XII. ELECTRON MICROSCOPY 397 



evaporation from aqueous or saline suspensions has commonly been 

 used, though more recently techniques of growing these directly on 

 the supporting membrane have been developed. Since the latter 

 technique seems to be much less susceptible to the introduction of 

 artifacts, it will probably soon replace the older technique. 



All the above methods of preparation lead to micrographs that 

 provide two dimensional representations of the specimen. In the 

 majority of cases this is sufficient, since the three dimensional 

 representation can be inferred from the appearance of the particles 

 and from a consideration of the method of preparation. In some 

 cases, however, the problem requires precise three dimensional in- 

 formation that cannot be deduced in this way. It is then necessary 

 to make a direct determination of the three dimensional shape of the 

 particles. Two methods are now available. 



The first is direct stereoscopic imaging of the specimen. Owing 

 to the great depth of field of the electron microscope it is possible to 

 obtain high magnification stereomicrographs by the simple method 

 of making two micrographs of a given field of view, the specimen 

 having been tilted, between exposures, about an axis that lies in its 

 plane. Under ideal conditions this method is capable of resolving 

 approximately 20 m/x in depth. 



The second is the recently developed shadow-casting technique 

 (17), which is more suitable for smaller particles. This involves 

 placing the prepared specimen, prior to examination, in a vacuum 

 chamber where a light layer of heavy metal is condensed on its sur- 

 face. The heavy metal is evaporated in the vacuum chamber 

 from a small source, usually a heated filament, basket, or foil 

 boat, so placed that the vapor strikes the specimen obliquely. 

 The condensed layer of metal has then a distribution on the specimen 

 exactly analogous to that which would obtain for the illumination 

 of an enlarged model of the specimen produced by a similarly placed 

 small light source. However, in a positive electron micrograph — due 

 to its greater scattering power — the metal appears darkei' in those 

 regions where it is more heavily deposited. For this reason, such 

 micrographs are usually printed as negatives whence they take on 

 the appearance of an obliquely illuminated surface, permitting direct 

 observation of the relief present in the specimen. The method is 

 particularly valuable for showing the existence and shape of small 

 particles that by virtue of their size and density are normally very 

 nearly indistinguishable from the supporting membrane, for resolving 



