72 MICRO-ORGANISMS AND DISEASE [CHAP. 



only but after seven days, and the number of colonies in the 

 ]5lates was dver twelve hundred per i cc. The plates ought 

 to be kept in the incubator always at a temperature of 20 to 

 31° C, if counted after two days it will be found that even 

 under these conditions not all colonies have sufficiently 

 developed as yet, and for these reasons the counting ought 

 to be repeated after three or four days. It seems to me that 

 the low figures of bacteria present in London waters as 

 pubHshed by the public analyst of the Local Government 

 Board as also by the analysts of the London water com- 

 panies are not to be accepted without hesitation, for the 

 reason that in neither case were the conditions for obtaining 

 a full and correct number of the bacteria fulfilled : the plates 

 were not kept at the most favourable temperature, or the 

 counting was done too early. 



Koch has devised a plate in equal squares of known area 

 which enables one to count the number of colonies easily. 

 But it is just as easy and just as accurate to count the 

 colonies in a plate-cultivation, by drawing in ink lines on 

 the outside of the plate- dish (not the cover) by which the 

 area is divided in two, four, eight, sixteen, and so on, and to 

 count under a magnifying glass on a black ground the actual 

 number of colonies in each division. If the number of 

 colonies in the plate-cultivation is small or moderately large 

 counting is easily and soon done, but if the number is ex- 

 cessively large, say several thousand, then only an approxi- 

 mate estimate can be made, by selecting two, three, or four 

 small subdivisions, say Jj- or J-g, representing a fair average 

 distribution of the colonies, to count them patiently in these 

 divisions, and then by calculation give the total. 



Messrs. Washbourne and Pakes have designed a print 

 which embodies the same principle, only is much simpler ; 

 it is a printed circular area in black of the size of a plate 



