Journal of Applied Microscopy. 



53 



An Apparatus to Facilitate the Counting of Colonies of Bacteria 

 on Circular Plates. 



H. W. Jeffers. 



140 



In speaking to his students concerning 

 the appliances for aiding one in counting 

 the colonies of bacteria in plate cultures, 

 Dr. Moore suggested that an apparatus 

 similar to Park's but with the surface 

 divided in addition to the circles and 

 sectors into small spaces of equal area, 

 would be especially convenient for count- 

 ing the colonies on plates made in Petri 

 dishes. 



From these remarks, there seemed to 

 be a need for an apparatus for circular 



plates corresponding to the excellent 

 one of Wolffhuegel for square or oblong 

 ones. 



Acting upon this suggestion, I have 

 constructed a figure which seems to ful- 

 fill all the requirements. It consists of 

 concentric zones which are divided into 

 small sections, each having an area of 

 one square centimeter. To determine 

 the position of the circles marked 10, 20, 

 40, 60, 100, and 140 in the diagram, whose 

 areas equal 10, 20, 40, 60, 100, and 140 



