824 
That it must be special properties of the preparations, that are 
the cause of such anomalies, becomes also evident from the fact, that 
the pseudo-symmetrical substances showing them, under favourable 
circumstances may occur in such well developed individuals, as to 
give perfectly wndisturbed RÖNrerN-patterns: thus e.g. with potassiwm- 
ferroeyanide in most cases certainly only bilaterally symmetrical 
images were obtained’), but occasionally there were found also perfect 
tetragonal patterns. And while we obtained an only bilaterally 
symmetrical ROnTGEN-pattern with an apparently irreproachable 
individual of benitoite *) cut perpendicular to its optical axis, RiNNw *) 
afterwards was able to obtain a quite normal trigonal diffraction- 
image of the same mineral. 
Moreover, it was found that the direction of the single plane of 
symmetry in the RöNrGeNpattern was completely analogous in the two 
cases studied in the above: its situation being in that of the hevagonal 
complex, as well as in that of the octogonal one, coinciding with the 
bisector of one of the angles of two’ subsequent lamellae of the 
mica-piles. As the optical and microscopical investigation of the 
preparations did not reveal any abnormality in these directions, the 
only possible conclusion was, that the cause of this phenomenon 
must be attributed to some peculiarity in the lamellar arrangement. 
In the case of the preparations with lamellae crossing at 60°, 
the explanation of the phenomenon may be given in the simplest 
way as follows. 
The preparation of such mica-piles was hitherto executed only 
with the purpose of demonstrating the optical effects of such com- 
plexes: the apparent uniaxiality and the rotation of the plane of 
polarisation of the incident rays. Because of the fact that the optical 
orientation of each lamella does not differ appreciably from that of 
a rhombic crystal cut perpendicularly to its first bisector, it could 
be considered hitherto of no interest to the prepairer of such mica- 
piles, whether he piled up these lamellae in the same position as 
they were cut from the original crystal, or whether he turned 
them accidentally through 180° about an axis perpendicular to the 
plane of the lamella. For the final optical effect of the preparation 
will not be affected in the slightest degree by this turning. However, 
such a change of right and left, of the anterior or posterior part 
1) The plane of symmetry being in these and other cases often parallel to the 
direction of the composing lamellae, apntrary to what was observed here. 
2) F. M. JAEGER and H. Haga, Proceed. Acad. of Sciences Amsterdam. 17, 
1204 (1915), 
3) F. Rinne, Miner. Centralblatt, (1919), p. 193. 
