98 MANUAL OF BACTERIOLOGY. 



from the surface condenses on the inside of the lid, and runs 

 down over the surface of the agar. A round piece of filter 

 paper placed over the dish before putting on the lid may also be 

 employed, or the cover may be made of porous earthenware, as 

 recently recommended by Hill. Colonies develop usually in 

 from one to two days, more quickly, of course, in the incubator. 

 In plate No. i they will be very numerous, in plate No. 2 less 

 numerous and in plate No. 3 still less numerous. Where the 

 number is small the colonies will be widely separated and can 



Fig. 34. — Appearance of colonies on gelatin in Petri dish. 



readily be studied. They may be examined with a hand-lens, 

 or the entire dish may be placed on the stage of the microscope 

 and the colonies be inspected with the low power. The iris 

 diaphragm should be partly closed and the concave mirror 

 should be- used. Dilution-cultures prepared as described in 

 the next paragraph, where the principle is the same, are shown 

 in Fig. 31. In tube No. i the colonies are so numerous as to 

 look like fine, white dust. In tubes 2 apd 3 they become less 

 numerous and larger. 



