MONOCLINIC DOUBLE SELENATES OF THE NICKEL CROUP. 
211 
Molecular Optical Constants. —The values of the Lorenz and the Gladstone and 
Dale constants are given in the accompanying table. 
Lorenz 
Gladstone 
Axis of optical indicatrix . . . 
cl. 
A 
7- 
Specific refraction, — T— 7 = n . . -f 9 \ 
{n 1 + 2)d l G 
Molecular refraction, f —- . ^ = m . ■[ 
n 2 + 2 cl LG 
Specific dispersion, n Q - n c . 
Molecular dispersion, nr G - m c . 
^ _2 
Molecular refraction, —-— M . . . . C 
a 
0-1059 
0-1088 
65-59 
67-39 
0-0029 
1-80 
112-08 
0-1074 
0-1104 
66-57 
68-38 
0-0030 
1-81 
114-07 
0-1091 
0-1121 
67-61 
69-47 
0-0030 
1-86 
116-20 
Mean molecular refraction (Gladstone), L(a + /J -t-y) = 114* 12. 
Caesium Nickel Selenate, Cs 2 Ni (Se0 4 ) 2 .6H 2 0. 
Morphology. 
No previous measurements or description of this salt appear to have been made. 
It forms bright emerald green crystals of perfect transparency, possessing faces of 
high reflecting power. About thirty crops were prepared, derived from two distinct 
preparations. 
Twelve small crystals, in several cases no larger than a pin’s head, were measured, 
selected from four different crops. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes. — a:b :c = 07288: 1 : 0'4993. 
Axial Angle .— /3 — 106° IP. 
Forms Observed. — a {100}, b {010}, c {001}, p {110}, q {Oil}, r' {201}, and 
°' {T11} : 
Habit. —Very characteristic. Prismatic parallel to the inclined axis a, the prism 
being formed by the broad and elongated faces of q {011} and the narrower elongated 
faces of the basal plaue c {001}, as shown in fig. 5. 
The facial development and planeness, in the cases of the measured crystals, were 
exceptionally perfect, the signal reflections being so sharp and true that in the cases 
