84 Report of the Department of Bacteriology of the 



160 millimeters, which gave a field of such an area as to make 4,000 

 fields in one square centimeter and, since 0.01 of a cubic centimeter 

 of milk was taken, then each bacterium seen in one field represented 

 400,000 bacteria per cubic centimeter. For careful quantitative 

 work it was necessary to count several fields. Then if n equaled the 

 number of fields counted and m the total number of bacteria found, 

 the number per cubic centimeter was calculated thus: 



400,000 AT u u 



X?n = JN umber bacteria per cubic centimeter. 



The factor necessary for the transformation of results to the 

 cubic centimeter basis is not the same for all microscopes and varies 

 in the same microscope according to the lenses used and to the 

 length of the draw tube. It must therefore be accurately computed 

 for each case. To do this, it is necessary to determine the radius 

 of the microscopic field in millimeters with a stage micrometer 

 and calculate its area by the formula n R 2 . Then if x equals the 

 area of the smear in square millimeters and if 0.01 of a cubic centi- 

 meter of milk is used, the following formula may be derived to obtain 

 y, which is the factor necessary to transform the number of bacteria 

 found in one field of the microscope into terms of bacteria per cubic 



x 

 centimeter. ^ — X 100=?/. To simplify the calculation of 



the number of bacteria per cubic centimeter, it is desirable that y 

 consist of as many ciphers as possible. Therefore it is recommended 

 that the draw tube be so placed as to obtain some such a factor as 

 was employed in this work: i. e. 400,000. Then, if the draw tube 

 is always placed at this mark, the same factor can be used for all 

 subsequent work. Convenient factors will be obtained if the length 

 of R be 0.101 millimeter or 0.08 millimeter. 



A Sedgwick-Rafter eye-piece micrometer, or similar eye-piece mi- 

 crometer, is recommended for use where large numbers of bacteria are 

 present. This is so ruled that it shows a large square sub-divided into 

 quarters, one of which is further sub-divided. The area of the large 

 square is different from that of the whole microscopic field and conse- 

 quently the factor necessary for computation is different. The factor 

 can be determined by a modification of the formula given above. 



In comparatively fresh milk where the bacteria are few, it is more 

 convenient to count the whole microscopic field. 



