262 



PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



normal, so that the sulphur atoms remain on the (HO) planes of the zinc lattice. 

 But the (100) planes show the same sort of alternation as we observed just now 

 in the case of the (ill) planes of rock salt. The sulphur lattice must have moved so 

 that its (100) planes lie half-way between the (100) planes of the zinc lattice. Again, 

 the (ill) spectra show a new peculiarity in that the second order spectrum is 

 abnormally small. Such an effect is readily explained by supposing that the 

 (ill) sulphur planes divide the intervals between the (ill) zinc planes in the 

 ratio 1:3. 



All these conditions are fulfilled by the structure shown in fig. 12, in which the 

 sulphur lattice lias moved away from the zinc lattice in the direction of the cube 





*; 



M 



Zn 

 O S 



/* 



Fig. 12. 



diagonal, the distance of the movement being one quarter of the length of the cube 

 diagonal. 



Let us take yet one more illustration from the case of the diamond ; the spectra are 

 given in the figure. 



100 



no 



111 



2V2 



I 



a/2 



V3/2 3V3/2 



Fig. 13. 



5^3/2 



In this case it might appear that the relative positions of the first order spectra are 

 not those of the face-centred lattice, but this should be regarded as purely accidental. 



