3 8o 



SCIENCE PROGRESS 



There is another way of explaining why certain spots fail to 

 appear and I think that by its means the whole pattern may be 

 shown to be far more general than Laue considers it to be. The 

 point system in which the atoms are arranged for the purpose of 

 the above analysis is not the correct one. 



Point systems of cubic symmetry have three elementary 

 forms. There is that taken by Laue which has, as element of its 

 pattern, points at the corners of a cube ; another which has points 

 at the corners and one at the cube centre; a third with points at 

 the corners and also at the centres of the six cube faces. It 

 seems to me that it is this last system which is revealed as 

 characteristic of the structure of the zinc blende crystal by the 

 interference pattern. This different point system involves a 



Fig. 3. 



slight change in the analysis. Suppose, as before, that axes are 

 taken with origin at an atom and that the atom at the origin 

 send off a wavelet which is in the direction a, /3, 7, hj wave 

 lengths behind its neighbour along the x axis and so forth. 

 The distance between neighbouring atoms along the axis is 

 "a" as before but this is no longer the shortest distance 

 between atoms. 



The arrangement of the atoms in the X Z and Y Z planes will 

 be as in fig. 3. The previous equations (1) ensure that all atoms 

 such as O, A, B, C, etc., shall emit wavelets in phase. We must 

 now express the condition that atoms at points such as D, F, 

 the extra atoms at the centres of cube faces should also emit 

 wavelets in phase with those from O. 



The difference in phase of wavelets from D and O will be 



