406 G. S. WILSON 



this point exactly it was clear that a technique would have to be 

 evolved which would permit of as small an error as possible. 



A careful study of the Uterature seemed to show that few of 

 the methods which have been employed hitherto could claim to 

 conform to such a degree of accuracy. Generally speaking, the 

 methods which have been employed may be classified into (1) 

 the direct and (2) the indirect. In the former the organisms are 

 counted directly under the microscope, in the latter the number 

 of bacteria present is calculated from an enumeration of the 

 colonies which develop when an aUquot part of the emulsion in 

 question is mixed with a nutrient medium in a Petri dish, and 

 incubated for a variable period of time. The former is designed 

 to record the total number of organisms present, the latter only 

 the mmiber which happens to be \'iable at the moment of 

 sampUng. 



With regard to the direct or total count, Klein (1900) appears 

 to have been one of the first to realize the value of such a method 

 of estimation. His technique consisted in staining a moist prep- 

 aration of organisms with aniline gentian violet, spreading a loop- 

 ful of known capacity on a coverslip, drying, and clearing in xylol 

 balsam. The total number of fields on the coverslip was deter- 

 mined for a definite combination of lenses, fifty fields were counted 

 and the number of stainable organisms per cubic centimeter in 

 the original culture was calculated. This technique was followed 

 by Hehewerth and Eijkman, and in a somewhat modified form 

 by Anderson, Fred and Peterson. Zelikow introduced a method 

 whereby the number of bacteria was determined by estimating 

 the amount of dye they were able to adsorb from a solution of 

 fuchsin, the strength of the latter before and after adsorption 

 being determined by means of a Duboscq colorimeter. Winter- 

 berg conducted his count in a Thoma-Zeiss chamber. Wright's 

 method of counting against red blood cells is too well known to 

 need description. A modification of it was described by Harrison 

 (1905) who observed the mixture of bacteria and red cells in a 

 moist film instead of in the dried condition. Finally, a new form 

 of counting chamber — the Helbe — similar to the Thoma-Zeiss, 

 but measuring only 0.02 mm., in depth, and fitted with an opti- 



