266 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



reduce the danger of spreaders. While clay tops of 

 Petri dishes are useful for some purposes they are not 

 recognized in the standard methods." 



Incubation. " Only one period of incubation and 

 one temperature is regarded as standard, 48 hours at 

 37.5 C. In crowded incubators ventilation should be 

 provided." 



Counting. " If, among the different dilutions, there 

 are plates containing from 30 to 300 colonies these should 

 be counted, and the number, multiplied by the dilution, 

 be reported as the final count. All colonies on such 

 plates should be counted and the numbers averaged. If 

 there are no plates within these limits the one that comes 

 nearest to 300 is to be counted. No plate that contains 

 less than twenty colonies shall be counted, unless it hap- 

 pens that there are no plates with a larger number of 

 colonies, or unless the numbers in the plates check with 

 other dilutions. If the number of colonies on the plate 

 to be counted is over 300, a part of the plate may be 

 counted, and the whole plate averaged. 



" Counting shall be done with a lens, magnifying 

 2% diameters (or what the opticians call a 3% + lens). 

 Nearsighted persons should use their glasses in counting, 

 but farsighted persons should remove them. In case it 

 is doubtful whether certain objects are colonies or dirt 

 specks they should be examined with a compound micro- 

 scope." 



Reports. " In making reports it must be borne in 

 mind that with high numbers obtained by the routine 

 method only an approximation to accuracy can be ob- 

 tained. Only the left-hand figures of the final numbers 

 are of significance. It is best, therefore, to report only 

 the two left-hand figures of the results in order to avoid 



