416 
DR. A. E. H. TUTTON ON THE 
Determination of True Optic Axial Angle in Bromonaphthalene of RbFe Selenate. 
Light. 
Li 
Na. . 
T1 . 
Cd 
F . . 
No. of plate 
perp. 1 M.L. 
1 
2 
3 
1 
2 
3 
1 
2 
3 
Observed 
2H 
66 33 
66 38 
66 38 
66 31 
66 34 
66 36 
66 15 
66 17 
66 20 
65 58 
65 57 
66 2 
65 45 
65 47 
65 49 
65 30 
65 35 
65 36 
No. of plate 
perp. 2 M.L. 
4- 
1 a 
2 a 
3a 
la 
2 a 
3 a 
la 
2a 
3a 
la 
2a 
3a 
la 
2a 
3a 
la 
2 a 
3a 
Observed 
2IL. 
94 31 
94 30 
94 29 
94 29 
94 25 
94 25 
94 9 
94 1 
94 5 
93 47 
93 32 
93 38 
93 30 
93 16 
93 21 
93 15 
92 58 
93 5 
Calculated 
2V a . 
73 32' 
73 36 
73 36 
73 28' 
73 33 
73 34 
73 24' 
73 31 
73 30 
Mean 
2V 0 . 
73 35 
73 34 
73 32 
73 30 
73 28 
73 26 
Dispersion of the Median Lines .—This was determined by observations of the 
approximate true optic axial angle in cedar oil (/ul = 1‘5200) and monochlorbenzene 
(n — 1'5248), the refractive indices of which liquids are respectively just lower and 
just higher than the mean index of the crystals. The dispersion proved to be 
very small, only 10' between red C-light and green Tl-light, and is such that 
the first median line is nearer to the axis a by 10 r for red C-light than for green 
Tl-light. 
Effect of Temperature on the Optical Axial Angle .—Measurements of the apparent 
optic axial angle in air 2E at 85° C. showed that this angle increases almost exactly 
two degrees for 70° C. rise of temperature (from 15° to 85°). 
Refractive Indices .—Six excellent 6(T-prisms were ground, each symmetrical to a 
principal plane of the optical ellipsoid and with the refracting edge parallel to one of 
the axes, so as to afford directly two of the three indices a, /3, y. The separate results 
for each index were satisfactorily concordant, and the mean values are given in the 
next table. 
