500 -B^ CTERIOL OGY. 



phtalein neutral point — i.e., a feebly acid nutrient gel- 

 atin, and 500 colonies on a gelatin so alkaline as to 

 require 20 c.c. of a normal acid solution to bring it 

 back to the phenolphtalein neutral point. 



Throughout this part of the work it is to be borne 

 in mind that when one refers to plates it is not to a 

 set, as in the isolation experiments, but to a single 

 plate. 



Method of Counting the Colonies on Plates. 

 — For convenience in counting colonies on plates or in 

 tubes it is customary to divide the whole area of the 

 gelatin occupied by colonies into smaller areas, and 

 either count all the colonies in each of these areas and 

 add the several sums together for the total, or to count 

 the number of colonies in each of several areas, ten 

 or twelve, take the mean of the results and multiply 

 this by the number of areas containing colonies. This 

 latter procedure obtains, of course, only when all the 

 areas are of the same size. 



By this latter method, however, the results vary so 

 much in different counts of the same plate that they 

 cannot be considered as more than rough approxima- 

 tions. 



Note. — Prepare a plate; calculate the number of 

 colonies upon it by this latter method. Now repeat 

 the calculation, making the average from another set 

 of squares. Now actually count the entire number of 

 colonies on the plat,e. Compare the results. 



For facilitating the counting of colonies several very 

 convenient devices exist. 



