The Bacterial Suspension 



309 



blood and three volumes of physiological salt solution. 

 After thorough mixing a smear is made upon a slide, the 

 smear stained, and the number of bacteria and corpuscles in 

 successive fields of the micro- 

 scope counted until at least 

 200 red blood-corpuscles have 

 been enumerated. As the 

 number of red corpuscles per 

 cubic millimeter of blood is 

 5,000,000, the number of bac- 

 teria per cubic centimeter can 

 be determined from the re- 

 sults of the counting by a 

 simple arithmetical process. 

 To facilitate the counting the 

 eye-piece of the microscope is prepared by the introduction 

 of a diaphragm shown in Fig. 90. The prepared suspen- 

 sion must usually be greatly diluted before using, but the 



Fig. 90. Diaphragm of eye- 

 piece showing hairs in position 

 (Miller). 



Fig. 91. Photomicrograph showing cross-hairs, bacteria, and red blood- 

 corpuscles (Miller). 



reduction of bacteria is, of course, easily calculated. It re- 

 quires experience to determine the appropriate number of 

 bacteria to be employed. When this is once determined, 



