114 MICROBIOLOGY 



each of several (20 to 40) of the small areas, and ascertain 

 the average number on one area from which the number of 

 colonies on the plate can be approximately determined. 



For dividing the area of the plate into smaller, equal 

 areas, it is convenient to use WolffhiigeVs counting appa- 

 ratus. This was devised more particularly for square or 

 oblong plates (Koch). In counting the colonies on the Petri 

 dishes Parkes' 2 scheme modified by Jeffers 3 is more suitable. 

 It consists of a disk about 20 cm. in diameter, divided into 

 areas of a square centimeter each. Place the Petri dish over 

 the disk, taking care that it is centered. 



Count the number of colonies in several (10 to 40) of the 

 areas and multiply the mean number by the number of areas 

 covered. This product gives the approximate number of 

 colonies on the plate. 



Fig. 39. Characters of surface ele- 

 vation: 1, flat; 2, raised; 3, convex; 

 4, pulvinate; 5, capitate; 6, umbilicate; 

 7, umbonate. 



Examination of cultures in test tubes on solid media. In 

 examining the growth of the organism on slant agar the 

 extent of its development (whether feeble, moderate or vig- 

 orous), color, form, and surface appearance (whether glis- 

 tening or of a dull appearance), should be observed. The 

 character of the growth in the condensation water which 

 forms at the bottom of the inclined surface should be noted. 

 In stab cultures the appearance of the growth both on the 

 surface and along the needle track should be noted. In 



2 Parkes, Journal of Pathology and Bacteriology, Vol. IV, p. 173. 



3 Jeffers, Journal of Applied Microscopy, Vol. I, No. 3, 1898. 



