555 



appeared to be absent on coinparisoii with the former image. The 

 also superficially somewhat rougher ]jlate parallel to |0I0| however, 

 gave now undoubtedly also an ahnormal image, but as a very 

 remarkable fact : jiiM the other {second) phine of symmetry as be/ore 

 was now manifested in the pattern, notwitlistanding the same position 

 of the crystal plate.' Formerly this plate had given an abnormal 

 image, which was symmetrical after the plane {OOlj; now it showed 

 a symmetry-plane parallel to \\0()\. As b\oth plates of tlie tartrate 

 were superficially a little altered by a feeble efflorescence, we pre- 

 pared from a fresh, translucid ci-ystal of the salt three new plates, 

 which were examined in the same wa^'. 



With these plates we were now able to obtain RöNTOEN-patterns, 

 which were symmetrical after two planes; in this way it was 

 possible at the same time to compare the parlial symmetry of both 

 the abnormal images parallel to SOlOj with the normal pattern. The 

 three normal and I lie abnormal images are reproduced as stereo- 

 graphical projections in fig. 20 to 25 on Plate VII. 



^ 6. We have convinced ourselves by especially arranged ex- 

 periments of the fact that a deviation from (he true plane-pai'allel 

 shape of the hamheryiie-plate could not be the cause of the partial 

 symmetry of its pattern formerly obtained. Moreovei-, in our numerous 

 experiments with cordierite, with ma'erial from all localities, we 

 nerer obtained other images than abnormal ones, which were only 

 symmetrical after one single plane of symmetry. With (his mineral the 

 phenomenon thus seems to manifest itself constantly. \n our way of 

 experimenting, wi(h the use of fixed, on the lathe i-ectangularly 

 turned-otf crystal-supports, a somewhat appreciable deviation from 

 the true orientation is highly improbable. Moreover (he same 

 crystals, adjusted by tiie same apparatus, appeared formerly often 

 to give cpiite normal patterns, if radiated through in one or two of 

 the other principal directions, so that a .yy.v^^^ma^ica/ error of the whole 

 arrangement can hardly be considered to be the cause of the pheno- 

 menon. If this were (rue, or if deviations in the right orientation 

 of the prepared crystal-sections were the cause of the phenomenon 

 observed, it could furthermore not be undeistood, why never a 

 distortion of the normal pattern after another direction than only 

 after the two principal ones of the plate, were till now observed. 

 The fact, that the planes of symmetry of Ihe rhombic ci'ystals just 

 play the preponderant role in this, proves sufficiently, that no 

 accidental causes are responsible here, but that these are of such 



36 



Proceedings Royal Acad. Amsterdam. Vol. XVllI. 



