434 
DR. A. E. H. TUTTON ON THE 
have in addition shown cleavage parallel to the symmetry plane, b {010}, but this has 
not been observed in the case of ammonium ferrous selenate. 
Orientation of the Optical Ellipsoid. —-The optical ellipsoid (either the indicatrix 
or the optical velocity ellipsoid) rotates on the symmetry axis b of the crystal—which 
is identical with the intermediate axis of the ellipsoid (/3 of the indicatrix or b of the 
velocity ellipsoid) and is the only one of the three axes which is fixed, in accordance with 
monoclinic symmetry—when one of the alkali bases is replaced by another. The best 
mode of comparing its position is to state the inclination of the a axis of the indicatrix 
or a axis of the velocity ellipsoid to the vertical crystal-axis c, as is done in the next 
table. 
Inclinations of a-Extinctions of the Fe Double Selenates in front of Axis c. 
(2M.L. in all four salts.) 
AmFe selenate 5° 24' Pv,bFe selenate 13° 37', 
KFe „ 10° 27' CsFe „ 21° 4'. 
The table is also expressed graphically in fig. 11, which further shows the positions 
of the other perpendicular axis y also lying in the symmetry plane, which is that of 
cc 
the paper. The a axis is the second median line (obtuse bisectrix of optic axial 
angle) in all four salts, the y axis being the direction of the first median line 
(acute bisectrix of optic axial angle). 
The position of the ellipsoid is seen to be such that its axis lies close in front of 
(only 5° 24' from) the vertical axis c. in the case of the ammonium salt; it rotates 
so that its a axis is 10° 27' from axis c for the potassium salt; further, till the axis 
is 13° 37' inclined for the rubidium salt, and still further to 21° 4' for the caesium salt. 
