556 



a nature, as to l)e coimocted intimately witli tiie |)ropei-, inlei-iial 

 inolecnlar strneture of' the crystals. 



But a inillier and persuasive illustration of' this (|uestioii is given 

 also l)\' the case of' the ihoinbio :iiic-s)i//)/iiiit'. Here we used a 

 splendid Iranspai'eut crystal-plate, obtained by direct di'avoge of the 

 crystal along the plane of /jfr/ect clea\abilily S010|, whose perfectly 

 right orientation could be controlled very rigorously by optical 

 examination, the /y-axis being at the same time tlie first bisectrix. 

 Notwithsiaiiding this, however, the corresponding diffraction-image 

 appeared to be constantly nl)iu>nnid, and to possess only one single 

 plane of symmetry parallel to |0()lj, — i.e. parallel to the optical 

 axial plane. (Vid. Plate V, fig. 17 and Plate Vll, fig. 26). 



The above mentioned observations undoubtedly must bring the 

 conviction, that the cause of the observed phenomenon must be 

 ascribed to the crystal-plates themselves, — faultless as they may 

 appear eren on })u)re detailed e.ramination. Indeed further experiments 

 taught us, that also with other rhombic crystals than Avilh /(a??!/ic;ï/ifó, 

 it is eventually possible to obtain perfectly nornud patterns, with 

 the aid of the same ap|>aratus. In the following paper we will 

 reproduce the photos and projections of the images, which we 

 obtained with the plates of a number of biaxial minerals and arti- 

 ficial substances, cut parallel to the three pinaeoidal faces. They 

 will, besides some new cases of abnormal diffraction-patterns, also 

 show many, which indeed must be judged to be quite "normal" 

 ones; the fundamental exactness of the original theory thus being 

 convincingly proved. As Prof. Rinne of Ze//^^^^', who supposed already 

 some time ago, that special secondary causes might be connected 

 with the observed phenomena, wrote to one of us, — he obtained 

 in the case of the anhydrite as well normal as abnormal diffraction- 

 images, and with calamine parallel to {OlOj only abnormal ones. 

 With respect to our own results with these minerals, we can refer 

 here to the following paper. 



^7. As a result of our completed experiences, we finally can 

 make the statement, that the non) adopted theory of the dijf'raction- 

 phenoniena, really can describe sufficiently the general behaviour of 

 crystals tvith respect to R'CmioKH-rays ; and that the pecidiar partial 

 symmetry of the Ki'mTGM^- patterns, as observed till nou) in many 

 cases and especially tvith rhombic crystals, must be caused by secondary 

 circumstances, connected tvith a particular kind of disturbances of 

 the internal molectilar structure of the crystals, and ivhich at the 

 moment can be eMiinined by no other physical means. 



