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 80 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 
21% diameters (or what the opticians call a 314 + 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 
