LABORATORY AND TEACHING METHODS 



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were drawn as dotted circles, thus making the areas marked "a" and "6" equal to 

 0.5 sq. cm. The colonies in several areas can be counted, an average taken, and the 

 result multiplied by the number of square centimeters in each plate. 



A fine apparatus could be made by covering a plate of glass with a uniform layer 

 of wax and with a sharp instrument cut the figure in the wax and subject it to hydro- 

 fluoric acid for a few minutes which would etch the glass where exposed. Cleaning 



Fig. 224. — Jeffer's circular counting plate for Petri dish cultures. The entire 

 area (100 sq. cm.) is marked off into the equal sectors of ten sq. cm. each. {After 

 Schneider, Pharmaceutical Bact. p. 90.) 



off the wax and placing the glass plate over black velvet, the colonies could easily 

 be counted. 



Neisser's Marking and Counting Apparatus for Bacterial Colonics. — The apparatus 

 is employed for counting bacterial colonies and for marking off their position. 



When in nsC the apparatus is mounted on the lid of the box with which it is 

 supplied, thus the latter serves at the same time as a, base. 



