MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
407 
The mean result for these two plates, the adjustments for which were not quite so 
perfect as for Plate 1, on account of less true-planeness of the c-faces, is so close to that 
for Plate 1 that the mean of the two results, given in the next table, may confidently 
be accepted as very near the truth. 
Optic Axial Angle of KFe Selenate. 
Light. 
2V 0 . 
2V a 
in cedar oil. 
True 2V a 
(corrected for difference 
of refraction). 
Li. 
O f 
116 49 
63 11 
o / 
64 11 
C. 
116 48 
63 12 
64 12 
Na. 
116 42 
63 18 
64 18 
T1. 
116 35 
63 25 
64 25 
Cd . . . ‘. 
116 30 
63 30 
64 30 
F. 
116 24 
63 36 
64 • 36 
If the refractive index of the immersion liquid, cedar oil, were absolutely identical 
with the mean index of the crystals (l/3 (a+/3 + y) = 1‘5207 for sodium light), and 
the two dispersions were also identical, we could be content with the final value of 2V a 
given in the third column. After the completion of the work on these unstable 
crystals, however, when time was no longer so important, the refractive index of the 
cedar oil employed was carefully determined, with the following result:— 
Refractive Indices of Cedar Oil. 
Li . . . P5064 Na . . . P5102 F . . . P5175 
C . . . P5068 T1 . . . P5135 G . . . P5238 
Cd . . . P5153 
The index for Na-light is thus 0'0105 lower than the mean crystal index, but the 
dispersion between Li and G-light is practically identical (0'0174) with that of 
the crystals 0'0171. Another liquid, monochlorbenzene, has the index 1'5248 for 
Na-light, 0’0041 higher than the mean crystal index. But it has the disadvantage of 
dissolving the cementing balsam, which cedar oil does not. The angle 2Y 0 as observed 
in cedar oil would thus be slightly too large, while if observed in monochlorbenzene 
it would appear still more slightly too small. It has been possible to calculate the 
exact amount of the desirable correction, from observations made in both liquids at 
leisure, with the permanent crystals of rubidium ferrous selenate, the mean refractive 
index of which is almost identical (l‘5220) with that of potassium ferrous selenate. 
The measurement of 2V a for Na-light was 74° 28' in cedar oil and 73° 4' in monochlor¬ 
benzene ; the true value, as calculated from 2H a and 2H 0 in bromonaphthalene, 
was 
