20 
DE. A. E. H. TUTTON ON THE 
Mxjemann and Rotter found indirectly for yellow light a = 1*489, /3 = 1*494, 
and y = 1 ‘501 ; also f3 for red light = 1'492, and for green light = 1*497. 
General Formula for /3, corrected to a vacuum :—- 
0 , ., ooc , 441 773 , 449 400 000 000 , 
/ 3 = 1 4826 +—^^-+ ... 
The index a is reproduced also by the formula if the constant 1*4826 is diminished 
by 0*0051, and y if it is increased by 0*0079. 
Observations at 70° showed that the refractive indices of ammonium cobalt sulphate 
are diminished by about 0*0020 (for a) to 0*0025 (for y) by 55° rise of temperature. 
In the next table the refractive indices of the four cobalt-containing salts are 
compared. 
Comparison of the Refractive Indices. 
Index. 
Light. 
KCo 
sulphate. 
EbCo 
sulphate. 
NH 4 Co 
sulphate. 
■ 
CsCo 
sulphate. 
r 
Li. 
1-4780 
1-4833 
1-4871 
1-5028 
C. 
1-4784 
1-4837 
1-4876 
1-5032 
Na. 
1-4807 
1-4859 
1-4902 
1-5057 
“ • • ■ ■ 1 
T1. 
1-4830 
1-4882 
1-4930 
1-5079 
F. 
1-4861 
1-4910 
1-4964 
1-5112 
i 
Cl. 
1-4904 
1-4954 
1-5018 
1-5159 
- 
Li. 
1-4838 
1-4889 
1-4922 
1-5057 
C. 
1-4842 
1-4893 
1-4927 
1-5061 
Q 
Na. 
1-4865 
1-4916 
1-4953 
1-5085 
p . . . . - 
T1. 
1-4889 
1-4940 
1-4982 
1-5110 
F. 
1-4919 
1-4968 
1-5014 
1-5142 
i 
G. 
1-4961 
1-5011 
1-5067 
1-5188 
r 
Li. 
1-4973 
1-4985 
1-5001 
1-5102 
C. 
1-4977 
1-4989 
1-5006 
1-5106 
Na. 
1•5004 
1-5014 
1-5032 
1-5132 
Ti. 
1-5028 
1-5038 
1-5060 
1-5156 
F. 
1-5059 
1-5068 
1-5094 
1-5187 
l 
G. 
1-5105 
1-5114 
1-5148 
1-5237 
Mean refractive index l (a + B + y) 
CG 
00 
1-4930 
1-4962 
1-5091 
for Na light 
Double refraction, Na Y _ a . , . . 
0-0197 
0-0155 
0-0130 
0-0075 
The refractive indices of the ammonium salt, which are concisely represented by the 
mean index given at the foot of the table, are slightly higher than those of the 
rubidium salt, and the double refraction is slightly less than that of the rubidium 
salt, the ammonium salt thus standing between the rubidium and caesium salts as 
regards both properties, and much nearer to the rubidium salt. 
