of the Double Sclenates of the Scries rt a M(Se0 4 )o,6H 2 0. 83 



Besides the main conclusion regarding these axial ratios, given in 

 the Abstract, it will be observed that a notably less change occurs in 

 the length of the y axis of the indicatrix than along the other two 

 axes, when the total change is considered ; this is illustrated by the 

 less sweep of the corresponding curve of the dotted series shown in 

 fig. 3. When the relative change only is considered the reverse is 

 observed, the change along the 7 axis being the maximum, and its 

 curve of the continuous series in fig. 3 taking the greater sweep. The 

 relative relationships of the three axes to one another, in the case of 

 each salt, govern the magnitude of the optic axial angle of each salt, 

 and the fact that the positions for the rubidium salt are intermediate, 

 points to an intermediate optic axial angle for that salt, an expecta- 

 tion which the next table shows is fulfilled. 



The diminution of the double refraction as the atomic weight of the 

 alkali metal is increased is clearly shown by the ratios, and by the con- 

 vergence of the curves. 



Comparison of True Optic Axial Angles, 2Va. 



The symmetry plane is the common plane of the optic axes. The 

 double refraction is positive for all three salts, and the disposition of 

 the median lines is also identical, subject to the rotation of the whole 

 optical ellipsoid. 



Comparison of the Molecular Optical Constants. 



o -i 



Specific Refraction, = Jt . 



For ray C (Ua). 



For ray lly near G. 



KZu sel. 0-1170 0-1181 0-1210 0-1199 0-1211 0-1242 



119 119 130 121 122 134 



EbZnsel. 0-1051 0-1062 0-1080 0-1078 0-1089 0-1108 



60 66 76 62 67 78 



CsZnsel. 0-0991 0-0996 0-1004 0-1016 0-1022 0-1030 



