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BELL SYSTEM TECHNICAL JOURNAL 



colatitude 56° when the bombarding potential is 143 volts. For 

 diffraction into the C-azimuth we must regard the atoms in the 

 surface layer as arranged in lines normal to the plane of this azimuth 

 as illustrated in Fig. 5. The grating constant is 1.24 A., and the 

 similar gratings that make up the whole crystal are piled up without 

 lateral shift. For this reason the C-azimuth is si.\-fold instead of 

 only three-fold. For a beam occurring in this azimuth in colatitude 

 56°, n\ should be equal to 1.24 X sin 56° or 1.03 A. The value of 

 himv for electrons that have been accelerated from rest through 143 

 volts is (150/143)^''^ or 1.025 A. Again the beam does very well as a 

 first order diffraction beam. 



SINE e SINE e SINE e 



Fig. 6 — Plot of X against sin d for various beams 



The total number of such beams which we have observed in all 

 azimuths in explorations up to 370 volts is twenty-four — nine in the 

 A-azimuth, ten in the B-azimuth, and five in the C-azimuth. It 

 would be possible to calculate an observed wave-length for each of 

 these beams from n\ = d sin d, and to compare this in each case with 

 the theoretical wave-length calculated from X = hImv, just as we 

 have done already for three of the beams. We have chosen, however, 

 to display the results graphically rather than numerically. 



The data for the twenty-four beams are exhibited in diagrams in 

 Fig. 6, in which wave-length X is jilotted against the sine of the co- 



