60 AN INTRODUCTION TO THE STUDY OF VIRUSES 
restored either by filtration through collodion membranes or by high- 
speed centrifugation. 
Other plant virus inhibitors include the sap of certain plants such as 
Phytolacca and also the enzyme trypsin. The effect of trypsin on 
tobacco mosaic virus is reversible and infectivity can be restored by 
heating. This proves that the inactivity of the virus is not due to 
proteolysis. There seem to be two possible explanations of the 
inhibition of the activity of some plant viruses by certain substances; 
one is that the effect is on the plant rather than the virus and the other 
is that, in the case of trypsin at all events, some kind of reversible 
complex is formed between virus and inhibitor. 
In animal viruses a somewhat similar phenomenon occurs with 
the papilloma virus in domestic rabbits and there is an inhibitor 
associated with the chicken tumour I virus which Claude (1939) 
considers to be a protein. 
Absorption Spectra 
Ultra-violet light absorption spectrum measurements have been 
made on purified preparations of several viruses and they show in 
general an absorption maximum at about 2600 A and a minimum 
about 2500 A. This is what would be expected of substances containing 
nucleic acid, which is known to absorb strongly at 2600 A. 
The amount of the absorption is related to the nucleic acid content, 
for tobacco ringspot virus with a nucleic acid content of about 40 
per cent absorbs most strongly, and bushy stunt, tobacco necrosis, 
chicken tumour I, potato virus X, and tobacco mosaic virus prepara- 
tions with nucleic acid contents of about 20 to about § per cent absorb 
proportionally less (Stanley, 1940"). 
Size and Shape of Viruses 
There are six main methods of measuring the size of virus particles; 
these are by (1) ultra-violet light and electron microscopy, (2) X-ray 
diffraction, (3) sedimentation and diffusion, (4) ultra-filtration, (5) 
radiation inactivation, (6) measuring the light scattered by solutions 
of viruses. 
The most direct method of finding the size of a virus is by ultra- 
violet light or the electron microscope. The great merit of the use of 
the electron microscope is that the measurement of size is direct and 
is not subject to error due to the necessity of interpreting the measure- 
ments by theoretical formulae. 
