104 



BELL SYSTEM TECHNICAL JOURNAL 



It has been pointed out by Eckart that if the index of refraction 

 of the cr^^stal for the electron radiation is other than unity diffraction 

 beams will occur as if the separation between atom planes were other 

 than normal. We ha^•e computed the indices of refraction that would 



150 180 210 240 270 



BOMBARDING POTENTIAL 



330 360 390 



120 ISO 180 210 240 270 300 



BOMBARDING POTENTIAL 



330 360 390 



Fig. 8 — Plot of values of spacing factor and associated values of 

 refractive index for twenty-one beams 



give rise to the observed occurrence of beams and these are plotted 

 in the lower part of the diagram against bombarding potential. 

 Again the points fall very irregularly. While it cannot be said that 

 there is at present a satisfactory explanation of the peculiar occurrence 

 of the space lattice electron diffraction beams, it should be clearly 

 understood that this deficiency in no way affects either the wave-length 

 measurements of these beams or the agreement of these wave-lengths 

 with the values of hlmv. 



The electron diffraction beams which I have described are the 

 only ones observed when the surface of the crystal is free from gas. 

 When the surface is not free from gas still other beams api^ear. These 



