Chemistry. — “On the Symmetry of the RonteENpatterns Obtained by 
means of Systems Composed of Crystalline Lamellae, and on the 
Structure of Pseudo-Symmetrical Crystals’. By Prof. F. M. JAEGER. 
(Communicated at the meeting of April 23, 1920) 
§ 1. It is well known how SOoHNCKE *) and MArrLARD °), as a conse- 
quence of experiments formerly executed by NOrrempere and Von 
Revscn, have first tried to account for the optical properties of 
uniaxial, circularly-polarizing crystals, by the supposition that all 
such erystals are in reality only apparently higher-sy mmetrical inter- 
growths of very numerous, extremely thin, and often submicrosco- 
pical, crystalline lamellae of lower crystallographical symmetry. In 
many cases this supposition has afterwards been confirmed by 
experience; and just in the same way as in the experiment executed 
in 1869 by Von Revuscu, who demonstrated in a more or less 
perfect way the possibility of imitating the behaviour of uniaxial 
erystals endowed with rotatory power in the direction of their 
optical axis, by means of a number of mzca-lamellae, regularly 
piled-up clock-wise or oppositely, while crossing under the same 
fixed angle, — thus the behaviour of such pseudo-symmetrical crystals, 
built up from microscopical lamellae, also appeared to approach the 
more closely to that of true tetragonal, trigonal, and hexagonal 
erystals, as the composing lamellae were thinner and more numerous. 
The complexes thus obtained are either dextro-, or laevo-gyratory, 
be it that the piling-up of the suecessive lamellae has occurred clock- 
wise or in the opposed direction. During the investigations of Haga 
and myself about the specific symmetry of the Rénreenpatterns 
obtained by diffraction of ROnrGenrays in plane parallel plates of 
optically uniaxial crystals’), we had occasion to study also some 
psendo-symmetrical crystals of this kind, which were characterised 
by more or less evident optical anomalies; and, while some of them, 
— eg. the pseudo-tetragonal strychnine-sulphate, — gave ROn?TGEN- 
patterns of so perfect a symmetry, that they could not be distinguished 
from those obtained with real tetragonal crystals, — we also found 
with some other crystals of this kind (racemic triethylenediamine-cobalti- 
hj L. Sonnoxe, Zeits. f. Kryst, u. Miner, 19, 529, (1899). 
2) E. Marrarp, Ann. des Mines, (7), 19, 256 (1881); Traité de Cristallogra- 
phie, TL 262—804, (1881), H. Porncart, Théorie mathém. de la Lumiere, I, 
275. (1892). 
5 Of. ja: H. Haca and F. M. Jararr, Proceed. Royal Acad. of Sciences, 
Amsterdam, 17, 1204, (1915); 18, 558, 1355, (1916). 
ke 
54 
Proceedings Royal Acad. Amsterdam, Vol. XXII. 
