1216 
then fic. 14 must be symmetrical with respect to both perpendicular 
planes; and if the c-axis is not of that kind, fig. 14 should neces- 
sarily possess the same symmetry. 
In every case therefore, one plane of symmetry must have dis- 
appeared in fig. 14; here also no other supposition is possible than 
that there must be some reason why the expected spots in directions 
parallel to the intersection (OOOL) : (1010) are completely or partially 
suppressed. The real symmetry of the pseudotrigonal complex of 
lamellae can thus be regarded after this as a matter of secondary 
importance; for it is very well possible, that in tg. 12 also a 
second symmetry-plane, parallel to {OOL{ has disappeared, and in 
that case the resulting combination of symmetry-properties would 
be geometrically impossible too, just as in all preceding cases. 
§ 8. We here thus meet the extremely remarkable phenomenon, that 
in biavial crystals, in striking contradiction to the experience hitherto 
gathered from optical isotropous or uniaaial crystals if studied 
perpendicular to their optical axis, certain symmetry-elements of the 
RÖNrGENograms which were to be expected according to the Liaur-BraGg- 
theory absolutely vanish. Thereby a complex of symmetry-properties 
is revealed in the complete set of RoOnrexNpatterns of the same 
erystal, which is geometrically impossible, and which theretore cannot 
be a representation of the special symmetry of the crystal itself. 
As far as experience now goes, and provided that the more com- 
plicated case of the mimetic benitoite is for the present left out of 
consideration, the suppression of the spots occurred in two of the 
cases studied, in those images which are obtained by the trans- 
mission of the RÖNTGeN-rays parallel to the optical normal; te. the 
spots disappeared there in the plane in which the differences of the 
optical elasticity. of the erystal are as great as possible. In the case 
of the sodiumammoniumtartrate the suppression occurred for crystal- 
plates either parallel to the optical axial plane, or perpendicular to 
the second bisectrix; i.e. in the directions of the greatest and smallest 
elasticity, not however in the direction of the optical normal. 
One would be inclined to explain these phenomena, — just because 
they are observable exclusively in those crystals whose optical 
anisotropy is manifested in all directions, — by supposing some 
condition of polarisation of the generated secondary waves. which 
polarisation would finally find its expression, — somewhat in the 
same way as in the case of ordinary light-waves, — in an unequi- 
valence of perpendicular directions. Or one again would be inclined 
to suppose an anisotropy in the mode of motion of the particles 
