Frost—Bacteriological Control of Public Milk Supplies . 1313 
sample. Unless this is done it is necessary to make more than 
one dilution of each sample of milk. 
c) Use of Pipettes. 
I. Different Forms. The pipettes used for milk and water 
work vary in length, diameter and shape. The longer and nar¬ 
rower, of course, the more accurate the graduations. Some of 
the pipettes used are graduated with one mark and deliver 1 c.c. 
Others have two marks the right distance apart to deliver one c.c. 
Frequently the graduations are in fractions of a c.c. as tenths or 
hundredths. Some workers also measure fractions of a c.c. by 
counting the drops delivered to the c.c. by a one or two mark 
pipette, and then take the desired number of drops. There 
should, perhaps, be no dogmatic statement made in regard to 
the size, shape, graduation, etc., of the pipette. But it has seemed 
worth while to make some comparisons of the different forms, 
and especially the influence of the point or delivery end, since 
some laboratories have recently introduced and used perfectly 
straight tubes, not at all narrowed at the end. See fig. 4. 
Fig. 4. Forms of pipettes. A. Ordinary form of pipette used in milk and 
water work. B. Form suggested for use with “Pipettometer”. D. Pipette 
used in New York City Board of Health. 
The accuracy of these was tested in the following manner: 
One c.c. of a certified milk was added to 99 c.c’s of sterile wa¬ 
ter, thoroughly shaken, and then 4 c.c’s were taken out consecu¬ 
tively and plated, using first a pipette of ordinary form, and 
