4 STUDIES IK GELS I23 



sion that the objects are obHquely illuminated. On the photographic 

 negative the shadows are black, comparable to the shadows in a land- 

 scape cast by the sun, which is the reason why this method has been called 

 shadowing. Because this effect is very striking, the negatives of shadowed 

 preparations are reproduced and not the positives, as in ordinary photo- 

 graphy. This means that a positive film must be made of every photograph 

 before prints can be made. 



Metal shadowing permits even very flat objects to be pictured, for the 

 shadow can be accentuated by lowering the shadowing angle. A suitable 

 angle is ^\^ (1:6), furnishing pictures reminiscent of sunset or sunrise 

 illumination with its very long shadows. As we are accustomed to illuminate 

 relief maps from the left-hand top corner, shadowed electron micrographs 

 ought to be oriented so that their shadow points towards the bottom right- 

 hand corner. Only thus do we get the natural impression of a high-relief. 

 If such a picture is turned upside-down, the impression received is of 

 reversed rehef, all elevations seeming to be depressions. 



Wyckoff (1949) has found the most suitable metals for shadowing to be 

 chromium and palladium. The higher the atomic number of the element, 

 the thinner is the metal film yielding the same effect when deposited on the 

 preparation. Whereas the thickness of a chromium film must be 40 A, 

 a palladium film of 20 A will do. In this respect uranium would be better 

 still. 



As the scale of pubHshed electron micrographs varies from i : looo up 

 to 1:100000, it is as well to mark the magnification on every individual 

 picture. This is done by putting a black line on the micrograph which 

 represents the length of i /m; for a magnification of 10,000, its length is i cm. 



Results of electron microscopy. The improvement in the resolving 

 power for structures invisible in the ordinary microscope is most 

 evident from the electron optical images which have been obtained 

 from the silica wall of the diatom Pleurosigma angtilatum (Fig. 82a), the 

 well-known test object for the immersion objective of the ordinary 

 microscope. In the latter case the best objectives show three inter- 

 secting systems' of lines (Fig. 82b), which at the utmost give a vague 

 impression of a perforation (Fig. 82c; Ardenne, 1940b), whereas in 

 the electron microscope Fig. 83a is obtained. The surmised pores are 

 clearly imaged with sharp edges ; and, being so far apart, it is evident 

 from this "coarse" structure that the electron microscope is able to 

 resolve exceedingly minute details. It is shown that the pores do not 

 represent cyUndrical canals running through the silica walls, but that 

 the outer opening is in the form of a slit, while the inner one is elliptic 

 and closed by a sieve membrane. Stereoscopic pictures moreover 

 betray, not canals, but spacious caverns, whose outer openings re- 



