270 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
all, while a Grade C milk will show the field crowded with 
bacteria. In all doubtful cases, however, counting should 
be done, and there should never be less than thirty fields 
counted in order to have reliable results. Counting thirty 
fields is not so tedious a task as it would seem to be, since 
in ordinary milk the number of bacteria in each field is 
small, and the counting may be done very rapidly.” 
Counting. —“ Counting the bacteria in such a smear 
may be done in two ways: 1. The number of groups of 
one or more bacteria present. 2. The number of indi- 
viduals. The second, of course, is really the correct count 
of the number of bacteria, but the former will give a 
count much closer to that obtained by the plate count, 
since the colonies upon the plate represent groups of bac- 
teria rather than individuals, each group growing into 
a single colony only. Extensive tests have shown that 
there is a fair correspondence between the number of 
groups reported by experienced observers and the num- 
ber of colonies that may grow in plates made from the 
same milk, although there are occasionally discrepancies 
of considerable extent. These discrepancies are caused 
by variations in judgment as to what constitutes a 
group, variations in the extent to which groups break 
up in the dilution waters when the smears are made, 
and the presence of dead bacteria or of bacteria which do 
not grow on the plates. Some experience is needed by 
the microscopist in determining just what should be 
counted. In high-grade milks an inexperienced person 
is apt to fail to recognize differences between bacteria 
and other minute objects. This results as a rule in an 
overcount by inexperienced men. In milk containing 
many readily recognizable bacteria in each field the in- 
