78 MICRO-ORGANISMS IN WATER 



ported at a aaa the transparent glass plate B. The middle 

 portion of this glass plate is etched out into squares, 

 some of which, situated in various parts of the field, are 

 further divided up into nine smaller squares. When the 

 gelatine-plate is ready to be counted, B is raised, the 

 gelatine-plate so placed that it is covered by the etched 

 surface C, and a small magnifying glass resting on three 



FIG. 13. WOLFFHUGEL'S COUNTING APPARATUS. 



feet is placed on C, and the colonies enclosed by each 

 square counted. If the colonies on the plate are too 

 numerous to count individually, an approximate esti- 

 mate may be made of their numbers by counting those 

 contained in a few of the large squares (which is most 

 accurately carried out by using the squares divided into 

 the nine smaller squares) and then multiplying the 

 average number on these squares by the total number 

 of squares over which the gelatine-film extends ; a very 

 accurate result may be thus obtained if the water has 

 been well mixed with the gelatine in the first instance. 

 The number of colonies found is then calculated on 1 c.c. 

 of the original water. 



By using glass dishes l instead of plates there is much 

 less risk of aerial contamination, whilst by introducing 

 the water direct from the pipette into the melted 

 gelatine in the dish the loss is avoided of those bacteria 

 which must necessarily remain in the gelatine left ad- 



1 See note, p. 84. 



