498 



This is, so far as we know, the first example, in which atoms 

 are found in the centres of tlie prism-faces, not howevei- in those 

 of the bases. 



In excess of the above-mentioned triples, still other triples must 

 in the case of this structure give occasion to interference-lines. In 

 this case namely the structure-factor becomes -S' = i -|- t'~''^ + /'i) -|- 

 _[_ ^-i(A, + Ai). The value of this is 3, when all indices are either 

 even or odd and in the other cases 1 or — 1. So it is evident that 

 planes the indices of which do not fnltill the condition mentioned, 

 will be present as interference-lines, though, under equal cii'cum- 

 stances for the rest, with Vg only of the intensity of the otfier 

 lines. So one cannot wonder that a few of this kind of planes can 

 give sufficient intensity to the interference-lines and that their index- 

 triples are present in colums 7 — 10 of table I. 



In order to be able to jndge about the agreement between the 

 place and intensity of the intei-ference-lines, which are to be expected 

 on the basis of the given stincture and the observed ones, a table 

 has been drawn up, in which the values of .s'/vi' ^^, as well for 

 Cu,K'y-, as for 6'M\-^5-radiations, are collected, as computed for all the 

 iiidex-triples that come into consideration, with their relative inten- 

 sities. Of this table II foi'ms an extract in which are omitted in the 

 first place all the trij)les for which the structure-factor not equals 

 3, and in which for the rest out of each group of iiidex-triples 

 which can be esteemed to produce lines which lie so near to each 

 other that these together will give on our Röntgenogram oidy a 

 single line, only that triple is noted that is the origin of the most 

 intense line of that group. If siudi a group contains u- as well as 

 /J-lines, then the strongest oidy of both kinds is given- ') 



Deviations between computed and observed values of shi^ Yi ^> 

 which are larger than corresponds to the degree of accuracy of the 

 measurements, only occur in table II at the first thi-ee lines. About 

 this it must be remarked that these lines are very hazy, which, in 

 connection with the veiling of the film as a consequence of the 

 action of white radiation in the Rönfgenbeam makes the reading 



^) During Coram. Nr. 1 being under press we learned, that Dr. Scherrer and 

 Prof. JoHNSEN had taken a Röntgenogram of white tin and Prof. Johnsen proposed to 

 us, to exchange the observed values mutually. The results, then communicated to 

 us cordially by Prof. Johnsen, give, especially at lower values, all of them some- 

 what smaller values of sin^ Vo than ours. F'erhaps this might be ascribed to the 

 correction applied by us for thickness of bar being somewhat too small. In the 

 notes added to table 11 a few of the differences between the results of Scherrer 

 and Johnsen and ours are discussed. 



