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THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



only if they differ by some small fraction on the density scale. Correspond- 

 ingly, a finite difference in /« is required. For 50 kv electrons, the minimum 

 perceptible contrast difference is produced by a mass thickness difference of 

 about 0.7 Mg/cm^. By plotting eq. (4) for particular choices of 6a, <Pa and 

 assumed values of a and Aq/A, the geometrical conditions required to pro- 



te, e = o 



75° 



75" 



60° 45° 30° 15° 0° 15° 30° 45° 60° 

 e. COLATITUDE OF LOCAL SURFACE NORMAL 



Fig. 16 — Contrast-determining curves. (1) Qa =30", every atom sticks where it strikes. 

 (2) ea = 0**, half the atoms diffuse. (3) Q^ = 30°, half the atoms diffuse. (4) B^ = 0**, all 

 atoms diffuse over surface. 



duce detectable differences in shading can be displayed. To make the curves 

 more general, relative thickness rather than /, is plotted in Fig. 16 for azi- 

 muth ipa — 0. The four curves are, respectively, for 



AqIA has been chosen as 0.833. 



Let us assume that pi, when = is 7 /xg/cm'^. Then a thickness difference 

 of 0.1 on the ordinate scale is barely perceptible. For incidence shading, 



