NUMERICAL DETERMINATION OF BACTERIA. 



149 



COUNTING. 



Expression oj results. Since minor differences in milk counts are 

 within the working error of the methods and are of no significance in 

 practice, the following scale has been adopted for recording results of 

 market milk examinations : 



Counts below 100,000 are distinguished by ten thousands. 



Counts between 100,000 and 500,000 are distinguished by fifty thou- 

 sands. 



Counts between 500,000 aud 1,000,000 are distinguished by hundred 

 thousands. 



Counts between 1,000,000 and 2,000,000 are distinguished by two hun- 

 dred thousands. 



Counts between 2, 000, 000 and 5,000,000 are distinguished by five hun- 

 dred thousands. . 



Counts above 5,000,000 are distinguished by millions. 



Therefore only the following figures are used in reporting : 



Below 10,000 Above 

 Above 10,000 



20,000 



30,000 



40,000 



50,000 



60,000 



70,000 



80,000 



90,000 

 " 100,000 

 " 150,000 

 " 200,000 



Counts on ' ' certified " or " inspected ' ' milk shall be expressed as 

 closely as the dilution factor will allow. 



The whole number of colonies on the plate shall be counted, the prac- 

 tice of counting a fractional part being resorted to only in case of neces- 

 sity, such as partial spreading. 



Various counting devices have been recommended by different work- 

 ers. The more simple ones, where the whole plate can be seen at once, 

 are more desirable on account of there being less likelihood of recount- 

 ing colonies. Colonies too small to be seen with the naked eye or with 

 slight magnification shall not be considered in the count. 



