BACTERIOLOGICAL STUDY OF WATER. 455 



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 plate. Compare the results. 



For facilitating the counting of colonies several very 

 convenient devices exist. 



WOLFFHUGEL'S COUNTING APPARATUS. This ap- 

 paratus (Fig. 92) consists of a flat wooden stand, the 

 centre of which is cut out in such a way that either'a 



PIG. 92. 



WolffhiigeFs apparatus for counting colonies. 



black or white glass plate may be placed in it. These 

 form a background upon which the colonies may more 

 easily be seen when the plate to be counted is placed 



