40 
DR. A. E. H. TUTTON ON THE 
As the axial angle ac = 73° 
51', this direction, which is that of the first and not the 
second median line (the double refraction being 
negative), is 18° 47' from (in front of) the vertical 
axis c, and the other extinction in the symmetry 
plane is 2° 38' from (below) the axis a. The two 
mutually perpendicular extinction directions in the 
symmetry plane are thus not both in the obtuse 
angle of the morphological axes ac (as is the case in 
other groups than the copper group). The positions 
of the four optical ellipsoids of the copper group 
of salts are given in the next table, and expressed 
in fig. 10. 
Inclinations of a-Extinctions of the Copper Salt in front of Axis c. 
(lM.L. in Am salt and 2 M.L. in other three salts). 
AmCu sulphate 18° 47' RbCu sulphate 2G° 28' 
KCu „ 18° 33' CsCu „ 42° 57'. 
In this case of the copper group the ellipsoid of the ammonium salt is almost 
identically situated with the potassium salt, whereas in other groups it has not been 
inclined so much as the potassium salt from the vertical position. The whole of the 
positions of the ellipsoid (as indicated by its a-axis) for the four copper salts are 
considerably further forward than in the other groups. 
Refractive Indices. —The next table gives the results of the refractive index 
determinations, carried out with six excellent 60°-prisms, each ground so as to afford 
two indices directly. 
Refractive Indices of Ammonium Copper Sulphate. 
Light. 
a. 
/5. 
7- 
Li. 
1-4878 
1-4972 
1-5020 
C. 
1-4883 
1-4977 
1-5025 
Na. 
1-4910 
1-5007 
1-5054 
T1. 
1-4938 
1-5035 
1-5083 
Cd. 
1-4954 
1-5052 
1-5099 
F. 
1-4971 
1-5067 
1-5116 
C4. 
1-5024 
1-5121 
1-5171 
Mean of a, /3, and y for Na light = 1 -4990. 
a = Vibration direction parallel to first median line, 18° 47' in front of axis c. 
fi = ,, „ ,, symmetry axis b. 
y = ,, ,, ,, second median line. 
