408 
DR. A. E. H. TUTTON ON THE 
73° 28'. The angle in cedar oil was thus 1° 24' greater than that in mono¬ 
chlorbenzene, and we can consider this result as also applicable to potassium ferrous 
selenate, the conditions being so nearly alike. The difference between the indices of 
the two liquids is 0'0146, and between those of cedar oil and the crystals of KFe 
selenate 0'0105. Hence, the correction, to be subtracted from the angle 2V 0 as 
observed in cedar oil in order to give the true angle 2V 0 within the crystal, is 
84' x 0'0105 
-= 50 . 
0-0146 
The values in the last column of the table have, therefore, been obtained by adding 
exactly one degree to those for 2V a in the third column, which corresponds to the 
subtraction of a degree from the observed values of 2Y 0 in column 2, as 2V a is the 
supplement of 2V 0 . This final corrected value for 2V a is certainly very close to the 
truth, quite as close indeed as if the ordinary procedure of observing 2H a and 2H 0 in 
bromonaphthalene, with section-plates perpendicular to the first and second median 
lines respectively, had been employed. 
For the true optic axial angles in different crystals of the same substance frequently 
differ more than the few minutes of any possible error in these determinations. 
In confirmation, however, the true angle 2V a may also be calculated, now that the 
exact refractive index n of the immersion liquid is known, with the aid of the inter¬ 
mediate refractive index /3 of the crystals, from the observed value of the obtuse angle 
2V 0 , considered as 2H 0 , by means of the well-known formula :— 
cos V a = sin H 0 . 
Inserting the values for sodium light and making the calculation we have :— 
from which 
cos V a — 1 gin 58° 21/, 
a 1-5182 
V„ = 32° 8' and 2V„ = 64° 16'. 
This result differs by only 2' from the corrected value given in the last column of the 
table, thus affording a most satisfactory confirmation of the final column of values 
in the table. 
Refractive Indices .—A series of measurements of refractive index with a natural 
prism affording directly the index /3 had been carried out previous to the work 
just described with the plates, in order to obtain the necessary exact knowledge 
of the refractive power of the crystals to enable a proper choice to be made of an 
immersion liquid of like refractive index, for use with those plates. The index (3 is 
but slightly higher than the mean index of the crystals, and so enabled the 
3 
