DISTRIBUTION OF BACTERIA. I^-T 



medium has an important influence upon the development of 

 these water bacteria in plate-cultures. 



When the number of colonies is small, there is no diffi- 

 culty in counting them as they appear in the ordinary 

 Petri dish. When the number is large some kind of me- 

 chanical 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 

 may 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 sur- 

 face 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. 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 num- 

 ber contained in the whole plate may be calculated. Such 

 estimations, however, are likely to give results very wide 

 of the truth. 



The plating may be done by rolling the medium after the 

 manner of Esmarch. When the number of colonies is not 

 large this may serve very well. Counting may be assisted by 



