222 
DR. A. E. H. TUTTON ON THE 
General formula for (3, corrected to a vacuum:— 
(3 - D5191 + 
690 578 
A 2 
1 921 310 000 000 
A 4 + 
The a indices are equally well reproduced by the formula if the constant 1 ‘5191 is 
diminished by 0‘0086, and the y indices if it is increased by 0’0090. 
Observations at 70° indicated that the refractive indices of ammonium nickel 
selenate diminish by about 0'0020 for a rise of temperature of 55°. 
Axial Ratios of the Optical Ellipsoid .—The calculated values for the two types of 
ellipsoid are as under :— 
Indicatrix.a : /3 : y = 0"9945 : 1 : 1'0059. 
Optical Velocity Ellipsoid . . a : 1) : r = 1‘0055 : 1 : 0'9942. 
Molecidar Optical Constants .—These are given in the next table. 
Lorenz 
55 
55 
55 
Gladstone 
Axis of optical indicatrix . . . 
CL . 
A 
7■ 
Specific refraction, -A - = n . . <f ^ 
( ri 2 + 2 )d L 5- 
Molecular refraction, % —- . ^ = m . { ^ 
’ n 2 + 2 d X D 
Specific dispersion, n G - n c . 
Molecular dispersion, m G - nic . 
Molecular refraction, M . C 
d 
0-1366 
0-1404 
66-37 
68-18 
0-0038 
1-81 
113-70 
0-1385 
0-1422 
67-27 
69-08 
0-0037 
1-81 
115-56 
0-1404 
0-1442 
68-19 
70-03 
0-0038 
1-84 
117-45 
Mean molecular refraction (GLADSTONE), ±(a. + /3 + y) = 115 ‘ 57. 
Comparison of Residts. 
Habit .—The progression of habit which has been observed in all the other groups 
of double sulphates and double selenates of this large isomorphous series yet 
investigated (the work on the double sulphates having been completed by the author’s 
last communication), is very clearly exhibited in this group of the double selenates 
containing nickel. The progression is characterised by the relative development of 
the faces of the forms c {001} and g {Oil}. In the potassium salt the basal plane 
c {001} is largely predominating, the faces of the clinodomal prism q {011} on each 
side of it being relatively small, as shown in fig. 1. In the caesium salt, on the 
contrary, the two faces of g{011} predominate, and indeed confer a prismatic 
character on the crystal parallel to the inclined axis a, while the basal plane c{00l} 
forms only a central strip, as exhibited in fig. 5. Distinctly intermediate between 
these two extreme types come the crystals of rubidium nickel selenate, in which the 
two forms are more nearly approached in relative size, the g{01l} faces being larger 
