874 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



It will be realized that the detailed shapes and location of the maxima 

 are quite sensitive to thickness and more often than not a calculation from 

 the fringe spacings cannot be made with certainty. The limiting value of 



the fringe separation is found from (2) to be given by -^ for large values 



of the integer n. The fringe pattern indicated in Fig. 4 is simply a rocking 

 curve for the crystallites with a displacement due to their difference in 

 orientation. 



The absence of extinction contours from the electron image of a crystal 

 may indicate that one or more of the following conditions exists: 



(1) No bending or thickness changes. 



(2) The thickness is sufl&ciently small that the argument of the sin^ in 



n = 2 



DEVIATION IN DEGREES 



Fig. 5- 



electrons. 



-(200) rocking curve calculated for an aluminum crystal 250-4 thick for 50KV 



(1) can replace the sine function thus suppressing the periodic fea- 

 tures; this can occur for D less than about lOOA. 

 (3) The crystal is sufficiently distorted that the assumption of a periodic 

 potential function in the Schrodinger equation is not valid. 

 Of these causes for the absence of extinction contours, the first is very 

 unlikely as mentioned previously. The second is quite obvious since a sec- 

 tion too thin to produce contours would give a very high transmitted in- 

 tensity and would be immediately apparent. The third is the most likely 

 reason and is thought to be the case in all the thin sections examined to 

 date. This is particularly important here since crystals that have been sub- 

 jected to plastic deformation are of primary interest. The incorporation of 

 strains or lattice disortion into the potential function for the Schrodinger 

 equation appears to be a formidable task and will not even be attempted. 

 A more or less semi-quantitative approach to the effect of lattice distortion 



