552 



thickness of 0.70 m.m. The obtained diffraction-image was extremely 

 feeble: the spots, which, — as in the case of the quartz, — were 

 all cloub/)'-ones, — were situated very far from the centre and were 

 so feeble, as lo make any reproduction impossible. It was however 

 possible to see, that (he pattern possessed a senary axis (schematical 

 projection in tig. 17, Plate VII); 7io vertical planes of symmetry 

 were present. 



The plate parallel to {1010| was 0,78 m.m. thick, and gave a 

 rather good image, which as a stereographical projection 'is reproduced 

 on Plate VII, l!g. 18. All spots here were also doubled, and the 

 axes of the oval impressions were inclined to each other, giving 

 to each pair of spots the shape of an arrow-point; this seems to 

 indicate a twin-formation of the used mineral. The pattern was 

 merely symmetrical after a horizontal plane. The third plate was 

 too disturbed and inhomogeneous, to give any suitable image. 



/. That cinnabar, if radiated through in the direction of the c-axis, 

 will give a RöNTOKN-paftern, whose symmetry is in full concordance 

 with the theory, was already formerly recorded '). The stereographical 

 projection of the RöNTGKN-ogram is reproduced here once more in 

 fig. 19, Plate VII. Finally in fig. 15, Plate V, the very beautiful 

 photo of pennine is reproduced; although this mineral' is only 

 mimetic and clearly shows optical abnormalities, the structure of the 

 lamellae is evidently here a so regular and perfect one, that the whole 

 pile cannot be distinguished from a real trigonal crystal. Attention 

 must be drawn to the remarkable fact, just as formely stated in 

 the case of sylviiu', thai the central spot seems to irradiate in about 

 eighteen directions ; it seems, that this irradiation is connected in 

 some way with the presence of certain gliding-planes in the cr3'Stal. 

 The thickness of the dark green, positively birefringeut, and clearly 

 optically anomalous crystal-plate, was 0,81 m.m. 



§ 4. If we now review all the results hitherto obtained in these 

 researches, it becomes clear, that, — with the exception of the 

 phenomena observed in the case of the two quartz-plates, which 

 phenomena undoubtedly are to be considered as true "abnormalities", — 

 the symmetry of the RöNTGKt^-ijatterns is always in aïjreenient loith 

 that predicted by the now adopted theory of the dijf'raction-phenomenon. 

 On the other hand the correctness oï {\\& f,u^\)Oëed centrical symmetry 

 of the said [)henomenon is thus sufticiently proved in this way. 

 Our experience can be considered evidently as a strong argumenta- 



1) These Proceed. 17. p. 1204: vid. Plate IV, fig. 5. 



