LABORATORY AND TEACHING METHODS 



629 



were drawn as dotted circles, thus making tlie 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 



lO^T — .TT. 



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, P-harmaceutical Bad. 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 use the apparatus is mounted on the lid of the bo.x with which il is 

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



