MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
229 
The optic axial angle of ammonium nickel selenate is slightly greater than that of 
the rubidium salt. 
Comparison of the Refractive Indices. 
Index. 
Light, 
*5 
KNi 
selenate 
RbNi 
selenate. 
NIRNi 
selenate. 
CsNi 
selenate. 
Li. 
1-5142 
1-5161 
1-5246 
1-5358 
C. 
1-5147 
1-5166 
1-5251 
1-5363 
Na. 
1-5181 
1-5198 
1-5285 
1-5395 
a. .... ^ 
T1. 
1-5212 
1-5231 
1-5320 
1-5428 
Cd. 
1-5232 
1-5248 
1-5339 
1-5447 
F. 
1-5251 
1-5268 
1-5360 
1-5467 
G. 
1-5314 
1-5335 
1-5423 
1-5533 
r 
Li. 
1-5232 
1-5253 
1-5332 
1-5412 
C. 
1-5237 
1-5258 
1-5337 
1-5417 
Na. 
1-5272 
1-5291 
1-5370 
1-5450 
/3 .... \ 
T1 ..... . 
1-5305 
1-5325 
1-5405 
1-5483 
Cd . 
1-5325 
1-5342 
1-5425 
1-5504 
F . 
1-5344 
1-5362 
1-5447 
1-5525 
G . 
1-5407 
1-5429 
1-5510 
1-5589 
r 
Li . 
1-5387 
1-5351 
1-5419 
1-5451 
C . 
1-5392 
1-5356 
1-5424 
1-5456 
Na . 
1-5427 
1-5390 
1-5460 
1-5489 
y .... ^ 
T1 . 
1-5464 
1-5424 
1-5496 
1-5526 
Cd. 
1-5486 
1-5443 
1-5517 
1-5547 
F. 
1-5507 
1-5466 
1-5539 
1-5568 
- 
G. 
1-5571 
1-5534 
1-5602 
1-5631 
Mean refractive index 4 (a + (3 + y) 
1-5293 
1-5293 
1-5372 
1-5445 
for Na light 
Double refraction, Na y _ a . . . . 
0-0246 
0-0192 
0-0175 
0-0094 
Refractive Indices .—The refractive indices are compared in the above table. It 
will be seen that the a and /3 indices for the rubidium salt are intermediate 
between those of the potassium and caesium salts. Owing, however, to the great 
diminution in double refraction (y — a), which progressively diminishes as the atomic 
weight of the alkali metal present increases, the y indices for the rubidium salt 
do not quite reach the value of these indices for the potassium salt. As another 
consequence of this the mean refractive index (mean of all three indices a, (3, y for 
Na light) for the potassium and rubidium salts works out to be only just the 
same. As, however, the optical change is an accelerating one with the atomic 
weight of the alkali metal, in spite of the further diminution in double refraction 
when the caesium salt is reached, all the indices (including the mean) for the caesium 
salt show an advance. 
