48 ELEMENTS OF WATER BACTERIOLOGY 



Counting. The number of bacteria is determined by 

 counting the colonies developed upon the plate, with the 

 aid of a lens magnifying at least five diameters. For 

 convenience in counting the plate may be placed upon 

 a glass plate ruled in centimeter squares and set over a 

 black tile; or the tile itself may be ruled. As has 

 already been said, it is desirable that the number of 

 colonies should not exceed 200, for when the number 

 is very high the colonies grow only to a small size, 

 making counting laborious and inaccurate, and many 

 do not develop at all. The best results are obtained 

 with numbers ranging from 50 to 200. 



When it is possible to do so, all the colonies on the 

 plate should be counted. When they exceed 400 or 

 500 it is often easier, and fully as accurate, to count 

 a fractional part of the plate and estimate the total 

 number therefrom. This should not be done, however, 

 except in case of necessity. 



Ayers (1911) has suggested two counting devices 

 which will be found very useful where a great many 

 plates have to be handled. For getting the best possible 

 transmitted light, he places his plate on the ground- 

 glass top of a wooden box, 7 inches square, with one side 

 open to admit light, which is reflected upward by a 

 plane mirror set in the box at an angle of 45 degrees. 

 An ordinary graduated-glass counting plate may be 

 placed between the ground-glass and the Petri dish, and 

 the eyes are protected from direct light by a screen 

 rising from the open side of the box. For picking 

 colonies from a gelatin plate in a warm room, he places 

 between the ground glass and the Petri dish a copper 



