816 
bromide (+ 3 H,O), benzile, apophyllite, etc), patterns, showing only a 
single plane of symmetry, and having, therefore, really the aspect of the 
patterns commonly obtained with monoclinic crystals, if cut parallel 
to a plane of their orthodiagonal-zône. On that occasion we also 
emphasized, that the cause of this abnormal behaviour might probably 
be ascribed to an imperfect orientation of the lamellae in one of 
the directions of intergrowths, these for the rest being equivalent. 
This imperfect orientation might then consist of either a slight 
rotation of the lamellae about one of their axes in the special 
direction mentioned above, or of a twinning of some of these 
lamellae. In all cases, however, it appeared necessary to make the 
supposition, that those particulars should have occurred more frequently 
in one of the direations of intergrowth, than in each of the others. 
As it was our purpose to obtain a more exact insight into the 
real behaviour of such pseudo-symmetrical crystals composed of 
intergrown lamellae, with respect to the phenomenon of. the diffrac- 
tion of transmitted RöÖNreeNrays, we have undertaken the study of the 
specific symmetry of the R6nrGENpatterns, which could be obtained 
by means of systems of regularly piled-up mica-lamellae, in its 
dependency on the special structure of the used mica-complexes. 
The results obtained, which are reviewed in the following pages, 
have in the first place shown some pecularities, pointing to a close 
analogy with the anomalies formerly found by us in the case of real 
pseudo-symmetrical crystals; on the other hand, however, the expe- 
rience gained must necessarily lead to the conclusion that the views 
of SoHNCKE and Marrarb, — at least in so far, as tetragonal crystals 
endowed with circular polarisation be considered — cannot yet be 
considered to give a final explanation of the phenomena observed. in 
these cases. 
The Rönrerenpatterns used here, have all been obtained in the 
Physical Laboratory of the University of Groningen by my colleague 
Haca, to whom I wish once more here to offer my sincere thanks 
for his kind and expert help during this investigation. 
§ 2. In these experiments, thin cleavage-lamellae of mzscovite: 
KH,Al(SiO,), were continually made use of. As is wellknown, 
this mineral has monoclinic-prismatic symmetry, with the parameters : 
a:b:¢e: = 0.577: 1: 2,217. and = 84°55’. This symmetry, however, 
very closely approaches a hexagonal one, the prism-angle of ms- 
covite being 120°11’. A perfect cleavage occurs parallel to {001}, 
the preparation of very thin lamellae being thus extraordinarily 
facilitated. In this m2ca-species the optical axial plane is perpendi- 
