436 
DR. A. E. H. TUTTON ON THE 
Comparison of the Refractive Indices. 
[ 
Index. 
1 
Light. 
KFe 
selenate. 
RbFe 
selenate. 
NH 4 Fe 
selenate. 
CsFe 
selenate. 
Li. 
1•5059 
1-5099 
1-5177 
1-5269 
C. 
1-5064 
1-5104 
1-5182 
1-5274 
Na. 
1-5095 
1-5133 
1-5216 
1-5306 
a .... ^ 
T1. 
1-5127 
1-5165 
1-5251 
1-5339 
Cd. 
1-5145 
1-5184 
1-5271 
1-5359 
F. 
1-5164 
1-5202 
1-5291 
1-5379 
G. 
1-5224 
1-5264 
1-5356 
1-5442 
' 
Li. 
1-5144 
1-5165 
1-5241 
1-5317 
C. 
1-5149 
1-5170 
1-5246 
1-5322 
Na . . . . . 
1-5182 
1-5200 
1-5280 
1-5352 
T1. 
1-5215 
1-5233 
1-5314 
1-5385 
Cd. 
1-5234 
1-5252 
1-5334 
1-5405 
F. 
1-5253 
1-5272 
1-5354 
1-5425 
G. 
■ 
•1-5314 
1-5334 
1-5422 
1-5488 
r 
Li. 
1-5306 
1-5290 
1-5343 
1-5379 
C. 
1-5311 
1-5295 
1-5348 
1-5384 
Na. 
1-5345 
1-5328 
1-5381 
1-5414 
\y • • • ■ I 
T1. 
1-5379 
1-5363 
1-5416 
1-5450 
Cd. 
1-5399 
1-5382 
1-5437 
1-5470 
F. 
1-5421 
1-5404 
1-5457 
1-5491 
* 
G. 
1-5483 
1-5467 
1-5524 
1-5555 
Mean refractive index ^ (a + /? + y) 
for Na light 
1-5207 
1-5220 
1-5292 
1-5357 
Double refraction, Na y _ a . . . . 
0-0250 
0-0195 
0-0165 
0-0108 
Double Refraction .—This property has been shown to diminish at a rate accelerating 
with the rise of the atomic weight of the alkali metal. The total diminution between 
potassium and caesium ferrous selenates is 0'0142. For the two analogous salts of the 
nickel group last studied* it was found to be 0'0152; while for the zinc and magnesium 
groups previously studied it was less and still less, 0'0128 and 0‘0112. This smaller 
change in the double refraction in the two last-mentioned groups was only just 
adequate to reverse the progression of the y indices in RbZn and RbMg selenates, these 
indices being lower than those of KZn and KMg selenates by 0'0004 only. 
Axial Ratios of the Optical Indicatrix. 
a 
: P 
: 7 
cc : 
P '■ 
y 
KFe selenate . 
. . 0T943 
: 1 
: 1-0107 
0-9943 : 
1 : 
: 1*0107 
RbFe ,, 
. . 0-9956 
: 1 
: F0084 
0-9968 : 
F0012 : 
: 1-0096 
NH 4 Fe „ 
. . 0-9958 
: 1 
: F0066 
1-0022 : 
1-0064 : 
: 1*0131 
CsFe 
. . 0-9970 
: 1 
: 1-0040 
F0082 : 
1-0112 : 
: 1-0153 
* ‘ Phil. Trans.,’ 
A, 
vol. 217, p. 
229 (1917). 
