Frost—Bacteriological Control of Public Milk Supplies. 1319 
developed in a later paragraph, where it is shown that a curve 
representing the rate of development of the colonies in a good 
milk is quite different from the rate of development in a poor 
milk. See p. 1337. 
4. Counting Colonies. 
a) Methods. The plates were all counted under a five inch 
reading glass, and all of the colonies were counted including the 
lactic acid or pin-point colonies. Where possible, the entire 
plate was counted. On heavily seeded plates as large a propor¬ 
tion was counted as possible. The plates were always run in 
duplicate and the average taken. A self-registering hand 
counter was used to minimize errors due to personal equation. 
b) A New Form of Counting Apparatus. During the course 
of the work a new form of counting apparatus was constructed 
which may be briefly described as follows: It consists of two plat¬ 
forms. one below for the Petri dish, B., and one above to hold the 
magnifying lens R., Fig. 1, Plate XCV. The dish is supported 
on a screen of fine wire with one centimeter meshes instead of 
the usual ruled glass plate. The dish is centered by a mechani¬ 
cal arrangement which at the same time automatically indicates 
the area of the dish on a scale at the right, E. The lens is a five- 
inch reading glass. A metal shield prevents reflections from the 
surface of this. Below the lower platform is a black surface 
which furnishes the proper background. It is also provided 
with a self-registering hand counter, F. 
C. Tests for Bacillus Coli in Milk. 
I. Methods. 
a) Uses of Fermentation Tubes. 
I. Lactose Peptone Bile. This was prepared as recommended, 
put in Smith tubes, in which 25 c. c. were used, or in Durham 
tubes, where 10 c. c. were required. The inner tubes, in the latter 
ca^e, were one dram vials 65 mm. long. In either case readings 
were made after 24 and 48 hours of incubation at 37° C. For 
measuring the amount of gas, a gasometer was used for the Dur¬ 
ham tubes, as well as for the Smith tubes. Fig. 7. 
II. A. New Medium—Neutral Red Milk Broth. In this medium 
the milk sugar is furnished by the mdk itself. It was this idea, 
