MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
29 
A pair of section-plates parallel to the symmetry plane afforded the following angles 
for the extinction direction corresponding to the second median line 
Inclination of 2M.L. from normal to c 1001 
Plate 1 . . . 9° 24' Plate 2 . . . 9° 6' 
Mean . . . 9° 15' 
The direction is behind the normal. As the axial 
angle ac = 73° 9' this extinction direction is 7° 36' in 
front of the vertical axis c , and the first median line 
extinction is 9° 15' above the axis a, both axes of the 
optical indicatrix which lie in the symmetry plane 
being thus in the obtuse angle of the morphological 
axes ac. Mhumann and Rotter found the two 
extinctions 8° 36' in front of c, and 8° 26' above a. 
In the next table the positions of the optical indi¬ 
catrix of the three manganese salts are compared, 
and in fig. 7 they are graphically illustrated. 
C* 
Inclinations of a Axis of Indicatrix (2M.L.) of the Manganese Salts in front of Axis c. 
AmMn sulphate 7° 36' 
KbMn „ 16° 57' 
CsMn ,, 25° 27' 
Although the potassium salt is missing, the relations of the indicatrices of the 
ammonium, rubidium, and caesium manganese salts are similar to what has been 
observed in groups which are complete, the ammonium salt indicatrix standing with 
its a-axis nearest to the vertical axis of the crystal c. 
Refractive Indices. —The results with six prisms, each ground so as to afford 
directly two indices, are given in the accompanying table. 
Refractive Indices of Ammonium Manganese Sulphate. 
Light. 
a. 
P- 
7- 
Li. 
1 -4770 
1-4810 
1•4882 
C. 
1-4775 
1-4815 
1-4887 
Na . '. 
1-4801 
1-4840 
1-4913 
T1. 
1-4827 
1-4865 
1-4940 
Cd. 
1-4842 
1-4881 
1-4956 
F. 
1-4858 
1-4897 
1-4971 
G. . . . 
1-4912 
1-4951 
1-5025 
Mean of a, /3, and y for Na light = 1 ‘4851. 
a = Vibration direction parallel to second median line, 7' 36' in front of axis c. 
ft = ,, ,, ,, ,, symmetry axis b. 
y = ,, ,, ,, ,, first median line. 
