MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
417 
Refractive Indices of Rubidium Ferrous Selenate. 
Light. 
a. 
ft- 
y- 
Li. 
1-5099 
1-5165 * 
1-5290 
C. 
1-5104 
1-5170 
1-5295 
Na.. 
1-5133 
1-5200 
1-5328 
T1. 
1-5165 
1-5233 
1-5363 
Cd. 
1-5184 
1-5252 
1-5382 
F. 
1-5202 
1-5272 
1-5404 
0 . 
1-5264 
1-5334 
1-5467 
Mean of a, ft, and y for Na light = 1-5220. 
a = Vibration direction parallel to second median line, 13° 37' in front of axis c. 
ft = ,, „ „ ,, symmetry axis b. 
y — ,, ,, „ „ first median line, 1 ° 20' above axis a. 
Double refraction, Na y _ a = 0 - 0195. 
General formula for ft, corrected to a vacuum :— 
O , ir a j o 546 205 160 200 000 000 
ft = l 5048+ - — - - -+ ... . 
* 
The a indices are also very closely reproduced by the formula if the constant 
1‘5048 be diminished by 0‘0067, and the y indices also if the constant be increased 
by 0-0128. 
Observations at 65° indicated that the refractive indices are diminished by about 
0‘0016 for a, 0'0018 for ft, and 0'0021 for y, by a rise of temperature of 50° C. (from 
15° to 65°). 
Axial Ratios of the Optical Ellipsoid. —The values for the optical indicatrix and 
optical velocity ellipsoid are respectively as follows :— 
a : ft : y = 0-9956 : 1 : U0084. 
a : b : C = 1-0044 : 1 : 0-9916. 
Molecular Optical Constants,. —The values of these constants, calculated by both 
the formulae of Lorenz and of Gladstone and Dale, are given in the next table. 
Lorenz 
5) 
J • 
5) 
Gladstone 
Axis of optical indicatrix . . . 
CL. 
ft- 
7- 
Specific refraction, ~—---- = n . . -f £ 
(n 2 + 2)d L Lr 
Molecular refraction, f — \ . ~ = in . 1 ^ 
rr + 2 a L' J 
Specific dispersion, n G - n c . 
Molecular dispersion, m Q - m c . 
yi _2 
Molecular refraction, —-— M . C 
a 
0-1069 
0-1097 
65 • 93 
67-66 
0-0028 
1-73 
112-44 
0-1080 
0-1109 
66-65 
68-41 
0-0029 
1-76 
113-89 
0-1102 
0-1132 
68-00 
69-83 
0•0030 
1-83 
116-65 
Mean molecular refraction (Gladstone), ^(a + ft + y) = 114'33. 
