88 MANUAL OF BACTERIOLOGY. 



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 em- 

 ployed, 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 

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



Fig. 32. — Appearance of Colonies on Gelatin in Petki Dish. 



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 and 3 they become less numer- 

 ous and larger. 



