COUNTING THE RED CORPUSCLES 7 



hundred squares. It is well to count blocks of 25 squares in each 

 corner of the ruled space. Clean the counting chamber and cover 

 glass, place another drop and count the corpuscles in another 

 100 squares. The pipette should be shaken for a minute and a 

 few drops expelled before placing the second drop in the counting 

 chamber. If the two counts do not agree closely count another 

 100 squares in another drop. 



Computation. Divide the number of corpuscles counted by 

 the number of squares counted, divide this by the dilution and 

 multiply this quotient by 4000 as each square represents 40W 

 of a cubic millimeter of blood. The result will be the number of 

 corpuscles per cubic millimeter. 



No. Cornuscles counted x dilution x 4000 Xo. of corpuscles per 

 ^ Xo. of squares cmm. 



For example suppose 1321 corpuscles were counted in 200 squares 

 with blood diluted .5:100. The computation would be: 

 1321 x 1000 x 4000 



200x5 



52S4000 



The blood examined has 5,284,000 red corpuscles per cmm. 



Limit of error. U^ing such a high dilution and such a small 

 quantity of blood the error is at best rather high. Thoma and 

 Lyon obtained an error of five per cent, in counting 200 corpuscles, 

 two per cent, with 1250 corpuscles and one-half per cent, with 

 20,000 corpuscles. A variation of 100,000 corpuscles is not or- 

 dinarily important. This is close enough for clinical purposes. 



Cleaning the apparatus. It is important to clean the ap- 

 paratus as soon as the counts are made. If blood dries in the 

 pipette it may take several hours to remove, while if cleaned 

 promptly, it requires only two or three minutes. The counting 

 chamber is to be cleaned with pure water only. As the plate and 

 disc are cemented to the slide by Canada balsam, alcohol or any- 

 thing that will act on balsam must not be used. The cover glass 

 may be cleaned with water, then with alcohol. For drying the 

 counting chamber and cover glass about the best thing is a linen 

 handkerchief that is practically worn out. A new one is too harsh. 

 Japanese lens paper may be used but it is not so convenient as 

 soft linen. Anything more harsh than lens paper or old soft linen 

 should not be used. The pipette should be cleaned, after expelling 



