230 
DR. A. E. H. TUTTON ON THE 
The indices of the ammonium salt are intermediate between those of the rubidium 
and caesium salts. 
The Double Refraction .—This is a property which shows the optical accelerating 
progression with the atomic weight of the alkali metal particularly well, as will be 
obvious from the values given at the foot of the table. Its effect is greater in this 
nickel group than in the magnesium and zinc groups already studied, in which groups 
the mean refractive index of the rubidium salt showed a considerable advance on that 
of the potassium salt. 
Axial Ratios of the Optical Indicatrix. 
a 
: f3: 
7 
a. : /3 
7 
KNi 
selenate 
. 0'9940 
: 1 : 
roioi 
0-9940 : 1 
roioi 
RbNi 
D 
. 0-9939 
: 1 : 
1-0065 
0-9952 : R0012 
1-0077 
NH 4 Ni 
33 • 
. 0-9945 
: 1 : 
1-0059 
1-0009 : 1T064 
1-0123 
CsNi 
33 * 
. 0-9964 
: 1 : 
1-0025 
1-0080 : 1-0116 
1’0142 
Axial Ratios of the Optical Velocity Ellipsoid. 
a 
: b : 
f 
a : b 
r 
KNi 
selenate 
. roo60 
: 1 : 
0-9900 
1-0060 : 1 
0-9900 
RbNi 
33 * 
. roo6i 
: 1 : 
0-9936 
D0049 : 0-9988 
0-9923 
NH 4 Ni 
33 * 
. 1-0055 
: 1 : 
0-9942 
0-9991 : 0-9936 
0-9878 
CsNi 
33 * 
. 1-0036 
: 1 : 
0-9975 
0-9920 : 0-9885 
0-9860 
Axial Ratios of the Optical Ellipsoid .—The values of these ratios are given in the 
accompanying tables for both ellipsoids. The left hand series in each case are 
calculated with the (3 value for each salt taken as unity. The right hand series 
exhibit the total change in the dimensions of the optical ellipsoid on passing from 
one salt of the group, to another, the /3 value for the initial potassium salt being 
taken as unity for all four salts. The main fact that these latter ratios indicate is 
the accelerating nature of the change in dimensions of the ellipsoid as the atomic 
weight of the alkali metal rises. The slight set-back of the y values for the rubidium 
salt is due to the effect of the diminution in double refraction, as already explained. 
The dimensions of the ellipsoid of the ammonium salt are intermediate, another proof 
of the true character of the isomorphism, for they lie within the values for the 
extreme metallic salts of the eutropic series. 
Molecular Optical Constants .—These are by far the most valuable of the optical 
constants, and they are set forth in order in the three next tables. 
