817 
cular to the erystallographical plane of symmetry (010); moreover, 
the first bisector is almost perpendicular to the plane of cleavage, 
while the dispersion (@ > v) also, differs only unappreciably from 
that of a rhombic crystal. The mineral is strongly birefringent 
(about: 0,038), with negative character. 
In first instance now, the Rönreenpattern of a single lamella 
(d=0.32 mm.) was obtained, as reproduced in fig. 1 of Plate I; 
a stereographical projection of this beautiful and rich diffraction- 
image is given, moreover, in the diagram 1 of the text. It manifests 
the ordinary bilateral symmetry of the monoclinic crystals parallel 
to {001} or {100}; on more detailed examination, three directions 
may, moreover, be clearly discerned in it, which include angles 
of almost 60° with each other, and which are closely related to the 
hexagonal “radiation-figure’ of this mzca-species. obtained by pres- 
sure with a sharp object. The direction of the optical axial plane 
may also be discerned in it without much difficulty ; it is indicated 
by a row of numerous smaller spots, situated perpendicularly to 
the plane of symmetry of the diffraction-pattern. 
Fig. 1. Stereographical projection of the Rönraenpaltern of a 
single Muscovite-lamella, parallel to {O01}. 
54* 
