428 
DR. A. E. H. TUTTON ON THE 
(l'5248 for Na-light) is very close to the mean refractive index of the crystals. The 
median lines are so dispersed in the symmetry plane that the first median line lies 
nearer to the inclined crystal-axis a by 9' for red Li-light than for green Cd-light. 
Effect of Temperature on the Optic Axial Angle .—Determinations with section- 
plate No. 2 at 75° C. showed that the apparent optic axial angle in air 2E diminished 
by 11° 24' on heating the plate to 75°, the angle being 137° 40' for Na-light at this 
temperature, whereas for 15° it was 149° 4'. 
Refractive Indices .—Six 60°-prisms were ground, the two faces of each being 
symmetrical to (30° from) a principal axial plane of the optical ellipsoid, and the 
refracting edge of each being parallel to a principal axis of the ellipsoid. Each prism 
thus yielded directly two of the three refractive indices a, ft, y, and the combination 
of prisms was so arranged that a, 8, and y were each obtained four times in all. The 
four individual results for each index, for each of the seven wave lengths employed, 
were remarkably close, and the final mean values of the four are given in the next 
table. 
Refractive Indices of Ammonium Ferrous Selenate. 
Light. 
a. 
ft. 
y- 
Li. 
1-5177 
1-5241 
1-5343 
C. 
1-5182 
1-5246 
1-5348 
Na. 
1-5216 
1-5280 
1-5381 
T1. 
1-5251 
1-5314 
1-5416 
Cd. 
1-5271 
1-5334 
1-5437 
F. 
1-5291 
1-5354 
1-5457 
G. 
1•5356 
1-5422 
1-5524 
Mean of a, ft, and y for Na light = 1 • 5292. 
a = Vibration direction parallel to second median line, 5° 24' in front of vertical axis c. 
ft = ,, „ „ „ symmetry axis b. 
y = „ ,, ,, ,, first median line, 10° 45' above inclined axis a. 
Double refraction, Na v _ a = 0-0165. 
Only the /3-indices were determined directly by Tops0e and Christiansen, for C, 
Na, and F-light, the values obtained being respectively 1*5226, 1'5260, and 1'5334. 
General formula for ft, for light of any wave-length X, corrected to a vacuum 
(correction + 0*004) :— 
ft = 1*5076 + 
862 174 
X 2 
4 865 900 000 000 
X 4 
The a indices are equally well reproduced by the formula if the constant 1*5076 be 
diminished by 0*0064, and the y indices if it be increased by 0*0102. 
Observations at 65° O', indicated that the index a diminishes by 0*0013 on heating 
to this temperature, ft by 0*0016, and y by 0*0018. 
