550 



now was radiated througli in anotliei' place, its abnormal symmetry 

 was fonnd once more. 



Here now we could, for the first time, observe in the case of 

 a uniaxial crystal a very particular abnormality: indeed it appears, 

 that properly a plane of symmetry perpendicular to the trigonal 

 axis seems to lie added to the crystal, which involves at the same 

 time the addition of three new verticai fiJanes of symmetry passing 

 thi'ough the e-axis, making this axis necessarily a senary one. In 

 the original negatives this different symmetry in both cases is very 

 evident, somewhat less, however, in the reproductions (Plate IV, 

 fig. 13 and 14); but the differences between the normal and the 

 abnormal pattern are clearly expressed in the stereograjihical [irqjections, 

 which here are given together in fig. 3 and 4. 



Thé same abnormality, i.e. the addition of a horizontal plane of 

 symmetry perpendicular to the ternary axis, seems to be also present in 

 the RöNTGEN-ogram, oblained with a cjystal-plate parallel to {lOlOj; 

 this plate had a thickness of I, JO m.m. Alihoiigh this plate was 

 parallel to the c-axis, it appeared to be not completely parallel to 

 the prismface; the pattern, which therefore very probably did not 

 show a true vertical symitietry-plane, is here not reproduced. The 

 stereographical projection of the normal patterns are given in fig. 14, 

 15 and 16, Plate VII. 



A careful microscopical examination of both the plates parallel 

 to {121 0|, did however not reveal any optical differences. 



One might be inclined to suppose, that the plate parallel to (1210) 

 which had given the abnormal pattern, were realty a twin-formation 

 after the brasilian rule; i.e. with a plane of (1210) being the t winning- 

 plane. Because perpendicularly to (1210) there is a binary axis 

 present, the RoNTGKN-ogram therefore should indeed show a symmetry 

 with respect to two planes, perpendicular to each other. But by this 

 supposition it could never be made e\ident, that the difl'raction- 

 pattern obtained with a |)late cut from the same crystal parallel to 

 (1010), shows very probably also a horizontal phtne of sijminetry. 

 Thus the said explanation can hardly be considered a final one 

 already for the peculiar RoNTGEN-patlerns which were obtained 

 parallel to (12lO) and to (ToTO). The observed abnormality therefore 

 cannot be said to be explained fully, and we intend to make 

 further experiments on this phenomenon in future. 



h. Nepheli.te. From a small, clear crystal from the Vesuvius, 

 three crystal-plates were investigated. The ])late perpendicular to 

 the optical axis showed a well-centred, uniaxial interference-image, 

 possessing only a slightly abnormal character. The plate had a 



