56 



GENERAL MICROBIOLOGY 



9. Place the plates on a level surface until the medium 

 solidifies. 



10. Incubate at the desired temperature. 



EXERCISE 14. METHODS OF COUNTING COLONIES 

 IN PETRI DISH CULTURES 



Apparatus. Jeffer's counting plate; black glass plate or 

 cardboard; tripod counting lens, magnifying four diam- 

 eters; plate cultures. 



Note. In Jeffer's counting plate (see illustration) each division 

 has an area of 1 square centimeter. The figures denote the number 

 of square centimeters in the respective circles. 



123156789 10 



FIG. 17. Jeffer's Counting 

 Plate. 



FIG. 18. Wolfhugel'g 

 Counting Plate. 



Method. 1. Invert the Petri dish culture to be counted 

 upon the black glass plate or upon some black surface. 



Note. If liquefiers are present on the gelatin plate, place the Petri 

 dish right side up upon the counting plate; this necessitates refocusing 

 the lens. The cover may be removed to facilitate counting if the 

 plate is to be discarded. 



2. Place the counting plate upon the Petri dish, making 

 the circumference of the Petri dish coincide as nearly as 



