MONOCLINIC DOUBLE SELENATES OE THE NICKEL GROUP. 
205 
Refractive Indices of Potassium Nickel Selenate. 
Light. 
a. 
A 
7- 
Li. 
1-5142 
1-5232 
1-5387 
C. 
1-5147 
1-5237 
1-5392 
Na. 
1-5181 
1-5272 
1-5427 
T1. 
1-5212 
1 - 5305 
1-5464 
Cd. 
1-5232 
1-5325 
1-5486 
F. 
1-5251 
1-5344 
1-5507 
G.” . 
1-5314 
1-5407 
1-5571 
Mean of a, /3, and y for Na light = 1 ’5293. 
cc = Vibration direction parallel to second median line, 6’ 59' in front of axis e. 
/3 = ,, „ „ „ symmetry axis b. 
y = „ ,, „ ,, first median line, 7° 28' above axis a. 
Double refraction, Na Y _ a = O'0246. 
General formula for /3, corrected to a vacuum :— 
0 .._ n , 0 1 034 256 7 821 960 000 000, 
P = 1 5043 +- - a ---+ ... . 
The a indices are equally well reproduced by the formula if the constant 1'5043 is 
diminished by 0'0092, and the y indices if it is increased by 0'0156. 
Observations at 70° indicated that the refractive indices of potassium nickel 
selenate diminish by about 0'0018 for a rise of 55 c of temperature. 
The values of Topsoe and Christiansen* for the refractive indices were : for f3, line 
C 1'5207, line D 1'5248, line F P5315; the a and y values were only obtained 
indirectly, and for line D only ; they are given as 1'5199 and 1'5339. 
Axial Ratios of the Optical Ellipsoid .—These values work out as under : 
Indicatrix . . . . a : /3 : y = 0'9940 : 1 : l'OlOl. 
Velocity ellipsoid . . a : i) : C = 1'0060 : 1 : 0‘9900. 
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. 
Axis of optical indicatrix . 
a. 
7- 
Lorenz 
/c 
0-1178 
0-1195 
0-1227 
0-1225 
0-1258 
(n l + 2) a 
n 2 1 M 
l G 
0-1210 
rc 
62-14 
63-04 
64-60 
Molecular refraction, ——- . — = m . 
n 2 + 2 a 
IG 
63-82 
64-75 
66 - 37 
5? 
Specific dispersion, it G - ltc .... 
0-0032 
0-0032 
1-71 
0-0033 
1-77 
J > 
Molecular dispersion, m G - m c • • • 
1-68 
Gladstone 
Molecular refraction, " - M ... 
d 
. C 
106-10 
107-96 
111-10 
Mean molecular refraction (Gladstone), 
lO + h + y) = 
108-39. 
- .—. 
* ‘Ann. Chinn Phys.,’ series 5, vol. 1, p. 82 (1874). 
