56i 



stereographical projection P by means of the existent zone-relations, 

 and to investigate, which reticular planes will give tlie most intense 

 impressions on the photographic film; or, what is the same: it 

 will be possible to find out, in what way the spectral dissolving- 

 power of the different molecular planes is connected with their 

 reticular density. 



We have chosen the case of a radiation perpendicular through a 

 plate parallel to |001j. In the same way however it can be found, that : 

 On [iOOj the spot corresponding with jlOij, is situated in the direction 



^ 100 c 



of 6-axis, at a distance of m.m 



a 



On {lOOj the spot corresponding with |ilOj, is situated in the direction 



100 



of the ij-axis, at a distance of m.m. 



a 



On jOlOj the spot corresponding with |OHj, is situated in the direction 



of t'-axis, at a distance of 100 c m.m. 



On |Ü1UJ the spot corresponding with jllOj, is situated in the direction 



of yl-axis, at a distance of 100 a m.m. 



all distances reckoned from the image-centre C). 



§ 3. If in this way the indices of the molecular planes are deter- 

 mined, it is possible to construct a stereograi)lucal projection of them, 

 and by means of this projection to execute the ordinary calculations, 

 as usual in crystallography. However it is also possible to construct 

 directly the stereographical projection of the reticular planes, with 

 which the spots on the photo correspond, from the stereographical 

 projection of the RoNTGKN-pattern itself. The way to do this, is as follows: 



The part to the right of fig. 2 relates to the original stereogra- 

 phical projection of the RoNTGEN-patlern oi iiotitssiumckloride, oh{a.\nQÓ. 

 by Bragg, if the plate, cut parallel to a face of the cube, is 

 radiated through in a direction perpendicular to that face. The part 

 to the left of fig. 2 represents the stereographical projection of the 

 molecular planes of the crystalline structure, corresponding with the 

 spots in the part of fig. 2, sitiiatetl to the right; the plane of projec- 

 tion is parallel to the same face of the ciilie. If now we again 

 pay attention for a moment to fig. 1, we shall find that in this figure 



1) The relation between the siluation of the spcls on Üie photograpliical fi m, 

 and that of the corresponding points in llie stereographical projection on P, can 

 also easily be seen from the fig. 1. l-'or CS = 2A.tg'p and CV= A.tg 'iip, il Fis 

 the original spot, and S its stereographical projection on P. If CI (= tt) is measured 

 on the photography, then ig "2 = 0,ü!2 a ; from this and ty ■ are calculaled, 

 and thus (76' = WO ly y is found in mm.. 



