MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
221 
Determination of True Optic Axial Angle in Bromonaphthalene (continued). 
Light. 
No. of plate 
perp. 1 M.L. 
Observed 
2H„. 
No. of plate 
perp.' 2 M.L. 
Observed 
2H 0 . 
Calculat 
2V a . 
f 
1 
o t 
78 21 
1 a 
o / 
84 40 
o 
86 
/ 
20 
Na. . . A 
2 
78 21 
2 a 
84 41 
86 
20 
l 
3 
78 22 
3 a 
84 39 
86 
22 
r 
1 
78 2 
la 
84 17 
86 
22 
T1 . . . A 
2 
78 5 
2 a 
84 13 
86 
25 
l 
3 
78 5 
3 a 
84 14 
86 
25 
f 
1 
77 50 
la 
84 3 
86 
22 
Cd-. . . .< 
2 
77 56 
2 a 
83 57 
86 
28 
1 
3 
77 53 
3 a 
83 54 
86 
28 
r 
1 
77 45 
la 
83 45 
* 86 
28 
¥ ... .1 
2 
77 39 
2 a 
83 38 
86 
29 
L 
3 
77 '39 
3a 
83 35 
86 
30 
Mean 
2 V„. 
86 21 
86 24 
86 26 
86 29 
Topspe and Christiansen found 86° 14/ for 2V a . 
Dispersion of the Median Lines. —Measurements with sections 1, 2, and 3 immersed 
in methyl salicylate, the refractive index of which (l‘5363) is almost identical with 
the mean index of the crystals, indicated that the dispersion of the two mutually 
rectangular median lines lying in the symmetry plane is such, that the first median 
line lies nearer to the morphological axis a by about 12' for red C light than for 
greenish blue F light. 
Refractive Indices. —Six 60°-prisms were prepared as usual, to afford two indices 
each, a and (3, (3 and y, or a and y, the bisecting plane being parallel to a principal 
plane, and the refractive edge parallel to one of the three principal axes, of the optical 
ellipsoid. The results are embodied in the accompanying table. 
Refractive Indices of Ammonium Nickel Selenate. 
Light. 
CC. 
P- 
7- 
Li. 
1-5246 
1-5332 
1-5419 
C. 
1-5251 
1-5337 
1-5424 
Na. 
1-5285 
1-5370 
1-5460 
T1. 
1-5320 
1-5405 
1-5496 
Cd .. 
1-5339 
1-5425 
1-5517 
F. 
1-5360 
1-5447 
1-5539 
G. 
1-5423 
1-5510 ' 
1-5602 
Mean of a, f3, and y for Na light = 1 -5372. 
a = Vibration direction parallel to second median line, 2° O' in front of vertical axis c. 
— ,, „ „ „ symmetry axis b. 
y = „ „ ,, „ first median line, 14° 17' above inclined axis a. 
Double refraction, Na y _ a = O'0175. 
