CHAPTER VII 
ELECTRON MICROSCOPY: OB. VIRUSES 
Since the limit of resolution of the optical microscope in visible light 
is about 200 my, an object must be at least 250 my in diameter to be 
properly resolved. This is far larger than most animal, and so far as 
we know, all plant viruses. 
The wave-length of ultra-violet light which can be used with a 
quartz objective is not much below 2500 A and so only the larger 
viruses of 100 my and above can be measured by this means. For 
the examination of the small viruses, therefore, it is clear that a radia- 
tion with a wave-length shorter than that of light is necessary. In 
the electron microscope the illuminating radiation is provided in the 
form of a swiftly-moving beam of electrons which corresponds to 
the beam of light in the optical microscope. 
The “lenses” of the electron microscope cannot be made of glass 
which is opaque to the electron beam but electric or magnetic “lenses”’ 
are used instead, to focus the electrons. Because electrons are scattered 
by gas molecules it is necessary for the electron optical paths to be 
evacuated and the specimen must be dry. The specimen is mounted 
on a thin film, usually of collodion, supported on a fine metal grid. 
The focal length of a magnetic lens is a function of the current 
through the lens coil; hence the magnification is controlled by varying 
the current through the magnetic lenses. It is necessary, of course, 
for the current to be constant to avoid blurring of the image and to 
obtain high resolution. The source of electrons is a hot filament and 
the beam of electrons is focused on to the object by the first, or 
condenser lens, coil. Electrons from the central portion of the inter- 
mediate image formed by the objective or second magnetic lens coil 
are re-imaged by the third or projection lens coil and the final highly 
magnified image of the specimen may be viewed directly on a 
fluorescent screen or photographed by means of a film sensitive to 
electrons (Stanley, 1943) (Plate XI). 
A great improvement in the technique of electron micrography 
has been described by Williams and Wyckoff (1946). This is known 
as metallic shadow-casting, and the method is, briefly, as follows: 
The sample is coated with a semi-transparent layer of metal deposited 
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