244 CLINICAL BACTERIOLOGY AND ILEMATQLOGY 



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. 49). 



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

 if the blood was taken from a patient with leucocytosis or 

 leucocythaemia there will be many. They may be dis- 

 tinguished from the red corpuscles by their greater refrac- 

 tivity, 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 num- 

 ber 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 ^ millimetre deep, ^V millimetre wide, and 

 ^o millimetre long; its cubic capacity is therefore yV x A X OT 

 == win7 cubic millimetre. Therefore the ^Vo part 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 T ^ part of j^Vo cubic millimetre. 



Therefore the number of corpuscles which has been deter- 

 mined as being the average per square is contained in T ^OTT 

 of T ^o 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. 



It may be expressed as a formula, thus : 



If n is the total number of corpuscles counted, 



j is the number of squares counted, 



and if the dilution is i in d, 



then the number of corpuscles per cubic milli- 



. n , 

 metre is - x d x 4,000. 



