METHODS OF EXAMINING MILK 269 
of oculars or adjusting the draw tube, or both, bring the 
diameter of the whole microscopic field to .205 mm. 
When so adjusted the microscopic field will cover almost 
exactly 1/300,000 of a cubic centimetre of the milk 
(actually 1/302840). This means that if the bacteria 
in one field only are counted the number should be 
multiplied by 300,000 to give the total number in a cubic 
centimetre. If the bacteria in a hundred fields are to 
be counted the total should of course be multiplied by 
8000. 
“Inasmuch as it is difficult to count bacteria lying 
near the margin of the microscopic field, it is much better 
to have an eyepiece micrometer with a circular ruling 
8 mm. in diameter and divided into quadrants. This 
will give, in the microscopic field, a smaller area within 
which the bacteria may be seen most sharply, and which 
may be more easily counted. Such eyepiece micrometers 
are now manufactured by laboratory supply houses and 
may be easily obtained. In the use of this eyepiece 
micrometer the inner circle, by the adjustment of the 
draw tube, should be made to cover a circle with a diam- 
eter of .146 mm. In this case this inner circle will cover 
1/600,000 of a cubic centimetre of milk, meaning, of 
course, that the number of bacteria in a single field should 
be multiplied by 600,000, or, if a hundred fields are 
counted, by 6000, to obtain the number per cubic centi- 
metre. 
“ The number of microscopic fields to be counted will 
depend somewhat upon the kind of data that is desired. 
If this method is to be used simply for the purpose of 
dividing milk into grades, it will in most cases be un- 
necessary to do the actual counting, since a Grade A 
milk will show field after field without any bacteria at 
