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THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



rated from the surface replicated, usually by dissolution of the latter. This 

 process subjects the film to considerable strain. Finally it must be removed 

 from the solvent, usually on a piece of 200-mesh screen, rinsed at least 

 once, and finally allowed to dry. It is not surprising that films sometimes 



Fig. 3 — Silica replica of particles and associated shadows, da = 60". Diffusing component 

 replicates particles within shadows. Oval hole shows presence of film in shadow. Black 

 lines = 1 At. 



exhibit cracks and holes. Stereoscopic examination of good replicas show 

 that despite all the processing violence, a faithful picture of surface to- 

 pography is obtained, at least for features up to a few microns in size. 



The nature of silica condensation is indicated by the micrographs of Fig. 

 3, of a silica replica of particles of an alkaline earth carbonate. These were 

 dispersed on a plastic-coated microscope slide, and silica evaporated at 



