METHODS OF OBSERVING BACTERIA. 145 



ground slide is used, and with the aid of a small camel' s- 

 hair pencil a ring of vaselin is drawn on the slide about, 

 not in, the concavity at its centre. A drop of the mate- 

 rial to be examined is placed in the centre of a large 

 clean cover-glass, and then placed upon the slide so 

 that the drop hangs in, but does not touch, the concavity. 

 The micro-organisms are now hermetically sealed in an 

 air-chamber, and appear under almost the same condi- 

 tions as in the culture. Such a specimen may be kept 

 from day to day and examined, the bacteria continuing 

 to live until the oxygen or nutriment is exhausted. By 

 means of a special apparatus (Fig. 7), in which the mi- 

 croscope is stood, the growing bacteria may be watched 

 at any temperature, and very exact observations made. 



FlG. 6. — The " hanging drop " seen from above and in profile. 



The hanging drop should always be examined at the 

 edge, as the centre is too thick. 



In such a specimen it is possible to determine the 

 shape, size, grouping, division, sporulation, and motility 

 of the organism under observation. 



Care should be exercised to use a rather small drop, 

 especially for the detection of motility, as a large one 

 vibrates very readily and masks the motility of the 

 sluggish forms. 



When the bacteria to be observed are in solid or semi- 

 solid culture, a small quantity of the culture should be 

 mixed up in a drop of sterile bouillon or water and ex- 

 amined. 

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