INORGANIC REPLICATION IN ELECTRON MICROSCOPY 817 



sequence of these factors, the precise interpretation of density variations in 

 micrographs is not practicable. Also many replicas contain artifacts, i.e., 

 features in the micrograph not due to the original surface, but introduced 

 somewhere in the processing. RecrystalUzation, plastic granularity, specks 

 of dirt or other foreign material, tears in the replica films, and defects in 

 the photographic emulsion are examples of artifacts. Few micrographs are 

 completely free of these effects. 



The interpretation of micrographs therefore has as its object not so much 

 a detailed topographical map of the surface, but rather its characteristic 

 features, repeated in many micrographs. For example, the micrographs 

 presented show that sintered NiO-Mn203 flakes develop grains with exten- 

 sive crystallographic planar surfaces but that thick disks develop a striated 

 or hill-and-valley surface structure on individual crystallites. In both cases 

 the structure is very compact, pores between grains being ahnost non- 

 existent. (Incidentally, these materials can be subjected to a heating cycle 

 in which pores are a predominant feature.) Naturally, the greater the range 

 of contrast and the better the resolution, the more surely can characteristic 

 features on an exceedingly fine scale be detected. In general, the method 

 of rephcation which portrays best the characteristic features under study 

 should be selected. Even in the study of a single material, more than one 

 method may be desirable. For example, a porous structure is probably most 

 easily reproduced by a silica replica using the two-step process, on account 

 of the fact that the diffusing component forms films over reentrant regions 

 not exposed directly to the source; but fine surface detail might best be 

 revealed by a germanium or chromium replica using the one-step process. 



5. Stereoscopy 



Electron microscopy has a fundamental advantage in that, because of 

 great depth of focus, stereoscopic study of surfaces at high magnification 

 is possible. This advantage is sometimes indispensable; for example, porous 

 structures result in complex micrograms which can be understood only by 

 stereograms. More generally, stereographic portrayal, by fully delineating 

 surface topography, achieves the chief purpose of microscopy and makes un- 

 necessary the precise interpretation of density variations. However, resolution 

 and contrast are important factors in stereograms.^^ It is obvious that the 

 replica must retain the third dimension; inorganic replicas in general do, but 

 thin film plastic replicas, unless heavily shadow-cast to make them effec- 

 tively inorganic, change under the electron bombardment and draw down 

 to a planar film of variable thickness.^ 



22 A. W. Judge, "Stereoscopic Photography," p. 28. 



23 C. J. Calbick, //. App. Phys., 19, 1186 (1948). 



