MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
421 
Optic Axial Angle .—Three pairs of section-plates were ground, 1, 2, and 3 
perpendicular to the first median line and la, 2a, and 3a perpendicular to the second 
median line. The optic axial angle in air 2E is so large that it could not be measured, 
it being impossible to get the hyperbolic brush-vertices quite to the cross-wires before 
disappearance. In monobromonaphthalene excellent measurements of 2H a with 
sections 1, 2, and 3, and of 2H 0 with sections la, 2a, and 3a were obtained, the 
central rings being small and the brush-vertices very sharp. The values obtained, 
and those of the true optic axial angle 2V a calculated therefrom, together with the 
mean 2V a , are given in the next table. 
Determination of True Optic Axial Angle, in Bromonaphthalene, of CsFe Selenate. 
Ibserved 
2H a . 
No. of plate 
perp. 2 M.L. 
Observed 
2H 0 . 
Calculat 
2V*. 
0 
75 
38 
la 
« 
87 
18 
o 
83 
14 
75 
25 
2 a 
87 
48 
82 
50 
75 
21 
3 a 
87 
44 
82 
50 
75 
33 
la 
87 
15 
83 
12 
75 
20 
2 a 
87 
46 
82 
48 
75 
17 
3a 
87 
42 
82 
48 
75 
2 
la 
87 
8 
82 
56 
76 
0 
2 a 
87 
32 
82 
42 
74 
59 
3 a 
87 
25 
82 
44 
74 
27 
la 
86 
51 
82 
42 
74 
25 
2a 
87 
16 
82 
28 
74 
25 
3a 
87 
12 
82 
30 
74 
8 
la 
86 
43 
82 
34 
74 
8 
2 a 
87 
8 
82 
20 
74 
6 
3a 
87 
4 
82 
22 
73 
53 
la 
86 
34 
82 
28 
73 
50 
2 a 
87 
0 
82 
13 
73 
56 
3a 
86 
52 
82 
20 
Light. 
No. of plate 
perp. 1 M.L. 
Li . . 
C 
Na. . 
T1 
Cd 
F . . 
1 
2 
3 
1 
2 
3 
1 
2 
3 
1 
2 
3 
1 
2 
3 
1 
2 
3 
Mean 
2V a . 
82 58 
82 56 
82 47 
82 33 
82 25 
82 20 
Dispersion of the Median Lines .-—The mean refractive index of the crystals for 
sodium light will be shown to be 1'5357. Methylsalicylate has almost exactly the 
same refractive index for sodium light, 1'5363, so that this substance was employed as 
the immersion liquid for the determination of the dispersion of the median lines, with 
the two best sections 1 and 3 perpendicular to the first median line. The first median 
line proves to lie very slightly nearer to the inclined axis a for greenish blue F-light 
than for red Li-light, the amount indicated by section 1 being 11', and that indicated 
by the measurements with section 3 being 13', the results thus in the mean affording 
the amount 12'. The mean values of the true optic axial angle 2Y a , afforded by 
3 K 2 
