44 



BACTERIOLOGICAL METHODS 



90 cc. of distilled water to 10 grams (or 10 cc.) of the substance. 

 Place the thumb firmly over the opening of the graduate; the con- 

 tents are thoroughly mixed by shaking vigorously for about 20 sec. 

 By means of a slender glass rod slipped well into the mixture, take 

 up a droplet of the mixed material and touch the end of the rod 

 lightly and quickly upon the middle of the ruled area of the hem- 

 acytometer. All this must be done rapidly, before the organisms 



FIG. 7. FIG. 



FIG. 7. Zappert ruling of the Thoma-Zeiss hemacytometer. This form of 

 ruling is especially convenient for making bacterial counts and counts of fat globules 

 in milk. (Carl Zeiss.) 



FIG. 8. Turck ruling of the Thoma-Zeiss hemacytometer. This is especially 

 useful if it is desired to combine the bacterial count with the spore and yeast count. 

 The smaller areas (1-400 sq. mm.) may be used for making the bacterial counts, while 

 the larger areas (1-25 sq. mm.) may be used for making the spore and yeast counts. 

 (Carl Zeiss.) 



have had time to settle to the bottom of the graduate, and before 

 they have had time to accumulate at the end of the glass rod. 



Making the Count. After having cleaned the hemacytometer 

 (do not use alcohol), it is sometimes desirable to rub a very soft, 

 grit-free graphite pencil over the ruled area so as to render the 

 lines more readily visible. Usually, however, this is not necessary. 

 After placing the droplet of material as above described, cover 

 with a No. 2 cover glass and orientate the ruled area by 



