80 AN INTRODUCTION TO THE STUDY OF VIRUSES 
the convection currents kept the tubes agitated. The bath was illumi- 
nated from behind in order that the tubes could be continuously 
observed. The optimal tube was taken as that which first showed 
any visible flocculation and the amount of serum is directly pro- 
portional to the amount of antigen. Bawden found that a close 
relationship existed between the antigen content, as measured by the 
optimal flocculation point with antisera, and virus content as measured 
by an inoculation method. 
