MICROSCOPIC EXAMINATION OF PREPARATIONS 193 



down upon the preparation before one is aware that the 

 focal distance has been reached. This may be avoided by 

 bringing 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 immersion for the dry system, when the edge 

 of the drop is readily detected with the higher power lens 

 somewhere near the centre of the field. 



In examining bacteria by this method there is a possibility 

 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 



Fig. 35 



Longitudinal section of hollow-ground glass slide for observing bacteria in 

 hanging drops. 



are not so, their movement in the fluid being only this 

 molecular tremor. 



The difference between the motion of bodies undergoing 

 this molecular tremor and that possessed by certain living 

 bacteria is. that the former particles never move from their 

 place in the field, while living motile bacteria alter their' 

 position in relation to the surrounding organisms, and may 

 dart from one position in the field to another. In 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 

 13 



