408 G. S. WILSON 



Zelikow, Neisser, Lane Claypon, Penfold, and ]\Ioore (1915). 

 Graham Smith used a dissecting microscope, while Frost (1921) 

 has evolved a microplate method for purposes of rapid enumera- 

 tion of organisms in milk. The foreging summarj^ of the litera- 

 ture is only intended to indicate the main outlines which have 

 been followed, a complete survey lying beyond the scope of this 

 paper. 



In perusing the results of previous workers, it was striking to 

 observe the peculiar lack of attention which was paid to the 

 estimation of the experimental error involved in the methods 

 employed. Probably this is to be attributed to the fact that in 

 many cases in which the enumeration of bacteria was undertaken, 

 a relative, rather than an absolute accuracy was essential. It 

 was felt that the successful accomphshment of this object could 

 only be attained by working out a technique in which the errors 

 inherent in every step should be known with certainty. The ful- 

 filhnent of this requirement has been kept constantly in mind 

 throughout the course of the present work. 



Technique of the total count 



After careful consideration and personal experience of several 

 methods — especially those of Wright and of Brown and Kirwan 

 (1915) — it was decided to adopt the method which has been in 

 use in this laboratory during the past two years. Practically, 

 this resolves itself into the adaptation of the Helbe counting 

 chamber to dark-ground illumination. Instead of examining the 

 organisms in a stained condition with open illumination they are 

 observed in their natural state against a dark background. For 

 use with a parabolloid condenser, the only alteration of the cham- 

 ber which is required, is a selection of a shde of such a thickness 

 that the distance between the lower surface of the chamber and 

 the upper surface of the condenser shall lie between 0.9 and 1.1 

 mm. With a slide of other dimensions than these the correct 

 focussing of the rays of light becomes impracticable. The depth 

 of the chamber is 0.02 mm., while the surface is ruled into small 

 squares.^ The best combination of lenses has been found to be 



'■ A slide of the dimensions quoted was made by Messrs. Hawksley & Son. 



