216 
DR. A. E. H. TUTTON ON THE 
Refractive Indices of Caesium Nickel Selenate. 
Light. 
CL. 
(3. 
7- 
Li. 
1-5358 
1-5412 
1-5451 
C. 
1-5363 
1-5417 
1-5456 
Na. 
1-5395 
1-5450 
1-5489 
T1 ........ 
1-5428 
1-5483 
1•5526 
Cd . .. 
1-5447 
1•5504 
1-5547 
F. 
1-5467 
1-5525 
1-5568 
G. 
1-5533 
1-5589 
1-5631 
Mean of a, /3, and y for Na light = 1'5445. 
a = Vibration direction parallel to first median line, 16° 5T in front of axis c. 
f3 = ,■, „ „. symmetry axis b. 
y = ,, ,, ,, ,, second median line, 0° 40' below axis a. 
Double refraction, Na y _ a = O’0094. 
General formula for the intermediate refractive index (3, corrected to a vacuum 
(correction + O'OOOf):— 
1'525° ! 837 474 4 729 540 000 000 , 
A 
The a indices are also reproduced very closely by the formula if the constant 
1‘5252 is diminished by 0'0055, and the y indices if the constant is increased 
by 0-0040. 
Observations at 70° showed that the refractive indices of caesium nickel selenate are 
diminished by 0‘0013 for a, 0’0016 for (3, and 0‘0018 for y for 55° rise of temperature. 
Axial Ratios of the Optical Ellipsoid .—The calculated values for both the optical 
indicatrix and the optical velocity ellipsoid are given below 
a \ (3 : y = 0-9964 : 1 : D0025. 
a : t> : C = 1"0036 : 1 : 0’9975. 
Molecidar Optical Constants .—These work out as under :— 
Lorenz 
>> 
?? 
Gladstone 
Axis of optical indicatrix . . . 
CL. 
A 
7- 
Specific refraction, — -—, = n . . <(9 
( n 2 + 2)d ID 
Molecular refraction, ~~ M = m . <T 9 
n 2 + 2 d L H 
Specific dispersion, n G - n c . 
Molecular dispersion, m G - m c . 
Molecular refraction, —— M . . . . C 
d 
0-1002 
0-1028 
71-49 
73-37 
0-0026 
1-88 
122-90 
o-ioio 
0-1037 
72- 09 
73- 98 
0-0027 
1-89 
124-14 
0-1016 
0-1043 
72-52 
74-44 
0-0027 
1-92 
125-03 
Mean molecular refraction (Gladstone), I(a + /3 + y) = 124 • 02. 
