersion exhibited by the y indices, these values are reproduced with 

 slightly less accuracy when the constant is increased by the average 

 amount of 0-0050. 



Alteration of Refraction by Rise of Temperature. Measurements of 

 the prism angle and minimum deviation at 60, indicated that the 

 refractive indices are reduced by 0'0015 for 45 rise of temperature 

 (15 to 60). 



Axes of the Optical Ellipsoid. These values are as follows : 



Axes of optical indicatrix : a : /3 : y = 0*9977 : 1 : 1'0033. 



Axes of optical velocity ellipsoid : a : ft : t = 1'0024 : 1 : 0-9968. 



Kefractive Indices of Caesium Zinc Selenate. 



Molecular Optical Constants. These are as under : 



