568 



three lallier lliick ci yslal-plales were prepared, wliose thickness was 

 rewp. 1.64 nun., 1,72 mm. and 2,09 mm. In tliis case and all others 

 here we experimented again witli the tinorescent screen "Eresco" ; 

 the time of e.xpositipn was ordinarily about 2,5 hours. In this case 

 of ihe anhydrite we used more particularly a CooiiDGE-tube, with a 

 */)o//Vf//Hi(iH(-anticathode and sejiaratc heating-spiral. 



Tlie three photographs are reproduced in fig. 1 — 3 on Plate I; 

 their stereographioal projections ah'eady in fig. 3, 4 and 5 of the 

 text. All three images appear to be quite normal, and every one 

 has two perpendicular planes of symmetry ; the normal to the plate 

 (direction of radiation) is thus at the same time a binary axis of the 

 RöNTGKN-patterns. 



b. Avrnyonite {CaCO,). Our clear, lustrous crystals were from 

 HoRSCHKNZ in Bokenim. The crystal-[)late parallel to JlOOj liad a 

 thickness of 0,96 mm., tiiat parallel to JOIOJ 1,06 mm., and that 

 parallel to jOOIj 1,10 mm. The photos are reproduced in fig. 4 on 

 Plate I, and in fig. 5 and 6 on Plate II ; their stereographical pro- 

 jections in fig. 1-3 on Plate IV. Also in this case the patterns 

 appear to be symmetrical with respect to two planes of symmetry 

 perpendicular to each other, just as might be predicted from theory. 

 In the image parallel to jOOIj moreovei' the well-known pseudo- 

 ditrigonal symmetry of the mineral is clearly recognisable. 



c. Topaz. {Ah{F,üH)^Siü,). The tnpaz-crys\Si\ used by us was 

 very homogeneous, vitreous and translucid ; it possessed a yellowish 

 hue, and originated from Saxony. The thickness of the three plates 

 parallel to JJOO|, jOlOj and {OOJ \, was from 1,20 mm. to 1,27 mm. ; the 

 time of exposition again two and a half hours. The plate parallel 

 to {010{ afterwards appeared to be a little inclined; therefore the 

 corresponding photo was not reproduced here, but solely those of the 

 other sections in fig. 7 and 8 on Plate II; their stereographical 

 projections are to be found in fig. 4 — 6 on Plate IV. 



Also in this case all three patterns appear to be symmetrical after 

 two perpendicular planes, as might be expected from the theory. 



To this same class belong furthermore the crystals of conUerite 

 and of hamberi/ite, already previously ') discussed. 



(/. In the ease of cordierite Ihe patterns of crystal-plates parallel 

 to jlOOj and jOlOj appeared to be, till this moment, '//iw??/^ abnormal, 

 notwithstanding the fact that crystals of several localities were used 

 in the experiments, and among these were present splendid, lustrous 

 crystals. Only the pattern obtained with a crystal-plate parallel to 

 jOOlj, appeared to exhibit the normal symmetry. 



1) These Proceed. 17, 430, 1204. (1915). 



