MICROSCOPIC EXAMINATIONS. 165 



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 

 can now easily be found. 



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 " Brownian 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 

 it is rapid and darting. The molecular tremor may be 

 seen with non-motile and with dead organisms. 



Note. — Prepare three hanging-drop preparations — 

 one from a drop of dilute India-ink, a second from a 

 culture of micrococci, and a third from a culture of 

 the bacillus of typhoid fever. In what way do they 



differ ? 



8* 



