APPLICATION OF ELECTRON DIFFRACTION 



599 



time that the intensities of the beams shown in these figures remained 

 sensibly unchanged for several hours. 



The scattering curves near the right-hand ends of these three 

 figures show weakened diffraction beams of the third type, which we 

 recognize readily. The curve second from the right in Fig. 8 shows 

 also a diffraction beam of the first type, a beam arising from the space 

 lattice of the nickel crystal. This is a (311) reflection according to 

 X-ray terminology. The other electron beams shown in Figs. 6-8 

 can be most easily considered by correlating the wave-length of each 

 beam with the sine of its co-iatitude angle. This is the procedure 

 which was followed in Fig. 17 of our original paper in considering the 

 diffraction beams of the first type. (Loc. cit. Fig. 17.) Fig. 9 is 



(100} AZIMUTH 



Fig. 8 — Electron diffraction beams of the fourth type in a (100) azimuth 



similar to Fig. 17 of the original paper. The diagonal lines in Fig. 9 

 are the plots in the various orders of the plane grating formula n\ = d 

 sin 6, where in each azimuth the grating constant d is equal to the 

 separation between lines of nickel atoms on the surface of the crystal 

 normal to the azimuth. On this figure are plotted as dots all of the 

 originally reported diffraction beams of the first type and as crossed 

 circles the diffraction beams of the fourth type shown in Figs. 6-8.^ 



It is clear from Fig. 9 that the diffraction beams of the fourth type 

 are "plane grating" beams of "one-half order." The obvious interpre- 

 tation of the occurrence of such beams is that, for these beams, the 

 value of the grating constant is really 2d instead of d. 



It was known from the consideration of data similar to those shown 

 in Fig. 5 that the dift'raction pattern of the fourth t>^pe had its origin 



^ In Fig. 9 there is plotted a crossed circle for each of the separate curves shown in 

 Figs. 6-8. In this respect the crossed circles of Fig. 9 are not similar to the dots. 

 Each dot represents a diffraction beam of the first type at its intensity maximum, 

 whereas the crossed circles represent all the beams of the fourth type which were 

 found. The intensities of some of these were very small. 



