OF THE DOUBLE SELENATES OF THE SERIES R,M(Se04)2,6H20. 
277 
the symmetry axis and tire minimum along the vertical axis. Also the change is 
greater for the replacement of I'ubidimn by csesium tlian for that of potassium by 
rubidium. 
Comparison of Orientations of the Optical Indicatrix. 
Inclination of Axis of Indicatrix to Vertical Axis c. 
For potassium magnesium selenate . . 11° 18' 
,, rubidium ,, ,, . . IG 24 
,, carsium ,, . 36 47 
The optical indicatrix rotates about the symmetry axis when one alkali metal is 
replaced by another, and by an amount which is very much greater when caesiinn 
replaces rubidium than when rubidium replaces potassium. 
This rotation of the optical ellipsoid is graphically represented in fig. 4, the dark 
lines representing the axes of the ellipsoid lying in the symmetry plane, namely, the 
first and second median lines. 
I Dilf. 5° 6' 
I I)ilf. 20 23 
Fig. 4. 
The Refractive Indices are com})ared in tlie accompanying table. 
It will be observed that the a and yS indices of rubidium magnesium selenate lie 
between those of the other two salts. The y indices of the potassium and rubidium 
salts are almost identical, and, indeed, those of the rulfidium salt are slightly the 
lower. The reason for this apparent anomaly in the case of tlie y indices is 
connected with a progressive change of dispersion, as will be shown under the next 
heading. That there is, however, a true progression of refraction is best exhibited by 
comparison of the values of the mean refractive index of each of the three salts, that 
is, the mean of the three indices of each salt for any one and the same wave-length 
of light. 
Such mean indices for Na-light are compared in the table following tliat of the 
actual indices. 
