144 MANUAL OF BACTERIOLOGY. 



This observation illustrates the fact that there are doubtless 

 many kinds of bacteria that do not find favorable conditions 

 for development on ordinary culture-media. The reaction of 

 the medium has an important influence upon the develop- 

 ment of these water bacteria in plate cultures. 



When the number of colonies is small, there is no difficulty 

 in counting them as they appear in the ordinary Petri dish. 

 When the number is large, some kind of mechanical device 

 may be used to assist counting. The Wolffhiigel plate is a 

 large square of glass resting in a wooden frame painted black. 

 The glass plate is ruled in squares. It was designed particularly 

 with reference to the form of plate-cultures first made by Koch. 

 The Petri dish, however, may be placed upon the glass plate 

 and the cross lines be used to assist in counting. Lafar, 

 Pakes and Jeffer recommend a surface painted black, ruled 

 with white lines which represent the radii of a circle, which m.ay 

 be still further subdivided by other lines. Many find counting 

 easier when a black surface divided into squares is employed. 

 An ordinary card with a smooth black surface divided into 

 squares by white lines may be placed under a Petri dish and will 

 be found to serve very well.* For the mere examination of the 

 colonies no better surface can be devised than the ferrotype 

 plate used by photographers. The examination of the colonies 

 will be easier if a small hand-lens be used. Care must be 

 taken not to mistake air-bubbles or particles of dirt for colonies 

 of bacteria. 



In any case, if possible, all the colonies in the plate should be 

 counted. But if this is not possible, the number contained 

 within several squares may be counted and the average taken; 

 knowing the size of the squares and the area of the plate, the 

 number contained in the whole plate may be calculated. 



The plating may be done by rolling the medium after the 



book 



♦Specially ruled cards will be found after the Appendix II., at the back of the 



