268 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



If you exclude the spaces which are thus marked with a double 

 line, the whole area will be marked out into a series of large 

 squares, each consisting of 4x4=16 smaller squares (Fig. 44). 

 It is convenient to count the smaller squares in these groups of 

 sixteen. At least a hundred of the smaller squares — i.e., six of the 

 large groups and four small squares — should be counted. 



In counting one of the smaller squares it is convenient to begin 

 with the corpuscles which are lying in the middle of the square, 

 and then to count those which are lying on the lines. In dealing 

 with these you count those which are lying on the upper and left- 

 hand lines as being within the square, and those that are on the 

 lower and right-hand lines as being without it ; if you like you may 

 reverse this, but you must keep to the same method throughout 

 (see Fig. 44). 



A few white corpuscles will be met with in every case, while 

 if the blood was taken from a patient with leucocytosis or leuco- 

 cythsemia there will be many. They may be distinguished from 

 the red corpuscles by their greater refractivity, or, if a stain has 

 been used in the diluting fluid, by their being faintly tinged. It 

 is scarcely necessary to say that they should not be counted. 



6. The Calculation. — -The best way of calculating the number of 

 corpuscles present from the data thus obtained is the following : 



First add up the number of corpuscles in all the squares which 

 you have counted, and divide the sum by the number of squares 

 counted. This gives the average in each square. 



Now the space enclosed between each square and the 

 cover-glass above it is J„ millimetre deep, ^jj millimetre 

 wide, and -^-^ millimetre long ; its cubic capacity is therefore 

 iV '^ ^ ^ Tij = TWO cubic millimetre. Therefore the 4 qVtt P^''^ of a 

 cubic millimetre contains the number of corpuscles which we 

 have already found as the average. 



But the square contained diluted blood ; if the amount of 

 dilution was i in 100, the amount of blood contained in the space 

 over each square was y^ part of j^Vtt cubic millimetre. 



Therefore the number of corpuscles which has been determined 

 as being the average per square is contained in ^5^ of -^^ cubic 

 millimetre of undiluted blood, the dilution being taken as i in 100. 



Hence the number of corpuscles in i cubic millimetre of 

 undiluted blood is obtained by multiplying the average per square 

 by the number which expresses the dilution (in this case 100), 

 and then by 4,000. 



