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BELL SYSTEM TECHNICAL JOURNAL 



which we were able to reahze. Under our best vacuum conditions, 

 the intensity changes took place more than twenty times more slowly 

 than the changes shown in Fig. 5, and under our poorest vacuum con- 

 ditions they took place so rapidly that the diffraction patterns of the 

 third and fourth types could not be found. 



AZIMUTH 



Fig. 6 — Scattering curves in a (110) azimuth of the crystal showing the tran- 

 sient electron diffraction beams of the fourth type. On each arrow indicating the 

 primary beam is printed the wave-length of the electrons giving rise to the curve. 



The entire diffraction pattern of the fourth type is exhibited by the 

 curves of Figs. 6-8.^ These figures show a series of electron scattering 

 curves in each of the principal azimuths of the crystal. When these 



(lit} AZIMUTH 



Fig. 7 — Scattering curves in a (111) azimuth showing electron diffraction beams of 



the fourth type 



curves were taken, the crystal surface was in the transient gas-covered 

 condition corresponding to the maximum of the "Type-4" curve shown 

 in Fig. 5. The vacuum condition of the apparatus was so good at this 



^ It will be remembered that we were bombarding normally a (111) face of a nickel 

 crystal. Each azimuth of the crystal we designated by the Miller indices of the 

 densest plane of atoms the normal of which lies in the azimuth and in or above the 

 surface of the crystal. Diffraction beams were found only in the three most import- 

 ant azimuths, the designations of which are the (HI) azimuth, the (100) azimuth, 

 and the (HO) azimuth. (Other azimuths were explored without finding diffraction 

 beams.) 



