MONOCLINIC DOUBLE SELENATES OF THE IKON GROUP. 
411 
It is obvious that, as expected, the approximate values for y afforded by the q -faced 
prism were the true values within the limits of experiment (0'0002) ; and that the 
approximate a values derived from the p-faced prism were slightly higher than the 
true a. 
It is thus now possible to give with all confidence the true values of a, (3 and y, all 
obtained directly. They are as under, the G-values for a and (3 being included from 
the evidence of their difference from the F-values afforded by the approximate 
determinations with the natural prisms. 
Refractive Indices of Potassium Ferrous Selenate. 
Light. 
a. 
P- 
y- 
Li. 
1•5059 
1-5144 
1-5306 
C. 
1-5064 
1-5149 
1-5311 
Na. 
1-5095 
1-5182 
1-5345 
T1. 
1-5127 
1-5215 
1-5379 
Cd. 
1-5145 
1-5234 
1-5399 
F. 
1-5164 
1-5253 
1-5421 
G. 
1-5224 
1-5314 
1-5483 
Mean of a, /?, and y for Na light = 1‘5207. 
a = Vibration direction parallel to second median line, 10° 27' in front of axis c. 
/3 = ,, „ „ ,, symmetry axis b. 
y = ,, ' ,, ,, ,, first median line, 3° 23' above axis a. 
Double refraction, Na y _ a = 0-0250. 
General formula for (3, for any wave-length X, corrected to a vacuum (correc¬ 
tion + 0'0004):— 
(3 = U4984 + 
838 265 4 754 500 000 000 
X 2 
X 4 
+ . . . 
The a indices are also reproduced if the constant 1’4984 be diminished by 0’0087, 
and the y indices if the constant is increased by 0'0163. 
Axial Ratios of the Optical Ellipsoid. —These values work out as under :— 
Indicatrix .... a : (3 : y = 0‘9943 : 1 : 1'0107. 
Velocity ellipsoid . . a : t> : C = 1'0058 : 1 : 0‘9894. 
Molecular Optical Constants .—The values of these constants, calculated by both 
the formulte of Lorenz and of Gladstone and Dale, are given in the next table. 
Lorenz 
55 
55 
55 
Gladstone 
Axis of optical indicatrix .... 
CL. 
P- 
y- 
Specific refraction, -f -—, = n . . / £ 
(f + 2) cl l G 
Molecular refraction, f ^ = m . { 2 
ri 2 +2 d 1G 
Specific dispersion, n G - n c . 
Molecular dispersion, m G - m c . 
Molecular refraction, ?i - l M . ... C 
a 
0-1192 
0-1224 
62-55 
64-21 
0-0032 
1-66 
106-54 
0-1209 
0-1241 
63-43 
65-13 
0-0032 
1-70 
108-33 
0-1241 
0-1274 
65- 10 
66- 85 
0-0033 
1-75 
111-74 
Mean molecular refraction (Gladstone), |(a + /3 + y) = 108-87. 
VOL. CCXVIII.-A. 3 I 
