1 g4 BA CTERIOLOGY. 



examine bacteria in the unstained condition. The cir- 

 cumstances calling for this arise while studying the 

 multiplication of cells, the germination of spores, the 

 formation of spores, and the absence or presence of 

 motility. 



In this method the organisms to be studied are sus- 

 pended in a drop of salt solution or bouillon in the 

 centre of a cover-slip. This is then placed, drop down, 

 upon an object-glass in the centre of which a hollow or 

 depression is ground (Fig. 32). The slip is held in posi- 

 tion by a thin layer of vaselin, which may be painted 



Fig. 32. 



Hollow grouQd-glass slide for observing bacteria in Iianging drops. 



around the margins of the depression. This not only 

 prevents the slip from moving from its position during 

 examination, but also prevents drying by evaporation if 

 the preparation is to be observed for any length of 

 time. This is known as the " hanging-drop " method of 

 examination or cultivation. It is indispensable for the 

 purposes mentioned, and at the same time requires con- 

 siderable care in its manipulation. The fluid is so trans- 

 parent that the cover-slip is often broken and the face 

 of the objective injured by its being brought down upon 

 the preparation before one is aware that the focal dis- 

 tance has been reached. This may be avoided by grasp- 

 ing the slide with the left hand and moving it back 

 and forth under the objective as it is brought down 

 toward the object. As soon as the least ipressure is felt 

 upon the slide the objective must be raised, otherwise 



