70 AN INTRODUCTION TO THE STUDY OF VIRUSES 
hole in the centre, of such a size that the rings appear larger 
than the size of the individual virus particles. From the photo- 
eraphic density of the images it may be estimated that the crystalline 
fragments are only a few molecules thick in the direction of the electron 
beam. Evaluation of the angles and particle separation in different 
directions shows that it is a diamond-type lattice, giving a size for the 
dry particle of the virus of 19:5 mu on the assumption that it is rigid 
and undergoes no distortion on entering the lattice (Cosslett and 
Markham, 1948). 
In concluding this short account of the electron microscopy of 
viruses we may appropriately quote these words of Wyckoff (1946)— 
We span that range of organized matter which extends from the animate 
to the lifeless and which must be understood as a basis of what we intuitively 
mean by “living”; at the same time we acquire the ability to “see” the 
larger of the molecules that are the basic units of chemistry. It rarely 
happens that any new experimental technique allows as direct an approach 
to the problems of a single science as the electron microscope thus gives 
to the fundamentals of both chemistry and biology. 

