MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
215 
not measurable. The apparent acute and obtuse angles, 2H a and 2H 0 , are readily 
measurable in bromonaphthalene, and the results of the measurements, together with 
the calculated values of the true optic axial angle 2V a derived therefrom, are given 
in the accompanying table. 
Caesium Nickel Selenate. 
Determination of True Optic Axial Angle in Bromonaphthalene. 
Light. 
Li 
C . . 
Na. 
T1 . . 
Cd . . 
No. of plate 
perp. 1 M.L. 
Observed 
2H a . 
No. of plate 
perp. 2 M.L. 
Observed 
2H 0 . 
Calcula 
2 V a . 
1 
o / 
76 15 
la 
o / 
89 14 
o / 
82 38 
2 
75 50 
2a 
89 2 
82 28 
3 
76 15 
3a 
88 35 
82 58 
1 
76 13 
la 
89 8 
82 40 
2 
75 49 
2 a 
88 57 
82 30 
3 
76 14 
3a 
88 30 
83 0 
i 
76 7 
la 
88 4 
83 8 
2 
75 45 
2a 
88 1 
82 56 
3 
76 9 
3a 
87 45 
83 20 
1 
76 1 
la 
87 12 
83 32 
2 
75 40 
2a 
87 4 
83 22 
3 
76 3 
3a 
87 17 
83 30 
i 
75 57 
la 
86 55 
83 38 
2 
75 35 
2 a 
86 49 
83 27 
3 
75 59 
3 a 
86 42 
83 46 
1 
75 53 
la 
86 42 
83 42 
2 
75 29 
2 a 
86 28 
83 34 
3 
75 49 
3a 
86 20 
83 52 
Mean 
2V a . 
82 41 
82 43 
83 8 
83 28 
83 37 
83 43 
Dispersion of the Median Lines. —The dispersion of the two median lines in the 
symmetry plane is such that the first median line lies about 10' nearer to the vertical 
axis c for greenish blue F light than for red C light. The determinations were made 
with sections 1 and 3 immersed in oil of anis, the refractive index of which liquid 
(l'5540) is not far removed from that of the crystal ; the actual amounts observed 
with the two sections were respectively 12' and 8b 
Refractive Indices. —Six 60 -prisms were ground, each with its bisecting plane 
parallel to a principal plane of the optical ellipsoid and its refracting edge parallel to 
one of the principal axes of the ellipsoid, so as to yield two refractive indices directly; 
the directions were so chosen that each of the three indices a, (3 and y, was afforded 
by four different prisms. The results were very concordant, and the final mean values 
are recorded in the next table. 
2 H 
VOL. CCXVII.-A. 
