236 CLINICAL BACTERIOLOGY AND H^EMATOLOGY 



nearest the bulb. These pipettes are used in the same way, 

 and it is quite immaterial which is obtained ; we shall describe 

 the use of the first form.* 



The rationale of the method is this : Blood is sucked up to 

 one of the divisions on the lower part of the stem, and then 

 an inert diluting fluid is drawn up to the single mark above 

 the bulb, and the two mixed by rotating the whole apparatus 

 for a minute or two. This gives us a dilution of blood of 

 definite strength, the exact amount of dilution depending 

 upon the amount of blood which was taken. Thus, if blood 

 had been drawn up to the figure i, we should have a dilution 

 of i in 100; while if blood had been drawn up to the figure 

 5, the dilution would be O'5 in 100, or i in 200, and so on. In 



FIG. 46. THOMA'S H^EMOCYTOMETER. 



the case of the other form of pipette the dilution is read off 

 directly from the figures on the lower stem. 



The diluted blood thus obtained is spread out in a film of a 

 definite known thickness on the slide supplied on the instru- 

 ment (Fig. 46, a). This is ruled in squares, and the squares 

 are of known size. The amount of blood lying upon each 

 square is thus known, and the- number of corpuscles which 

 lie upon it being" counted under the microscope, all the data 

 for the calculation are obtained. 



In blood examinations it is absolutely necessary that all 

 points in the technique should receive the most careful atten- 

 tion, or the result will be worse than useless. For this reason 

 we shall describe each step in the process at some length, 

 * The latter form is almost obsolete. 



