553 



tion against the supposition, that the particular fact of the dis- 

 appearance of certain symmetry-planes in the RöNTGEN-patterns of 

 birefringent crystals would have anything to do with their optical 

 anisotropy. For if this were true, it would be hardly possible to 

 understand, why not one of the numerous patterns of uniaxial crystals, 

 which were radiated througli in the direction of their optical axis, 

 and thus likewise are birefringent plates, exhibited the formerly 

 described phenomenon. On the other hand the case of the quartz- 

 images makes prudence necessary: for evidently the symmetry of 

 the patterns can by yet partially imknown secondary causes, a,ppear 

 othenvise than may be expected from the theorj' of the diffraction 

 phenomena, — as well of higher symmetry fquartz) as of lomer 

 symmetry (rhombic crystals). 



^ 5. In connection with these considerations, we have recom- 

 menced onr studies with some optically biaxial (rhombic) crystals, 

 and have begun with a renewed investigation of /^//t^ Avr/Ht^, translucid 

 and very beautiful plate of hnmbergite parallel to {010}, which for- 

 merly') had given a so strongly abnormal image. After having radiated 

 through in another place, we now repeatedly obtained a perfectly 

 normal diffraction-image, quite symmetrical after two perpendicular 

 planes. The normal pattern is reproduced in fig. 16, Plate V, as a 

 photo, and both images by the side of each other as stereographical 

 projections, in fig. 5 and 6. Using the normal image as standard, it 

 may be called very remarkable, that the abnormal image aj)pears in 

 comparison to it as a "distorted" normal pattern, as if the crystal- 

 plate were rotated round the vertical principal direction at a certain 

 angle. Very striking indeed is the completely different intensity- 

 distribution of the spots, and also their altered position in both 

 cases. Microscopically no differences could be found in the one place 

 of the plate and the other: with a very strong enlargement the 

 crystals showed certainly very small and long-shaped inclusions, but 

 these were in precisely the same way and arrangement present also 

 in both the other hamberg ite-\)\ales, cut parallel to jlOOj and jOOlj, 

 which plates however in striking contrast to the one mentioned, 

 ahoays gave completelg nvriual patterns. From this it must follow, 

 that these inclusions cannot be the cause of the phenomenon observed. 



On repeating our experiments with the same plates of sodium- 

 ammoniuintartrate, as we used formerly, we obtained with the, — 

 now superficially somewhat rougher, — |)late parallel to {100}, the 

 same abnormal pattern as previously: only a few of the spots 



1) These Proceed. 17, 1204, Plate II, fig. 8; Plate IV, fig. 11. 



