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 

 3000. 



" 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 



