66 AN INTRODUCTION TO THE STUDY OF VIRUSES 
obliquely by evaporation in a vacuum. To do this the collodion film 
bearing the virus or other particles is put on an ordinary glass micro- 
scope slide. The slide is then fixed inside a suitable container, such as 
a bell jar made of thick glass, at an oblique angle to a filament charged 
with a weighed amount of the metal to be evaporated. The air is then 
evacuated and, when the vacuum is complete, the metal filament is 
heated to incandescence by an electric 
current which volatilizes the metal 
and deposits it obliquely on the virus 
particles (see Fig. 2). Various metals 
have been used, particularly gold, 
chromium, nickel, and uranium, but 
gold seems to be the one most suitable 
for small virus particles though it is 
prone to form artefacts when viewed 
(Afier Wyckoff) in the electron microscope (Mandle, 
Fic. 2. A Drawinc To Outune 1947). This gives a three-dimensional 
THE EXPERIMENTAL PROCEDURE : : 
RR OTe mG “pea effect to the image so that the heights 
Metal evaporated in vacuum from a or thicknesses of discrete objects 
hot filament A deposits obliquely é 
upon the specimen B which are not wholly flat can be 
estimated from the lengths of the 
shadows they cast. Shadow-casting brings out in great relief 
the surface contours of the specimen by enhancing the scattering power 
of the entire preparation. The type of detail brought out by shadowing 
can be varied by altering the thickness of the evaporated layer. In 
addition it makes clearly evident objects so small or thin that by | 
themselves they would not produce a minimum contrast against the 
background of the rest of the specimen (Williams and Wyckoff, 1946). 
Photographs of viruses taken by the shadow-casting method are 
illustrated in Plates XII, XIII, and XIV. One result then of the 
application of electron microscopy to the study of viruses has been to 
increase greatly our knowledge of the morphology of virus particles. 
But the electron microscope does more than define the shapes and 
sizes of virus particles; it can also reveal the apparent structure of the 
larger viruses. 
Under dark ground illumination with the optical microscope, 
vaccinia virus appears as a spherical particle, but when photographed 
by the electron microscope the particles are seen to be brick-shaped 
with rounded corners. Occasionally a particle is photographed 
standing on end and it appears to be circular in cross-section. Dense 

