16 THE PHYSICS OF VIRUSES 



leave electron microscopy to other experts, the day soon comes 

 when he is making concentrated preparations and waiting for 

 the visual knowledge which the electron microscope can give. 

 There is no effective limit to the resolution of the electron 

 microscope, and without doubt this instrument dominates the 

 whole field of study of virus size. As ordinarily used, 50-kv 

 electrons are employed. These are focused on the virus prepara- 

 tion, which is deposited on a thin collodion membrane held on a 

 fine wire mesh. It is usual to shadow the preparation with 

 chromium, gold, or uranium to provide areas which are definitely 

 opaque to the passage of electrons. Such a shadowing technique 

 has the advantage of providing a third dimension which is 

 dependent on the thickness of the virus and on the angle of 

 shadowing. 



Shadowing technique consists of placing the collodion film 

 preparation in a high vacuum (1()~"' mm mercury, requiring an 

 oil or mercury diffusion pump) together with a heater containing 

 the element used for shadowing. This may consist of a tungsten 

 metal strip with an indent to take the evaporated metal, or may 

 simply be a coil of tungsten wire with chips of metal held loosely 

 in the coil. After the vacuum is established, the tungsten is 

 heated by passing a current through it, and the metal atoms 

 then evaporate. If the vacuum is excellent, the atoms make no 

 collision on the way to the virus preparation and accordingly 

 produce an accurate geometrical shadow just as would be seen 

 for oblique light. The spaces in the shadow are transparent, and 

 the focusing action of the microscope produces an image which 

 gives an idea of the shape and thickness of the virus. By using 

 comparison latex particles of known size, the microscope can 

 give fairly accurate dimensions. The method has been applied to 

 many viruses, notably the T-series bacteriophages, tobacco 

 mosaic virus, southern bean mosaic virus, vaccinia, influenza, 

 and polio viruses. In Fig. 2.1 are shown electron micrographs of 

 T-1 bacteriophage, southern bean mosaic virus, and tobacco 

 mosaic virus, taken by Dr. D. J. Fluke. 



In view of the success of this technique, some of its short- 

 comings need to be discussed. The greatest of these lies in the 



