433 



evidently often causes disturbances in two possible ways: 1*^* . by 

 increasing ajjpreciablj the diniensions of the central spot, because 

 of the ditFuse light-emission of the screen; which fact may render 

 some of tiie spots situated in the immediate vicinity of the central 

 part invisible in the reproductions; and 2"'^ because (he impossi- 

 bility of pressing the phosphorescent screen over its whole surface 

 quite equally against the photographic plate, eventually will cause 

 some differences in the intensities of the black spots, wliich apparently 

 create an accidental dissymmetry in the obtained photograph. Also 

 both these disturbing effects must be taken into account together with 

 the above given arguments, to explain the inevitable imperfection of 

 the RöNTGEN- radiograms, thus prepared. 



The RöNTGEN-patterns, Avhich. now are reproduced in fig. 3, 4, and 

 5 of plate I, can teach us the following facts: A somewhat more 

 accurate study of these photographs will immediately show, that 

 the images obtained by radiation through the crystalplates |100| and 

 jOlOj, possess only a hllateral symmetry: the molecular arrangement 

 of the crystal, seen in the two directions i)erpendicular to these 

 faces, can thus possess only one sinqk plane of syinmetry, in the 

 first case perpendicular to {10()|, in the last one perpendicular to jOlOj 

 and passing through the r-axis; hy hotk image.-^ koiuever it is proved 

 indubitably, that axes of binary syniinetry are completely absent. The 

 image, obtained by radiation through the crystal in a direction perpen- 

 dicular to |001| however, must be considered doubtlessly to be 5//?«7?Jé?- 

 trical with respect to a set of two synimetry-jdanes, perpendicular to each 

 other; of course the intersection of these two planes, being the c-axis, 

 needs to be an axis of binary symmetry too. On Table / we have repro- 

 duced a RöNTGENogram of this ciise, which shows some dis.symmetries 

 by a very small error in the normal orientation; the distribution 

 of spots of equal intensity however, etc., suggests the symme- 

 trical nature of this radiogram with respect to the mentioned planes 

 without any doubt. Of this same crystalplate we obtained some 

 more radiograms, which were however not sufficiently intense for 

 reproduction; they were somewhat more symmetrical than (he pho- 

 tograph reproduced here, which fact apparently was caused by a 

 somewhat better adjustment of the crystalplate with respect to the 

 RöNTGEN-tube. But an optical investigation of the crystal-plates in quite 

 the same position as in which they were during the experiment, allowed 

 no distinction of the orientation in the several cases: it must there- 

 fore be considered a fact of mere chance, if one gets accidentally 

 the right position of the plate, necessary to obtain a pattern, whose 

 symmetry approaches the pure one with more or less perfection j 



