INORGANIC REPLICATION IN ELECTRON MICROSCOPY 



823 



edge by an obstacle of height h, whose profile is normal to the plane of the 

 paper. From a point source, Fig. 17(a), the resolution i?, d = la sin da = 

 te tan Ba. More generally, if the profile makes an angle jS with the plane of 

 the paper, one obtains d = h tan ^a ! sin /5 | , identical with eq. (8). But 

 now, because the contrast is large, the factor | sin /3 | is eflfective in im- 



(a) ATOM BEAM FROM 

 A POINT SOURCE 



ATOM 

 BEAM 



CONDENSED 

 MATERIAL — 



(b) ATOM BEAM FROM 

 A LARGE SOURCE 



■PENUMBRA 



V///////////////////////////////////////// 



(C) ATOM BEAM FROM 

 A SMALL SOURCE 



Fig. 17 — Forms of shadow-edges when every atom sticks where it strikes. 



proving the resolution. For example, one side of the shadow in the en- 

 larged micrograph of Fig. 3 is very sharp. 



Actual sources are of finite size, and in the cases illustrated in Fig. 17(b) 

 and (c) the penumbra extends over a horizontal distance p = h sec^ Ba tan 5, 

 where b is the angle subtended by the source. If the source were of proper 

 geometry and uniform intensity, one could set /> = f/ to obtain a vertical 

 shadow-edge, whose intrinsic resolution would be zero. Actual sources are 

 neither of proper geometry nor of uniform intensity, resulting in shadow- 

 edges such as that of Fig. 17(c). If one takes into account a profile angle 



