210 BACTERIOLOGY 



The slip adheres and the preparation may then be handled 

 without fear of disturbing the drop or the position of the 

 slip over the depression. When completed it has the appear- 

 ance shown in Fig. 35. The drop hangs in an air-tight 

 chamber so that both evaporation and contamination are 

 prevented. 



This is known as the "hanging-drop" method of exami- 

 nation or cultivation. It is indispensable for the purposes 

 mentioned, and at the same time requires considerable care 

 in its manipulation. The fluid is so transparent that the 

 cover-slip may be broken by the objective being brought 

 down upon the preparation before one is aware that the 

 focal distance has been reached. This may be avoided by 



FIG. 35 



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

 hanging drops. 



bringing the edge of the drop into the center 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 

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

 molecular tremor. 



