Slue!.}/ of tlie Double Sdenates of the Series R 3 M(Se0 4 ) 2 ,6H 3 0. 81 



Fro. 2. 



The best mode of comparing the refraction of the three salts is 

 probably to take the mean of all three indices, for an intermediate 

 wave-length, of each salt, and to set these mean indices side by side. 

 Such values for sodium light are given below. 



Mean refractive index of KZn selenate for Na light... 1*5212 

 ,, RbZn 



CsZn ,, 



Diff. 26 

 , 129 



The mean refractive indices of the K, Kb, and Cs zinc sulphates are 

 1-4859, 1-4897, and 1-5054". The differences, 38 and 157, are thus 

 reduced when sulphur is replaced by selenium. 



Comparison of Double Refraction, 

 For KZn selenate ............ 0-0214 



RbZn 



CsZn 



0-0169 

 0-0086 



Diff. 45 

 83 



The difference for the first replacement, 45, is thus nearly double that 

 of the mean refraction, 26. This explains why, in spite of the general 

 progression in refraction, the y indices of RbZn selenate are slightly 

 less than those of the KZn salt. 



The relations of the refractive powers of the three salts are con- 

 cisely expressed by the axial ratios of the optical ellipsoid, which are, 

 compared beneath, and even more clearly by the continuous curves in 

 fig. 3, which express them graphically. 



VOL. LXVII. 



