220 APPLIED IMMUNOLOGY 



mersion objective. Recently, Glynn, Rees, Powell 

 and Cox have recommended a special counting-cham- 

 ber 0.02 mm. deep, instead of the common 0.1 mm. 

 depth, with which a heavier, 0.18 mm. thick, cover- 

 slip, suitable for all achromatic oil-immersion lenses, 

 even with a free working distance as low as 0.09 mm., 

 can be used. It is claimed that with the shallower 

 chamber the bacteria settle to the bottom in fifteen 

 minutes, while with the deeper they are still in motion 

 after one-half hour, therefore, with the special hsemo- 

 cytometer they are more easily enumerated for focal 

 reasons, their definition is clearer cut and the heavier 

 cover-slip is more durable. Any hsemocytometer 

 method is more accurate than Wright's method, which 

 underestimates the number of bacteria in suspension 

 and possesses an average and maximum error three 

 times as great as the h^emocytometer. 



Wright's method consists in mixing a unit volume 

 of the bacterial suspension with an equal volume of 

 normal blood known to contain approximately 5,000,- 

 000 erythrocytes to the cubic millimetre or 5,000,000,- 

 000 to the cubic centimetre. This is diluted by the 

 addition of six or seven volumes of 0.15 per cent, 

 sodium citrate solution to prevent clotting. After 

 thorough mixing, fairly thick smears are made on 

 glass slides and stained preparatory to the count. 

 Fields throughout the smear are counted for the sake 



