204 
DR. A. E. H. TUTTON ON THE 
Determination of True Optic Axial Angle in Bromonaphthalene. 
No. of plate 
perp. 1 M.L. 
Observed 
2H 
No. of plate 
perp. 2 M.L. 
Observed 
2H 0 . 
Calculated 
2 V«. 
Mean 
2V a . 
1 
o /. 
66 7 
la 
o / 
95 46 
G / 
72 40~] 
o / 
2 
66 13 
2a 
95 37 
72 48 
> 
72 45 
3 
66 17 
3 a 
95 45 
72 48 J 
1 
66 5 
la 
95 42 
72 401 
2 
66 11 
2 a 
95 34 
72 48 
72 45 
3 
66 15 
3 a 
95 40 
72 48 J 
1 
65 53 
la 
95 18 
72 421 
2 
66 2 
2a 
95 9 
72 52 
72 48 
3 
66 0 
3a 
95. 10 
72 50. 
I 
1 
65 41 
la 
94 54 
72 431 
2 
65 46 
2a 
94 38 
72 54 
72 51 
3 
65 48 
3a 
94 34 
72 57 j 
1 
1 
65 31 
la 
94 35 
72 441 
2 
65 38 
2 a 
94 20 
72 56 
^ 72 53 
3 
65 39 
3 a 
94 14 
73 0^ 
1 
65 20 
la 
94 15 
72 45^ 
0 
65 31 
2a 
93 56 
73 1 
72 56 
3 
65 29 
3 a 
93 51 
73 2 
I 
Light. 
Li . 
C . 
Na. 
T1 
Cd . 
The values obtained by Topsoe and Christiansen* were, for 2E 129 56' and for 
2V„ 72° 56'. 
Dispersion of the Median Lines. —This is exceedingly small, namely, about 7' only. 
The direction is such that the first median line is nearer to the axis a by 7' for 
red C hydrogen light than for greenish blue F hydrogen light. It was determined 
by observations with the section-plates 2 and 3 immersed in monochlorbenzene, the 
refractive index of which, 1'5248 for D-light, is nearly identical with the mean index 
of the crystals. 
Effect of Temperature on Optic Axial Angle.— The apparent optic axial angle in 
air 2E increases with rise of temperature. The amount of the increase was deter¬ 
mined with section 3, and proved to be 3 16' for a rise of 50° (from 18° to 68°) of 
temperature. 
Ref ractive Indices. —Six 60 prisms were prepared with the aid of the cutting and 
grinding goniometer, each to yield two of the three indices directly, the bisecting 
plane and refractive edge being parallel respectively to a principal plane and principal 
axis of the optical ellipsoid. Four different values for each index were thus obtained. 
The mean results are given in the accompanying table. 
* ‘Ann. Chim. Phys.,’ series 5, vol. 1, p. 83 (1874). 
