558 



llie surface of llie crystal-platcH in some (ypical way, eg. as irregu- 

 larities of that surface. In this connection it may i)e of interest to 

 mention the fact, that really in some few cases we found such 

 ahnoi'uial |)lienomena with crystalpiates, cut parallel to some of such 

 "striated" faces of the crystal. 



Moreover the cpiestion may arise : why is this abnornu»! iiehaviour 

 observed relatively so often in the case of hiaxial crystals, while it 

 occurs evidently hardly ever in ihe case of unia.xial crystals? 



Finally we may yet draw atlentioi) to the following case . If a 

 pseuilo-si/mini'triral (niiu)etic) crystal is i)uilt-up by lower-symmetrical 

 lamellae, it cannot a priori be understood, why such a combination, 

 radiated through in the direction of the (new) optical axis, would 

 in any way manifest its polysynthetic twin-structure. Indeed this con- 

 clusion appears to be verified here by our experience with the penninc. 

 But if that lamellar structure can cause in any way the presence 

 of such "internal vicinal [)lanes", so thai the molecular layers can 

 be turned a little round these two, three, four of six directions of 

 intergrowth, Ihe po.ssibility can then be foreseen, that these irregu- 

 larities will be brought dccidentalli/ in one of these directions more 

 strongly to the fore, than in the remaining ones : that one direction 

 will then appear in the di fraction- pattern as a single plane of sym- 

 metry of it, and in this way the appearance of this can be considered 

 to be an indirect proof for the lamellar structure of the investigated 

 crystal. This was evidently the case with apophyllite '), henitoite '), 

 and the racemic triethyltnrdiamine-cobahi-bromide'); moreover we found 

 it a short time ago also in the case of benzil, if cut perpendicular to 

 the optical axis of the pseudo-trigonal complex. We expect to elucidate 

 in every case these questions by systematical experiments, and especially 

 to determine finally the true nature of these internal disturbances, evi- 

 dently intimately connected with the normal molecular structure. 



University- LaboratO) ies for Physics and 

 Inorganical avd Physical Chemistry. 

 Groningen, August 1915. 



') These Proceed. 4. 438. 



') Ibidem, 17. 1204 (1915), Plate IV, fig. 14. (1915). 



S) Ibidem, 18. 5o. (1915). 



