176 BACTERIOLOGY. 



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 pressure is felt 

 upon the slide the objective must be raised, otherwise 

 the cover-slip will be broken and the lens may be ren- 

 dered worthless. 



A safer plan is to bring the edge of the drop into the 

 centre of the field with one of the higher power dry 

 lenses. When this is accomplished, substitute the im- 

 mersion for the dry system, and the edge of the drop 

 should now be somewhere near the centre of the field. 



In examining bacteria by this method there is a pos- 

 sibility of error that must be guarded against. All 

 microscopic insoluble particles in suspension in fluids 

 possess a peculiar tremor or vibratory motion, the so- 

 called " Browuian motion. " This is very apt to give 

 the impression that the organisms under examination 

 are motile, when in truth they are not so, their move- 

 ment in the fluid being due only to this molecular 

 tremor. 



The difference between the motion of bodies under- 

 going this molecular tremor and that possessed by cer- 

 tain living bacteria is that the former particles never 

 move from their place in the field, while the living 

 bacteria alter their position in relation to the surround- 

 ing organisms, and may dart from one position in the 

 field to another. With some cases the true movement 

 of bacteria is very slow and undulating, while in others 



