MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
5 L 
O' 
The direction is behind the normal. As the axial angle ac — 73 C 19' this 
extinction direction, the second median line, is 11° 27' from and in front of the 
vertical axis c ; and the other extinction in the symmetry plane, the first median 
line, is 5° 14' from and above the axis a, both median 
lines thus lying in the obtuse angle of the morphological 
axes ac. 
The positions of the optical indicatrix for the three 
cadmium salts are compared in the next table, and 
illustrated in fig. 13. 
Inclinations of a-Axis of the Indicatrix (2M.L.) of the 
Cadmium Salts in front of Axis c. 
AmCd sulphate 11 27' 
RbCd „ 15° 53' 
CsCd „ 26° 56' 
The order of these positions of the indicatrix for the three salts is similar to what 
has been observed in the other groups, in which the potassium salt also has been able 
to be compared. 
Refractive Indices. —Determinations with six prisms, each ground to yield two 
indices directly, gave the results summarised in the accompanying table. 
Refractive Indices of Ammonium Cadmium Sulphate. 
Light. 
a. 
A 
7- 
Li. 
1-4816 
1-4855 
1-4927 
C. 
1-4821 
1-4860 
1-4932 
Na. 
1-4847 
1-4887 
1-4959 
T1. 
1-4875 
1-4915 
1-4987 
Cel. 
1 4891 
1-4931 
1-5003 
F 
1•4907 
1-4947 
« 1-5019 
G. 
1-4961 
1-5001 
1-5073 
Mean of a, [3, and y for Na light = 1 • 4898. 
a. = Vibration-direction parallel to second median line, 11° 27' in front of axis c. 
[3 = „ „ „ „ symmetry axis b. 
y = „ ,, ,, ,, first median line. 
General formula for (3, corrected to a vacuum :—- 
« , 605 763 2 351 170 000 000, 
p = 1 4736+——-^-+... . 
The a indices are equally well reproduced by the formula if the constant I’4736 is 
diminished by 0'0040, and the y indices if it is increased by 0’0072. 
H 2 
