96 ESSENTIALS OF BACTERIOLOGY 



Counting. Two crossed hairs are placed on the dia- 

 phragm in the eye-piece of the microscope and the slide 

 examined under the oil-immersion lens. The number of cor- 

 puscles and bacteria in a number of fields are counted until 

 at least 200 red corpuscles have been enumerated. As the 

 number of corpuscles per cubic centimeter is 5,000,000,000 

 by simple proportion, the number of bacteria per cubic centi- 

 meter can be determined. For example, 200 red corpuscles 

 and 150 bacteria are counted in the same fields. Then 



200 corpuscles : 150 bacteria : : 5,000,000,000 : x x = 3, 750,000,0000 

 (Number of corpuscles is to number of bacteria as the total number 

 of corpuscles in a cubic centimeter is to the quantity to be determined.) 



Any number of bacteria per cubic centimeter can then be 

 obtained by simple dilution with sterile normal saline solu- 

 tion. When the final dilution is made, 0.2 per cent, of tri- 

 kresol is added as a preservative. 



