118 EXPERIMENTAL DAIRY BACTERIOLOGY 



off with absorbent cotton. The aspirating is continued until 

 but J- cc. of milk remains. The sediment is well mixed in 

 the liquid by means of a glass rod. 



The time of centrifugalization must be sufficient to cause 

 as complete a separation of the fat as possible, since large 

 numbers of fat globules interfere seriously with the counting, 



since they rise to the 

 top of the cell. A 

 speed and time neces- 

 sary to remove the 

 fat will be sufficient 

 to throw down all leu- 

 cocytes. 



A T h o m a-Z e i s s 

 blood-corpuscle-count- 

 ing cell is used. This 

 is formed of a heavy 

 glass slide on which is 

 cemented a square 

 piece of glass having 

 a circular hole in the 

 center. In the center 

 of this hole is cemented 

 011 the slide a piece of 



glass bearing on its upper surface a ruled area made up of 

 400 squares. The entire ruled area comprises 1 sq. mm. The 

 thickness of the glass ring is 0. 1 mm. greater than that of the 

 glass bearing the ruled area. Thus, when a perfectly plane 

 piece of glass is used as a cover, the depth of the cell over 

 the ruled area is 0.1 mm.; thus the volume of liquid over 

 the rulings is equal to 0.1 cu. mm. Each of the 400 squares 

 represents a volume of 1/4000 cu. mm. 



FIG. 17. THE RULED AREA OF THE TIIOMA- 

 ZEISS BLOOD COUNTER 



The area of all the squares is 1 sq. mm. The 



four hundred squares are separated into groups 



of sixteen by the double lines 



