MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
19 
As the axial angle ac = 72° 58', this extinction direction of the second median line 
is 6° 19' in front of the vertical axis c, and the first median line is 10° 43' above the 
axis a, both median lines being in the obtuse angle of the morphological axes ac. The 
positions of the optical ellipsoids of all four cobalt-containing salts are compared in 
the following table, and are graphically expressed in the accompanying fig. 5. 
Inclinations of a Axis of Indicatrix (2M.L.) of the Cobalt Salts in 
front of Axis c. 
AmCo sulphate 6 19' 
KCo „ 10° 5' 
RbCo „ 16° 1' 
CsCo „ 26° 8' 
The ammonium salt ellipsoid lies with its a axis nearest the vertical axis c, and the 
ellipsoid rotates further away from this position as 
ammonium is replaced by potassium, rubidium, and 
caesium in turn, according to the order of the atomic 
weights of those alkali metals. 
(iOO) 
iefractive Indices. —The following table gives the 
results of measurements with six excellent 60°-prisms, 
each ground accurately to afford two indices directly, 
the prism edge and bisecting plane being parallel to 
a principal axis and principal plane of the indicatrix. 
There is considerable absorption in the green and blue 
parts of the spectrum, but the illumination of the 
images of the spectrometer slit for Tl, Cd and F light 
was quite adequate to ensure accuracy, using the electric arc with the monochromatic 
illuminator, as usual. 
Fig. 5. 
Refractive Indices of Ammonium Cobalt Sulphate. 
Light. 
OL. 
A 
7- 
Li. 
1-4871 
1-4922 
1-5001 
C. 
1-4876 
1-4927 
1-5006 
Na. 
1-4902 
1-4953 
1-5032 
Tl . . . 
1-4930 
1-4982 
1-5060 
Cd. 
1-4946 
1-4998 
1-5076 
F. 
1-4964 
1-5014 
1-5094 
G. 
1-5018 
1-5067 
1-5148 
Mean of a, /3, and y for Na light = 1 ‘4962. 
a = Vibration direction parallel to second median line, 6° 19' in front of axis c. 
fi = . „ „ „ ,, symmetry axis b. 
y = „ „ ,, „ first median line. 
