MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
9 
given in the next table. This is followed by another table, in which the refractive 
indices of the four nickel-containing salts are compared. 
Murmann and Rotter obtained indirectly the values « = 1*489, j3 — 1*498, and 
y = 1'508 for the middle of the spectrum. 
There is a considerable amount of absorption by this green salt at the red end of 
the spectrum, but the electric arc, used as a source of light with the author’s spectro¬ 
scopic monochromatic illuminator, # affords ample light for the observations in Li and 
C light. 
Refractive Indices of Ammonium Nickel Sulphate. 
Light. 
a. 
ft 
7- 
Li. 
1-4916 
1-4975 
1-5046 
C. 
1-4921 
1-4980 
1-5051 
Na. 
1-4949 
1•5007 
1-5081 
T1. 
1-4976 
1-5037 
1-5110 
Cd. 
1-4993 
1■5052 
1-5125 
F. 
1-5007 
1-5069 
]-5142 
C. 
1-5060 
1-5123 • 
1-5196 
Mean of a, f3, and y for Na light = 1 - 5012. 
a = Vibration direction parallel to second median line, 5° 12' in front of axis c. 
f3 = ,, ,, „ symmetry axis b. 
y = ., ,, ,, first median line. 
General Formula for the intermediate refractive index /3, corrected to a vacuum 
(correction +0*0004):— 
0 . Q ko 472 952 , 212 960 000 000 , 
p — 1 48/ 6 H-—-1-—-1- ... 
A a 
A 4 
The a indices are also reproduced very closely by the formula if the constant 
1*4873 is diminished by 0*0060, and the y indices if the constant is increased 
by 0*0073. 
Observations at 70° indicate that the refractive indices of ammonium nickel sulphate 
are diminished by about 0*0018° (for a) to 0*0022° (for y) for 55° rise of temperature. 
The comparative table shows that the refractive indices of ammonium nickel sulphate 
are just slightly higher than those of the analogous rubidium salt, and not nearly so 
high as those of the caesium salt. This is also clearly apparent as regards the mean 
refractive index given at the foot of the table. As regards double refraction, the 
ammonium salt occupies a similar position between the rubidium and caesium salts and 
much nearer to the rubidium one ; as this property of double refraction diminishes 
* ‘ Crystallography and Practical Crystal Measurement,’ 1911, p. 698. 
VOL. COXVI.-A. 
C 
