1212 ° 
plane of symmetry i.e. is parallel to ,OOL. The plane of the optical 
axes of the crystal being parallel to {100}, while the c-axis coincides 
with the first biseetrix of negative character, it is evident, that the 
homologous spots are missing in the photograph which lie in the 
directions parallel to the a-axis, i.e. parallel to the direction of the 
smaller ontieal elasticity of the crystal plate. 
SiS ri a os OEE Oe are ae ee 
a:b:c=0.7988:1:0.7267. The crystals were glassy and splendidly 
homogeneous; they showed the forms {110}, {100}, {O10} and {011}, 
and had a prismatic aspect. A perfect cleavage is present parallel 
to $010%, a good one parallel to {100}. The crystals are very strongly 
birefringent; the birefringency is about: 0.072. The plane of the 
optical axes is parallel to {O10}; the first bisectrix, which has 
positive character, coincides with the vertical axis. The dispersion 
round the first bisectrix is only feeble, with 9 < wv. 
In fig. 9, LOand 11 the stereographical projections of the RÖNTGEN- 
