OF THE DOUBLE SELENATES OF THE SEIHES E,M(Se04)o,6H.20. 259 
The values obtained by Topsoe and Cpiristiansen, which are given in the last 
column, and of which only the /3 values were obtained directly, do not show such 
close agreement with the author’s values as was olDserved in the case of potassium 
zinc selenate. 
The following general formula represents the intermediate refractive index of 
potassium magnesium selenate, with great accuracy, throughout the whole length 
of the spectrum. The index is corrected to a vacuum (by the addition of 0‘0004 to 
the index as given in the table). 
^ . .nro . 490 058 , 222 100 000 000 
^ = 1-4852 + —— + 
A3 
A* 
The a indices are closely reproduced by diminishing the constant 1-4852 by 0 - 0022 , 
and the y indices by increasing the constant by 0-0148. 
Alteration of Refraction hy Rise of Temperature .—A series of determinations 
carried out at 75° indicated that the indices diminish by about 0-0020 for 60° rise of 
temperature. 
Axes of the Optical Ellipsoids. —Following are the calculated values of the axial 
ratios of the two optical ellipsoids :— 
Axes of optical indicatrix ; 
a : 13 :y = 0-9985 : 1 : 1-0099. 
Axes of optical velocity ellipsoid : 
a : h : f = 1 0015 : 1 ; 0-9902. 
Molecidar Opticcd Constants. —The values of these constants, calculated from the 
refractive indices with the aid of the density given on a preceding page, are as 
follows :— 
Axis of optical indicatrix. 
Specific refraction, ^= n . 
i ’bd + 2)d Ig 
Molecular refraction, -- 7 • ^ . I ^ 
+ 2 cl 1 G 
Specific dispersion, IG — Be • • 
Molecular dispei’sion, ug — lUc .... 
^ 
Molecular refraction, -— M . . . C 
0-1232 
0-1264 
61- 07 
62- 64 
0-0032 
1-57 • 
103-63 
0-1237 
0-1269 
61- 30 
62- 87 
0-0032 
1-57 
104-09 
7 - 
0 1267 
0-1300 
62-81 
64-44 
0-0033 
1-63 
10713 
Optic Axial Angle. —Following are the results of the measurements made with 
three excellent pairs of section plates, ground perpendicular to the first and second 
median lines. 
2 L 2 
