Fig. 8. A drop-pattern of poliomyelitis virus. X 12,000. The large spheres are poly- 

 styrene latex particles that have been mixed with the virus at a concentration of 3.2 X 

 10^" particles/ml. From the number of these particles and of the poliovirus particles in 

 this drop pattern it can be concluded that the virus was sprayed at a concentration of 

 3.8 X 1 0^1 particles /ml. 



