279 
OF THE DOUBLE SELENATES OF THE SEPJES EoM(SeO,).2,6H,0. 
Comparison of the Double Refraction^ Na^_^, 
For KMg selenate . . . . O'OIZO 
„ RbMg „ . . . . 0-0124 
„ CsMg „ .... 0-0058 J 
The double refraction is shown by the above table to decrease at an accelerating 
rate as the atomic weight of the alkali metal is increased. The amount of diminution 
for the first replacement is nearly double as much as the increase of mean refraction, 
which accounts for the fact already alluded to that the y indices of rubidium 
magnesium selenate are slightly lower than those of potassium magnesium selenate. 
This latter apparent anomaly, therefore, is the direct outcome of the general rule so 
far established for the series as regards progressive diminution of double refraction. 
The instructive axial ratios of the optical ellipsoid (calculated for Na-light) are 
compared in the following tables, and they are also graphically expressed in the 
curves of fig. 5. The table representing the values when is taken as unity, and 
the series of dotted cui'ves corresponding, exhibit the total change of the ellipsoid on 
passing from one salt to another, as distinguished from the change in the relations of 
the ellipsoidal axes of any one salt. 
-Diff. 46 
Comparison of the Oi^tical Ellipsoids. 
Optical Indicatrix. 
a. /?. 
y- 
Double 
refraction. 
KMo; selenate 
. . . . 0-9985 : 
1 
1-0099 
114 
RbMg selenate 
. . . . 0-9987 : 
1 
1-0069 
82 
CsMg selenate 
. . . . 0-9999 : 
1 
1-0038 
39 
KMg selenate 
Optical Velocity Ellij^soid. 
a. fa. 
. . . . 1-0015 : J 
c. 
0-9902 
113 
RbMg selenate 
. . . . 1-0013 : 
1 
0-9931 
82 
CsMg selenate 
. . . . 1-0001 : 
1 
0-9963 
38 
Indicatrix when 
a. 
/?. 
y- 
KMg selenate . 
0-9985 : 
1 
; 
1-0099 
RbMg selenate. 
28 
. . 1-0013 : 
27 
1-0027 : 
1-0096 
CsMg selenate. 
112 
. . 1-0125 : 
99 
1-0126 ; 
1-0163 
67 
