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BELL SYSTEM TECHNICAL JOURNAL 



instead of only spots. And if we could remove all but one of the 

 groups, and then in some magical way magnify the luminescence 

 which the waves that this survivor scatters produce at the wall of the 

 bulb, then we should see the wall shining all over with a continuous 

 brightness, sinking to zero perhaps nowhere, perhaps at occasional 

 points. 



Reverse the process, starting from the solitary atom-group. The 

 intensity of the scattered waves of which it is the source varies con- 

 tinuously with direction, and the brightness of the wall of the bulb 

 varies correspondingly from point to point. If this brightness is 

 proportional to that intensity, its distribution over the spherical wall 

 is the scattering-pattern or diffraction-pattern of the atom or group 

 of atoms. However it is immeasurably too faint to see. 





Fig. 3 — Illustrating how a beam of waves is split by a crystal (at K) into diffrac- 

 tion beams forming spots on a plane screen (P). (Apparatus of the first experiment 

 on X-rays by Friedrich and Knipping.) 



Add now to the solitary group a great number of similar and 

 similarly-oriented groups of atoms, forming altogether a long and 

 evenly spaced row. The luminescence now fades out everywhere 

 except over certain rings upon the wall, but along these rings it is 

 enhanced. The newly-added atom-groups have amplified the waves 

 scattered by the original one along the directions leading to these 

 rings; in compensation they have effaced those scattered in other 

 directions. These are effects of interference. 



Add next a great number of such rows of similar and similarly- 

 oriented groups, forming altogether a lattice in a plane. The rings 

 now all fade out except for occasional spots, which however are much 



