557 



Of course tlie question iniinedialelj arises; of what iiiud aie 

 these causes? On deviations in the right orientation of the crjslal- 

 plates, — (which are always present in a less or higher degree), — 

 it is iiardly necessary to expatiate: aftei' a longer practice one 

 learns to evaluate quite exactly the smaller and very typical distor- 

 tions, arising from that source, and to pass over them as over the 

 typographical errors in an ordinary text. But the anomalies here 

 considered are of a totally different order; they must be caused by 

 a breaking-u|) of the slratographic position of the molecular layers, 

 by which certain jiarls of the parallel |)lanes of the molecules will 

 be locally rotated round one of the principal directions in the crystal, 

 — in an analogous way, as on our earth the inversions and the 

 folding of geological strata may be observed. But in every case these 

 disturbances must be here of inoleculdr dinwnsions; ihej can evidenüy 

 not be studied or observed by other available means at the moment 

 than by the RöNTGEN-radiation. because the cryslal-medium, disturbed 

 in its molecular relations, behaves in respect to all oilier known 

 physical actions like a continuum, with exception just in respect to 

 the extremely small wave-lengths of the lloNTGEN-rays. 



If there are present in rhombic crystals some directions of higher 

 or less perfect deavability, which are parallel to the principal sections 

 of (he crystal, then it will be probable, that such "internal vicinal 

 planes" of the molecular layers will appear to be turned exactly 

 round these princi|)al cleavage-directions as axes, — here round the 

 one, and there round the second of them. It will then depend on 

 the place, where the crystal will be radiated through, if the dilfrac- 

 tion-image will show a symmetry after the one or after the perpen- 

 dicular plane, it must be remarked here however, that exactly in 

 the case of the sodiwnmnviouiumtartrate, where the mentioned 

 phenomenon was observed by us, no such directions of typical 

 cleavabilily are present. It seems therefore, that the principal direc- 

 tions of the molecular structure can play this remarkable rule also 

 in the case, that they are not at the same time directions of distinct 

 deavability. 



§ 8. We do not deny, thai the explanation given here has some 

 weak points, especially if it must be supposed, that all molecular 

 layers, contained in the whole thickness of the crystal-plate, contri- 

 bute their part to the final impression on the photographic film, 

 while notwithstanding that, only for a certain number of these 

 molecular layers the 'presence of such "internal vicinal planes" can 

 be accepted, because otherwise they would manifest themselves at 



36* 



