OF THE DOUBLE SELENATES OF THE SERTER RAi(SeO,)o,r,H,0. 
28:5 
of the field of the polariscope, to produce the uniaxial figure consisting of rectangular 
diagonal cross and circular rings. For still shorter wave-lengths the axial brushes 
separate in the vertical field of the instrument at right angles to the symmetry 
plane, and for G-light the separations are respectively 7° 0' and 12° 49'. The t(fial 
dispersion between Li and G amounts to 25° 10' and 3:3° 38' respectively, the selenate 
thus l)eing distinguished by considerably greater dispersion. All tlxese figures refer 
to the true angles within the crystals. 
The Effect of Rise of Temperature on the Optic Axial Angle in the case of ccesium 
magnesium selenate is likewise remarkably similar to that on the corresponding 
sulphate. In both cases the optic axial angle for wave-lengths on the red side 
of the crossing wave-length rapidly contracts as the temperature is raised, until tlie 
uniaxial figure is produced for each wave-length towards the red in turn. Tlie 
temperatures at wliich the uniaxial figure is produced for the different wave-lengths 
are slightly lower in the case of cresium magnesium selenate than in that of the 
sulphate. The temperatures for the latter salt given ixi the memoir concerning the 
double sulphates {loc. cit., p. 371) were the actual temperatures read off on the 
thermometers of the heating apparatus of the larger Fuess polaiiscopical goniometer. 
In order to render them strictly comparable with those given in this memoir for 
caesium magnesium selenate, they require to be corrected for the slight conduction 
of the crystal liolder, as has been done in the case of the latter salt. I'hese corrected 
temperatures are set forth Ijelow, and alongside them are given for comparison tlnxse 
of the selenate :— 
Temperatures at which the Uniaxial Figure is produced. 
Light. CsMg selenate. CsHg sulpliatc. 
F 
34' 
T1 
60 
Na 
78 
C 
91 
Li 
94 
47° 
G6 
80 
93 
90 
It is thus a fact that within the life-range of temperature of these dissociable 
water-containing salts the uniaxial figure is produced for every wave-length of the 
visible spectrum, and for each wave-length there is a definite temjxerature at wliicli 
these interesting monoclinic crystals simulate uniaxial symmetry as regards their 
optical properties while retaining morphologically their exterior monoclinic S 3 un- 
metry. 
The interference figures of csesium magnesium selenate given in the Plate op})osite 
p. 272 are almost ecpially applicable to caesium magnesium sulphate. 
With regard to the effect of rise of temperature on potassium magnesium and 
rubidium magnesium selenates, it was observed that the optic axial angle of the 
2 o 2 
