ESMARCH'S COUNTER. 183 



In Fig. 28 is seen the form of hand-lens commonly 

 employed. 



A plan that is frequently given for the counting of 

 colonies by the use of these devices is to count the num- 

 ber of colonies in each of a number (eight or ten) of the 

 squares, and take the average of these counts as the 



FIG. 28. 



average for each square on the whole surface of the 

 gelatin. The result is then obtained by multiplying 

 this average by the number of squares taken up by the 

 whole surface of the gelatin. 



The results vary so much in different counts of the 

 same plate, when made in this way, that they can hardly 

 be considered approximate. 



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. 



ESMARCH'S COUNTER. Esmarch has devised a coun- 

 ter (Fig. 29) for estimating the number of colonies pres- 

 ent when they are upon a cylindrical surface, as when in 

 rolled tubes. The principles and methods of estimation 

 are practically the same as those given for Wolff hiigel's 

 apparatus. If the number of colonies in an Esmarch 

 tube is to be determined, a simpler method to the use of 



