559 



Crystallography. - "On llie Stjnimplnj of tin' Rö^tgeh j^'ttti^rii.'! 

 of Rhombic Ori/s(aI>i". I. Bv Prof. H.'Haga and Prof. F. M. 

 Jakger. 



§ 1. Willi the purpose to test jet further by experinienis the 

 conclusions with respect to the special symmetry of the ditfiaciioii- 

 ]iat(eriis of R(')NTGKN-rays in crystals, as may he derived from the 

 theory adopted till now, we have moreover made a series of experiments 

 with optically l)iaxial crystals. In the following pages are recorded 

 the results obtained in the investigations with rhoiuhic crystals, and 

 especially with representatives from each of the possible symmetry- 

 classes of the rhombic system. As already formerly mentioned'), 

 the RoNTGEN-patterns of plates from rhombic crystals of /'^«^^/«('«('«//a?/, 

 pyramidal, or hipi/ramidul symmetvy , must always show the symmetry 

 which would be observed in tiie analogous crystal-sections of the 

 rhombic-bipyramidal class. From this is deduced, that crystal-plates 

 cut parallel to the three pinacoides 1100}, {01 Oj and {001 j, in the case 

 of crystals of each of the three possible classes of this system will 

 give RöNTGEN-|)atterns, which will nlioays appear symmetrical ivith 

 respect to iwoperpendiciilai- planes of symmetry, if they are radiated 

 through in the direction of the i)late-normal. 



^ 2. The way, in which the true situation of tlie spots in the 

 diffraclion-pattern, or more correctly : in its stereographical projection, 

 is related to the [)arameters a : b : c of such rhombic crystals, can 

 be elucidated more particularly as follows. 



Let P in P^ig. 1 be the plane of the pliotographic |)lale, and KC 

 the distance of the crystal from that plate. This distance (=J)\vas 

 in our experiments always 50 nmi.'); the diameter of the sphere 

 by means of which tiie stereographical projection on the plane P 

 will take place, is therefore always 100 mm. The viewpoint of lIh, 

 projection is 0; the planes AC )'(=/'), A' C'Z, and )'('Z are supposed 

 to coincide with (or to be i)arallel to) the three planes of symmetry 

 of rhombic-bipyramidal crystals. Let the parameter-quotient in the 

 directions C'A, CY and CZ be: a-.b-.c, of which a and c are 

 numbers, known by the measurements of the angular values, and 

 b is taken arbitrarily =1. 



If, now, Kz I'epresents a "possible" zone-axis of the crystal, the 



Cs 11. 



value of — must be: n . — , where — is the parameter-relcilion in 

 CK c c 



1) These Proceed. 17, 1204. (1915). 



5) However attention must here be drawn to tlie remarks in § 4 



