328 ROBERT S. BREED AND W. D. DOTTERRER 



clear that plates having less than 20 and more than 400 colonies 

 are so apt to be widely discrepant that counts from plates of 

 this sort should be disregarded. There are likewise clear indi- 

 cations that plates having between 40 and 200 colonies per plate 

 are as satisfactory as any that can be selected. However the 

 results secured in this investigation do not indicate that serious 

 errors would be introduced in routine work by extending these 

 limits to 30 and 400, or even to 20 and 400, thereby lessening 

 the amount of work necessary to secure acceptable counts. 



b. New York City analyses 



Another set of data which is more satisfactory in one way 

 because of the fact that a very large number of plates were 

 made from a single sample of milk but which is also less satis- 

 factory in another way because of the fact that it is more limited 

 in its application, has been secured from a set of analyses made 

 on November 19, 1915, by five New York State laboratories, ^ 

 under the supervision of Prof. H. W. Conn. In this series 20 

 samples of the same milk were sent to each laboratory for analy- 

 sis. Four laboratories made plate counts, one making them 

 in duplicate, so that five sets of plate counts are available. These 

 were made from two dilutions of 1 : 100 and 1 : 1000 each. Two 

 plates were made for each dilution. Three laboratories made 

 microscopic counts, one making them in duplicate so that four 

 sets of these counts are available. 



The average of the accepted plate counts was 4250. The aver- 

 age of the microscopic counts of clumps, or sources, was 5590. 

 The close correspondence in results obtained by these two very 

 different methods of counting makes it very probable that the 

 total number of groups of bacteria in this milk was close to 5000 

 per cubic centimeter. The 1: 100 dilution plates gave counts 

 in which the average number of colonies on the two plates varied 

 between 24 and 125. The 1 : 1000 plates gave counts in which 

 the average number of colonies from the two plates varied be- 



2 Lederle Laboratories, North's Sanitary Laboratories, N. Y. City Board 

 of Health Laboratory, Borden's Laboratory, N. Y. Agric. Exp. Sta. Laboratory. 



