220 
DR. A. E. H. TUTTON ON THE 
the obtuse axial angle ac , 14° 17' above the inclined axis a, and 2° 0 r below the normal 
to a {100}. These facts will be rendered clear by fig. 8. 
Tops0e and Christiansen give the position of the first median line as 16° 54' from 
the axis a and 89° 25 ; from the axis c. 
ot 
Optic Axial Angle.— Three section-plates perpendicular to the first median line, 
numbered 1, 2, 3, and three others labelled la, 2a, and 3a, perpendicular to the 
second median line, were ground. The apparent optic axial angle in air, 2E, is too 
large for measurement, the brushes being not quite capable of being brought to the 
cross-wires; but excellent measurements were obtained for 2H a and 2H 0 , the 
apparent acute and obtuse angles in bromonaphthalene. The results, together with 
the values of the true optic axial angle 2V a calculated therefrom, are given in the 
accompanying table. 
Ammonium Nickel Selenate. 
Determination of True Optic Axial Angle in Bromonaphthalene. 
Light. 
No. of plate 
perp. 1 M.L. 
Observed 
2H 
No. of plate 
perp. 2 M.L. 
Observed 
2H 0 . 
Calculated 
*2V„. 
Mean 
2V a . 
f 
1 
o / 
78 43 
la 
o l 
85 8 
o / 
86 18'] 
o / 
Li ... .1 
2 
78 40 
2 a 
85 2 
86 20 
86 19 
l 
3 
78 45 
3a 
85 6 
86 20 J 
r 
1 
78 41 
la 
85 5 
86 18 'j 
c ... A 
2 
78 38 
2 a 
85 0 
86 20 
86 19 
l 
3 
78 43 
3 a 
85 3 
86 20 J 
