214 
DR. A. E. H. TUTTON ON THE 
Optics. 
Nature and Orientation of the Optical Ellipsoid. —The plane of the optic axes is 
the symmetry plane h {010}, as usual for this series of double salts. But the sign of 
the double refraction is by exception negative, as was also found to be the case with 
caesium nickel sulphate. Extinction determinations made with two section-plates 
ground parallel to the symmetry plane afforded the following values :— 
Extinction Direction in the Symmetry Plane. 
Plate 1 .... 0° 55', Plate 2 .... 0° 24', 
Mean . . 0° 40' in front of normal to c {001}. 
This direction, nearly normal to the basal plane, is in this salt by exception the 
first median line, instead of the second median line as is usual in the series ; it 
corresponds, however, as usual, to the refractive index a, while the other extinction at 
right angles to it in the sj^mmetry plane corresponds to y and is the second median 
line. This will be clear from fig. 6. 
As the morphological axial angle ac is 73° 49', and the angle between the normal 
to c {001} and the vertical axis c is consequently 16° 11', the first median line lies 
16° 51' in front of the vertical axis c in the obtuse axial angle ac. The second 
median line lies 0° 40' below the inclined axis a in the acute morphological axial 
anode ac. 
Optic Axial Angle.-— Three section-plates, 1, 2, and 3, were ground perpendicular 
to the first median line, and three others, la, 2a, and 3a, perpendicular to the second 
median line. The optic axial angle is so large that 2E, the apparent angle in air, is 
