MONOCLINIC DOUBLE SELENATES OF THE NICKEL CROUP. 
20!) 
This direction is behind the normal to the basal plane c, {001 :, towards the vertical 
morphological axis c, and it is the second median line. Fig. 4 will make the 
position clear. As the morphological axial angle ac is 74° 40', and the angle between 
the normal to c {001} and the vertical axis c is consequently 15° 20', the second 
median line lies 9 J 59' in front of the vertical axis c, in the obtuse angle of the 
morphological axes a and c. The first median line lies 5° 21' above the inclined 
morphological axis a, also in the obtuse angle ac, and 9 59' below the normal to 
a {100}. 
Optic Axial Angle. —Three excellent pairs of section-plates were ground perpen¬ 
dicular to the first (l, 2, 3) and second (la, 2a, 3a), median lines, and they afforded 
good interference figures in monobromonaphthalene, from which the measurements 
of the acute and obtuse angles 2H a and 2H 0 and the calculated results for the true 
angle 2V a recorded in the accompanying table were derived. The apparent optic 
axial angle in air 2E is so large that it is not possible to measure it, the interference 
brushes and innermost rings disappearing before the brushes can be brought quite to 
the cross-wires of the polarising goniometer. 
Rubidium Nickel Selenate. 
Determination of True Optic Axial Angle in Bromonaphthalene. 
Light. 
No. of plate 
perp. 1 M.L. 
Observed 
2H a . 
r 
1 
o / 
74 36 
Li ... J 
2 
74 38 
l 
3 
74 53 
r 
1 
74 33 
c . . . J 
2 
74 36 
1 
3 
74 50 
r 
1 
74 14 
Na. . . J 
2 
74 19 
l 
3 
74 14 
r 
1 
73 52 
T1 . . . J 
2 
73 58 
l 
3 
73 47 
r 
1 
73 36 
Cd . . . A 
2 
73 36 
l 
3 
73 33 
r 
1 
73 23 
F . . . J 
2 
73 22 
l 
3 
73 IS 
No. of plate 
perp. 2 M.L. 
la 
2a 
3 a 
la 
2a 
3 a 
la 
2a 
3a 
la 
2a 
3 a 
la 
2a 
3a 
la 
2 a 
3a 
Observed 
2H 0 . 
87 37 
87 52 
87 53 
87 36 
87 50 
87 51 
87 29 
87 30 
87 38 
87 21 
87 9 
87 14 
87 12 
86 55 
87 0 
87 5 
86 39 
86 46 
Calculated 
2Y a . 
82 24' 
82 18 
82 27 . 
82 22' 
82 17 
82 26. 
82 14' 
82 16 
82 9 
Mean 
2Y a . 
82 23 
82 22 
82 13 
82 7 
vj .) •> 
oi o 
81 58 
Dispersion of the Median Lines .—The median lines show practically no dispersion. 
Section 1 showed 3' in one direction and section 3 showed 2' in the other direction, 
2 G 2 
