130 Report of the Department of Bacteriology of the 



The counting of the cells was done under an oil-immersion lens 

 where the area of the field was 0.0002 of a square centimeter." 

 This is obtained by adjusting the tube length of the microscope 

 until the diameter of the field as measured by a stage micrometer 

 is 1G millimeters. The need of making this adjustment carefully 

 should be emphasized became of the careless way in which several 

 Drevio»= investigators have referred to the field of an oil-immersion 

 lens as if it had a standard size. With all oil-immersion lenses it is 

 easily possible to secure fields much larger or much smaller than the 

 one designated by using different tube lengths and different oculars. 



The distinctness with which the cells show makes it possible to 

 use the whole field of the microscope even though the edge of the 

 field is hazy and indistinct. 



In all except a few of the counts here given, one hundred fields 

 of the microscope were counted on each of the duplicate smears 

 and the results so obtained averaged together. The number thus 

 obtained multiplied by 5,000 gave the number of cells per cubic 

 centimeter. All of the smears were made and counted by the author 

 himself unless otherwise noted. 



Samples were ordinarily taken from the pail of milk just as the 

 milking was completed, after careful stirring. The samples were 

 then taken to the laboratory at once and the smears made before 

 the cream began to form. All samples were thoroughly shaken at 

 the time the smears were made in order to prevent a concentration 

 of the cells at the top. 



I. CELL CONTENT OF NORMAL MILK. 



A. CELL CONTENT OF THE MILK OF THE ANIMALS IN THE STATION HERD. 



When the study of the cell content of the milk of the animals 

 in the Station herd was started, a preliminary examination was made 

 of samples taken as described above on the evenings of February 10, 

 13 and 14, 1911. The results of these tests are given in the column 

 of Table II which bears the caption " Count No. 1." After this 



« An unfortunate error crept into the first paper giving an account of this method 

 by Prescott and Breed (see footnote 25) where we state (page 634, 1. 14) that th, 

 area of the field covers approximately 0.005 sq. cm. This should read 0.0002 or 

 1/5000 sq cm Fortunately the remaining portion of the calculation is printed cor- 

 rectly so that the error is readily detectable. 



