1204 
Physics. — “On a New Phenomenon Accompanying the Diffraction 
of Röntgenrays in Birefringent Crystals.’ By Prof. Dr. F. M. 
JAEGER. (Communicated by Prof. H. Haca.) 
(Communicated in the meeting of March 27, 1915). 
§ 1. A short time ago Haca and JARGER') made some observa- 
tions on the diffraction of RÖNTGEN-rays in crystals of cordierite, 
from very beautiful, perfectly transparent and homogeneous examples 
of which suitable plates were cut parallel to the three pinacoidal 
faces 100}, SOLO} and SOOÏf. On this occasion the RÖNTGENOgram 
of the plate parallel to {0014 of this mineral hitherto considered 
rhombie-bipyramidal, appeared in fact to possess two symmetry- 
planes perpendicular to each other, as well as a binary axis; the 
patterns however, obtained by the transmission of RÖNTGEN-rays 
through the plates parallel to {100} and {010}, appeared to possess 
only one single symmetry-plane. This combination of symmetry- 
elements is just the essential of rhombic-hemimorphic crystals. 
It must be remarked however, that this fact is contrary to the 
consequences which follow from the theory of these phenomena, as 
far as it regards the expected symmetry of the ROnTGEN-patterns. 
The question, what will eventually be the symmetry of the 
RONTGENOgrams of crystals of a certain symmetry-class, can be 
answered comparatively easily. Deductions of this kind were made 
for the first time in 1913 by G. Frieper®), who concladed, that 
under no circumstances such symmetry of crystals, as were character- 
ized by the absence of a centre of symmetry, could be revealed in 
their RÖNTGEN-patterns. 
The reasoning of FrirpEL is principally as follows. He deduces 
the complex of symmetry-properties which is characteristic of hemi- 
hedrical and tetartohedrical crystals, from those belonging to the 
holohedrical forms, by the suppression of certain symmetry-elements 
in the latter groups, thereby making use of the wellknown fact, 
that in the holohedrical crystals every plane of symmetry corresponds 
to a binary axis perpendicular to it. This results from the fact, that 
all holohedrical crystals possess a centre of symmetry, and that such 
a centre, if combined with either a plane of symmetry or with an 
axis of pair period, necessarily wiil cause the presence of the other 
lb) H. Haca and F.M. Jaeger, Proc. of the R. Acad. Amsterdam, 17. 430. (1914). 
*) G. Frreper, Compt, rend. de I’\cad. des Sciences, Paris 157, 1533, (1913). 
