428 BELL SYSTEM TECHNICAL JOURNAL 



atom-groups reinforce the scattering from one another, so it may be 

 shown that for certain directions the different atoms of a single atom- 

 group destroy the scattering from one another. One has only to write 

 down the condition that the distance from source P to fieldpoint Q via 

 one atom of the group differs by an odd-integer multiple of \\ from the 

 distance via the other ; or if there are four atoms in the group, that the 

 waves scattered to Q from the four are so balanced in phase that they 

 annul one another. 



Now if it should turn out that one or more of the diffraction-spots 

 expected from the cubic lattice fall exactly where the effects of the 

 atoms of each individual group cancel each other out, then those spots 

 will be lacking. For the spots are due to amplification of the diffrac- 

 tion-pattern of the individual atom-group; but if at the location of a 

 predicted spot this pattern sinks to a vanishing intensity, there is 

 nothing to amplify. Well! owing to the neat and accurate way in 

 which the spacings between atoms of a group are related to the spacings 

 between the atom-groups, this sort of coincidence occurs for a re- 

 spectable fraction of the diffraction-spots — or, in the powder method, 

 it occurs for several of the diffraction-rings. From the missing spots 

 or rings therefore one identifies which style of atom-group prevails in 

 the cubic crystal. And there is much more yet to be learned; but that 

 will be material for another article. 



