548 



construction of tlie .stereograpbical projection in Plate VI, fig. 3. 

 Evidently there is only one ternary axis present, but no [danes ot 

 symmetry in tlie pattern. 



The plate parallel to jloTOj was 1,20 m.ni., thai parallel to [1 2T0 

 was 1,15 m.m. thick ; we obtained with them two very beautiful 

 photos, reproduced in Plate I, fig. 4 and Plate II, fig. 5 ; in these 

 photograms the direction of the c-axis is vertical. The diffraction- 

 patterns are wholly unsymmetrical ; the results are therefore exactl}' 

 what could be expected from the theory. 



c. In the same symmetry-group also Dolomite must be jilaced. 

 From a splendid, jjerfectly translucid crystal of Binnentlial in 

 Switzerland, three faultless plates parallel to jOOOlj, jlOlOj and 

 jl210j were carefully cut. The plate per|)endicular to the c-axis iiad 

 a thickness of 0,92 m.m.; the beautiful interference-image of negative 

 character appeared to be exactly centrical. The plate parallel to 

 {lOlOj was 1.14 m.m. thick ; that parallel to !T2rOj was 1,11 mm. 



The very beautiful ditfraction-pattenis obtained are reproduced in 

 fig. 6, 7 and 8 on Plate II, and in stereographical projection on 

 Plate- VI, in fig. 4 to 6. The image perpendicular to the c-axis 

 possesses only a ternary axis; both the other images are completely 

 unsymmetrical, just as in the case of phenakite. Also in this case 

 therefore experience and theoretical deduction are in full agreement 

 with each other. 



d. Calcite. From a lustrous calcite-ci-ystal from Iceland two plates 

 were cut: the plate parallel to jlOlOj, as well that parallel to {1210J 

 were 1,15 m.m. thick. Both images were too feeble to allow good 

 reproduction; they are however reproduced as stereographical projec- 

 tions in fig. 7 and 8 on Plate VI. The RöNTGEX-pattern for a plate 

 perpendicular to the c-axis was published some time ago by Bragg ^): 

 the image exhibits a ternary axis and three vertical planes of sym- 

 metry. The symmetry of all these patterns is the same, as was found 

 in the case of the turmah'nc, — just as could be expected from the 

 theory. It must be remarked that the image parallel to jlOlOj, 

 although possessiTig only a single (vertical) plane of symmetry, shows, 

 however, a very strong approximation to a case, wlieie two pei-pen- 

 dicular symmetry-planes were present. 



e. Beryl. We had veiy beautiful plates at our disposition, cut 

 from a splendid, colourless, translucid crystal i'vom the Aduntsckilon- 

 mountains in the Transbaical. The plate parallel to |0001j had a 



1) W. L. Bragg Vid. Zeits. f. Anorg. Chem. 90. 206. (191J); Proc. Royal 

 Soc. A. 89 248. (1913). 



