422 
DR. A. E. H. TUTTON ON THE 
these observations in a liquid of so very nearly the same refraction as the crystals, 
were 82° 41' for Li-light and 81° 33' for F-light, direct observations which amply 
confirm both the actual values given in the table for the true angle, by calculation 
from 2H a and 2H 0 , and the order of the dispersion. 
Refractive Indices.— The refractive indices were determined with six prisms of 
about 60° angle, ground so that the faces were symmetrical to the principal planes 
and the refracting edges parallel to the axes of the optical ellipsoid, so that each 
prism yielded two of the three refractive indices a, /3, y directly. The results for the 
same index from the different prisms were all remarkably concordant, and the mean 
values are given in the next table. 
Refractive Indices of Caesium Ferrous Selenate. 
Light. 
a. 
P- 
7- 
Li. 
1-5269 
1-5317 
1-5379 
C. 
1-5274 
1-5322 
1-5384 
Na. 
1-5306 
1-5352 
1-5414 
T1. 
1-5339 
1-5385 
1-5450 
Cd. 
1-5359 
1-5405 
1-5470 
F. 
1-5379 
1-5425 
1-5491 
G. 
1-5442 
1-5488 
1-5555 
Mean of a, f3, and y for Na light = 1'5357. 
a = Vibration direction parallel to second median line, 21° 4' in front of axis c. 
/3 = ,, „ „ ,, symmetry axis b. 
y = „ .,, ,, ,, first median line, 5° 2' below axis a. 
Double refraction, Na v _ n = 0 - 0108. 
General formula for the intermediate refractive index /3, corrected to a vacuum 
(correction + 0'0004):—• 
tkoao , 546 452 170 930 000 000 
The a indices are also reproduced by the formula if the constant 1'5200 be 
diminished by 0'0047, and the y indices if the constant be increased by 0'0063. 
Observations of refractive index at 65 0 with one of the prisms indicated the usual 
slight diminution of refraction with rise of temperature, the amount indicated for 
50° C. of rise of temperature (from 15° to 65°) being about 0'0012. 
Axial Ratios of the Optical Ellipsoid. —The values of these constants for the two 
types of ellipsoid are as under :— 
a : y = OT970 : 1 : F0040. 
a : b : C = 1 '0030 : 1 : 0’9960. 
