MONOCLINIC DOUBLE SELENATES OF THE IRON CROUP. 
427 
the true optic axial angle 2V a from these two latter determinations are given in 
the penultimate column of the second table, and the mean 2V a in the last column. 
Apparent Optic Axial Angle in Air, 2E, of AmFe Selenate. 
Light. 
Plate 1. 
Plate 2. 
Plate 3. 
Mean 2E. 
Li. 
o / 
145 35 
O f 
145 50 
O ! 
145 37 
o / 
145 41 
C. 
145 53 
146 18 
145 51 
146 1 
Na. 
148 56 
149 4 
147 46 
148 35 
T1. 
149 21 
149 40 
149 10 
149 24 
Cd. 
151 8 
150 7 
150 18 
150 31 
F. 
152 10 
151 0 
151 20 
151 30 
Determination of True Optic Axial Angle, in Bromonaphthalene, 
of AmFe Selenate. 
Light. 
Li . . 
C . . 
Na 
T1 . . 
Cd . 
No. of plate 
Observed 
No. of plate 
Observed 
Calculated 
Mean 
perp. 1 M.L. 
2H a . 
perp. 2 M.L. 
2H 0 . 
2V a . 
2V a . 
1 
o / 
70 28 
la 
o / 
91 45 
o 
77 
34 5 
O f 
2 
70 32 
2 a 
91 47 
77 
36 } 
77 36 
3 
70 32 
3 a 
91 42 
77 
38 J 
1 
70 27 
la 
91 43 
77 
35 5 
2 
70 31 
2a 
91 44 
77 
37 L 
77 37 
3 
70 30 
3 a 
91 39 
77 
39 J 
1 
70 14 
la 
91 10 
77 
42 5 
2 
70 24 
2 a 
91 15 
77 
46 l 
77 44 
3 
70 17 
3 a 
91 7 
77 
45 J 
1 
70 2 
la 
90 37 
77 
49 5 
2 
70 8 
2a 
90 42 
77 
50 l 
77 50 
3 
70 5 
3a 
90 36 
77 
52 J 
1 
69 53 
la 
90 21 
77 
505 
2 
69 59 
2a 
90 26 
77 
52 l 
77 52 
3 
69 55 
9 
3 a 
90 18 
77 
54 J 
1 
69 48 
la 
90 13 
77 
52 5 
2 
69 51 
2a 
90 12 
77 
54 l 
77 54 
3 
69 45 
3 a 
90 0 
77 
56 J 
The values obtained by Tops0e and Christiansen were for 2E 142° 50' and for 
2V a 76° 48'. 
Dispersion of the Median Lines .—This is very small, and was determined with 
section-plate No. 3 immersed in monochlorbenzene, the refractive index of which 
3 L 
VOL. OCXVIII. A. 
