825 
of the lamellae, etc., is by no means indifferent any longer, if the 
distribution of the intensities of the spots in the diffraction-image 
by RÖNTGEN-rays be considered. For the »wscovite-crystal has under all 
circumstances a true “monoclinic” molecular structure; the inten- 
sities of the diffraction-spots of a lamella parallel to {001} e.g., will, 
therefore, always be different to the left or to the right of the optical 
axial plane, while they will appear the same to the right or to the 
left of the plane of crystallographical symmetry. It must thus be 
evident, that the interchange of the two sides of a lamella in the way 
mentioned above, must really be of influence with respect to the 
special symmetry, which will be manifested in the distribution of 
the intensities of the diffraction-spots, as they will appear in the 
photographical image of the lamellar complex as a whole. 
If the subsequent lamellae of a hexade are numbered 1 to 6, 
while the longer side of the lamellae, — as was really the case 
with our preparations, — is parallel to the direction of the optical 
axial plane in each lamella, then, if in the piling-up of the lamellae 
at angles of 60° first the lamellae 1 to 5 be taken in their 
right position, but N°. 6 be turned now through 180° in its own 
plane, the thus obtained hexade will give a diffraction-image, in 
which the intensities of the spots will be no longer distributed 
symmetrically with respect to six planes of symmetry, but in which 
there is only a single plane of this kind, exactly bisecting the angle 
between the superimposed pairs of lamellae: (1—4+4) and (2—5), and, 
therefore, being perpendicular to the pair: (3—6). By means of 
schematic figures, in which the distribution of the intensities of the 
spots, as effected by a single lamella is taken into account, it is 
possible to deduce systematically the general symmetry-character 
of the final distribution of intensities in the diffraction-image resulting 
from the complete hexade. 
Undoubtedly such a reversion of-a lamella will have accidentally 
occurred during the preparation of the mica-piles considered, just 
because there was no need for the prepairer to draw special atten- 
tion to avoid such a reversion, and because with regard to his aim he 
was quite free to fix the subsequent lamellae in those positions, in 
which they accidentally were presented to him. Of course, there is 
a fair chance also, that during his work, he turned two or three 
lamellae in the way described; and it is necessary, therefore, also 
to consider the consequences of this for the final character of the 
diffraction-image, if all possible combinations of lamellae be in this 
way taken into account. In the case of such hexades, it appears 
unnecessary, however, to consider any combinations with a number 
