184 BACTERIOLOGY. 



not only prevents the slip from moving from its posi- 

 tion 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 indispen- 

 sable for the purposes mentioned, and at the same time 

 requires considerable care in its manipulation. The 

 fluid is so transparent that the cover-slip is often broken 

 by the objective being brought down upon the prepara- 

 tion before one is aware that the focal distance has been 

 reached. This may be avoided by grasping 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 rendered 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 " 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 only 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 



