METHODS OF OBSERVING BACTERIA. 



89 



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 con- 

 ditions as in the cul- 

 ture. Such a speci- 

 men may be kept 

 from day to day and 

 examined, the bac- 

 teria continuing to 

 live until the oxygen 

 or nutriment is ex- 

 hausted. By means 

 of a special appara- 

 tus (Fig. 7), in which 

 the microscope is 

 stood, the growing 

 bacteria may be 

 watched at any tem- 

 perature, and very 

 exact observations 

 made. 



The hanging drop 

 should always be ex- 

 amined at the edge, 

 as the centre is too 

 thick. 



In such a specimen 



it is possible to de- FIG. 7. Apparatus for keeping objects under 

 termilie the shape, microscopic examination at constant tempera- 



size, grouping, divis- tures< 



ion, 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 



