no 



BACTERIOLOGY 



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, or the entire dish may be placed on the stage of the micro- 

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

 iris diaphragm should be nearly closed and the plane mirror 

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

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

 Fig. 43. In tube No. i the colonies are so numerous as to look 

 like fine white dust. In tubes 2 and 3 they become less numerous 

 and larger. 



Esmarch's Roll-tubes. Use liquefied gelatin or agar. The 

 dilutions in tubes i, 2 and 3 are made as above. Tubes contain- 





FIG. 42. Manner of making Esmarch roll- tube. 



ing a rather small amount of the culture-medium are more con- 

 venient. A block of ice should be at hand, and, with a tube filled 

 with hot water and lying horizontally, a hollow of the size of the 

 test-tube should be melted on the upper surface of the ice. In 

 this hollow, place the tube of liquefied gelatin or agar; roll it rapid- 

 ly with the hand, taking care that the culture-medium does not 

 run toward the neck as far as the cotton plug. The medium is 

 spread in a uniform manner around the inside of the tube, where 



