36 TEXT-BOOK OF BACTERIOLOGY. 



glass, the latter will lie above the cavity of the former, and in the 

 hollow " hangs " the drop, protected from evaporation and isolated 

 from all that surrounds it by the vaselin. 



To examine bacteria which have grown on a solid medium, first 

 with the platinum loop place on the cover-glass a drop of distilled 

 water, and then "inoculate" it with a small number of micro- 

 organisms by means of a platinum needle. The fewer bacteria in- 

 troduced into the hanging drop, the better the preparation will be. 



Beginners almost always make the mistake that in well-meant 

 zeal they try to get as many germs as possible, and then they "can- 

 not see the woods because of all the trees." The fewer micro-organ- 

 isms present in the field the more exactly can their details be seen. 



For this reason it is also advisable, in the case of liquids which 

 contain bacteria, to put a few into a drop of distilled water. 



A microscopic examination of such an object is not without its 

 difficulties. If the immersion plan be used, and in order to get the 

 clearest possible picture, the smaller stop is employed, the field will 

 prove rather dark, and some difficulty will be met in merely finding 

 the drop. Seek and seek, push the slide backward and forward, 

 and at last the lens is brought down too far, the cover-glass is 

 smashed, and thus the examination is brought to a premature end. 

 It is, therefore, better to first place the preparation under a low 

 power, bring the edge of the drop into the field, and then use the 

 high power with immersion. Do not, however, make the mistake 

 of unnecessarily adding to the difficulty by using strong eye-pieces. 

 The edge will soon be found, appearing as a wavy line, clearly dis- 

 tinguished from its surroundings, and generally bordered by very 

 small bubbles which have settled on the glass. 



It is desirable for several reasons to devote special attention to 

 the edge. Here the liquid is thinnest, and the conditions for a 

 leisurely examination are most favorable; in the middle of the drop 

 it is almost impossible to see through its whole depth with the 

 lens, and non-motile bacteria, which by their own weight sink to 

 the bottom, escape observation. Further, the motile bacteria at 

 the edge are restricted in their very rapid movement from place to 

 place, and we can better observe their peculiarities of form. And 

 lastly, the great majority of motile micro-organisms, yielding to 

 their need of oxygen, proceed to the edge of the drop, and gener- 

 ally remain in its vicinity. 



The great advantages of examination in the hollowed slide will 

 now be apparent. 



It shows the bacteria in the nearest approach to natural condi- 

 tions, and enables us to take a glance at them in " real life." 



