THE MANUFACTURE AND USE OF SERA AND VACCINES. 137 



Fig. 61. — Counting the bacteria in standardizing bacterins. This chart shows the 

 count of bacteria and of red blood cells in twenty successive fields of the microscope. The 

 number of red cells counted (308) is to the number of bacteria counted (224) as the number 

 of red cells per cubic centimeter in normal blood (5,000,000,000) is to the number of 

 bacteria per c.c. in the suspension (3,636,000). This count (3,636,000) divided by the 

 count desired in the final dilution (400,000,000) gives the number of times (9) this sus- 

 pension must be diluted to bring it to the desired dilution. {Parke, Davis &= Co.). 



