549 



thickness of 1.10 mm., that parallel to jloTOj 1.17 mm., and that 

 parallel to {l2lOj 1.16 mm. 



The diffraction-image parallel to jOOOJ! (vid. Plate IllI, fig. 9), 

 shows a senary axis and six vertical planes of symmetry. Thns it 

 is again proved, that the benjl is really (/«'hexagonal, and that the 

 arguments against this supposition, formerly brought to the fore by 

 Viola '), can hardly be considered as valuable any more. 



The two remaining images (Vid. Plate III, fig. 10 and Plate IV, 

 fig. 11) are, quite in concordance with the theory, symmetrical after 

 two perpendicular |)!anes of symmeti'y. They are reproduced as stereo- 

 graphical projections in fig. 9 — 11, on Plate ^'J. The image of the 

 plate parallel to jlOlOj appears to be somewhat sloping, evidently 

 caused by not wholly jterfect orientation of tiie crystal-section. 



/. Apatite. From a beautiful crystal from Zillevthdl, in Ti/rol, 

 two plates were cut. The image of the |)late parallel to jOOOlj was 

 reproduced already previously ^). The second plate was parallel to 

 {lOlOj ; its thickness was 1,30 mm. The beautiful diffraction-pattern 

 is reproduced in fig. 12 on Plate IV, and both images as stereogra- 

 phical projections on Plate VI, tig. 12 and 13. The pattern parallel 

 to jlOKJj exhibits only one horizontal plane of symmetry, quite in 

 agreement with the theoretical expectations. 



(/. Quartz. From a translucid crystal from the St. Gothnrd four 

 j)lates were cut. The image of a i)late perpendicular to the c-axis 

 was too feeble to make reproduction by any means possible. A sche- 

 matical drawing of the most important, — and always double, — 

 spots, is given in fig. 14, Plate Vil. The pattern shows a ternary 

 axis and three vertical planes of symmetry. 



Two different plates, each of which was parallel to |1210j, and 

 having a thicknes.s of 1,12, resp. 1,05 m.m., gave particularly 

 remarkable patterns. For although both plates were very accurately 

 orientated, and did not manifest, with the microscope, any differences, 

 MOr any inhomogeneity discernible by optical means, — the image 

 obtained with the first mentioned plate appeared to be .nimmetrical 

 after tino perpendicular plane.i; the other image however, notwith- 

 standing its being composed of precisely the same spots, showed 

 quite another distribution of their intensities, in sucli a way, that 

 the pattern was mil;/ .•<i/))imelrical after a .s-inijle binari/ a.r/s. On 

 repeating the experiment with the tirstnientioned quartz-plate, which 



1) Viola. Z. f. Kryst. 34. 381. (1901) 



2) loco cit. 17. Plate I. 



