210 
DR. A. E. H. TUTTON ON THE 
while section 2 exhibited no measurable amount of dispersion. The section-plates 
were immersed during these observations in methyl salicylate, the refractive index of 
which liquid (1 ‘5363) is not far from the mean index of the crystals. 
Refractive Indices .—Six 60°-prisms were employed, ground so that each afforded 
directly two indices, a and /3, or /S and y, or a and y, the refracting edge not only 
being parallel to a principal axis of the optical ellipsoid, but the bisecting plane being 
also parallel to a principal plane of the ellipsoid. The mean results for the four 
independent values of each index thus obtained are given in the accompanying 
table. 
Refractive Indices of Rubidium Nickel Selenate. 
Light. 
CL. 
7- 
Li. 
1-5161 
1 • 5253 
1-5351 
C. 
1-5166 
1-5258 
1-5356 
Na. 
1-5198 
1-5291 
1-5390 
T1. 
1-5231 
1-5325 
1-5424 
Cd. 
1-5248 
1-5342 
1-5443 
F. 
1-5268 
1-5362 
1-5466 
G. 
1-5335 
1-5429 
1-5534 
Mean of a, /3, and y for Na light = 1 ’5293. 
a = Vibration direction parallel to second median line, 9° 59' in front of axis c. 
/3 = „ ,, „ ,, symmetry axis b. 
y — „ „ „ ,, first median line, 5° 21' above axis a. 
Double refraction, Na y _ a = 0-0192. 
General formula for /3, corrected to a vacuum :— 
o i.Kino . 760 688 3 270 190 000 000, 
p = 1 5103+ ——--+ ••• • 
A A 
The a indices are also reproduced by the formula if the constant 1 *5103 is 
diminished by 0'0093, and the y indices if it is increased by 0'0099. 
Observations at 70° indicated that the refractive indices diminished with rise of 
temperature, to the extent of 0'0017 for 55" rise of temperature. 
Axial Ratios of the Optical Ellipsoid. —The values for the optical indicatrix and 
optical velocity ellipsoid are respectively as follows :— 
a: (3: y = 0'9939 : 1: 1*0065. 
a : b : C = 1-0061 : 1 : 0‘9936. 
