184 BACTERIOLOGY. 



his apparatus may be employed. It consists in dividing 

 the tube by lines into four or six longitudinal areas which 

 are subdivided by transverse lines drawn about 1 or 

 2 cm. apart The lines may be drawn with pen and 

 ink. They need not be exactly the same distance apart, 



FIG. 29. 



or exactly straight. Beginning at one of these squares 

 at one end of the tube, which may be marked with a 

 cross, the tube is twisted with the fingers, always in one 

 direction, and the exact number of colonies in each square 

 as it appears in rotation is counted, care being taken not 

 to count a square more than once ; they are then added 

 together, and the result gives the number of colonies in 

 the tube. This method may be facilitated by the use of 

 a hand-lens. 



In all these methods there is one error that is difficult 

 to eliminate ; it is assumed that each colony represents 

 the outgrowth from a single organism. This is prob- 



