190 LABORATORY EXERCISES IN BACTERIOLOGY. 



gelatine is placed on the under surface, and with the point of the platinum needle 

 some of the organisms to be studied transferred to the drop. A thin ring of vaseline 

 is drawn about the margin of the concavity in the slide with a hair pencil by which, 

 when the cover is applied, with the drop suspended in the slide space from its under 

 surface, it may be retained in position, drying prevented, and the danger of contamina- 

 tion from the outside removed. When it is intended to observe the development 

 of the colonies of bacteria growing in the inclosed drop, the slide and cover should, 

 of course, have been previously sterilized (in the flame) and the medium used should 

 be gelatine or agar, the whole preparation then being submitted to the usual condi- 

 tions of temperature and atmosphere (if grown in anaerobic jar, the vaseline ring 

 should not have been complete, to allow the replacement of air in slide chamber with 

 hydrogen or nitrogen) as for any other culture preparation. Where prolonged ob- 

 servation of a selected field is desirable during a period of incubation, special incubators 

 permitting the introduction of the microscope into the interior, and allowing through 



FIG. 57. DIAGRAM OF THE HANGING DROP. 



glass sides the entrance of light, are sold ; but if one makes the examination in a warm 

 room and uses the mechanical stage for the location of the desired field, this rather 

 cumbersome and costly device is not required. In studies of the individual germs 

 requiring less prolonged observation the loopful of salt solution or bouillon inoculated 

 from the desired growth (or a loopful of an original liquid culture) is usually used 

 instead of the solid media, and there is no need beyond reasonable cleanliness for 

 sterilization. After use of such a slide and cover they should be sterilized and cleansed 

 by one of the methods prescribed for other containers. 



Care must always be observed, in the use of the high power with such a prepara- 

 tion, from the danger of breaking the unsupported cover-slip by inadvertent pressure 

 of the lens upon it in focussing. The edge of the hanging drop should be adjusted 

 with the low power, and thereafter the higher power substituted and the edge cautiously 

 focussed, after which it is usually easy, with the fine adjustment and movement of the 

 slide, to obtain the desired field in proper focus. 



EXAMINATION OF INDIVIDUAL BACTERIA. 



A loopful of physiologic salt solution or bouillon, or even clean water, may be 

 placed on a clean, plain slide, a few bacteria from some culture transferred and diffused 

 by drawing the needle once or twice through the drop (or a loopful of a culture in any 

 of the liquid media may be used instead). A cover is then adjusted and the preparation 

 at once examined (with moderately dark field) under the higher power of the micro- 

 scope. In such case the arrangement of the individual organisms is, of course, more 

 or less lost, the hanging drop being preferable when the latter feature is sought to be 

 studied. But examination of fresh, unstained material in this manner or by the 

 hanging drop is essential for determination of the motility of the organisms. Neither 

 is comparable to stained preparations for the most exact study of the morphology 

 of the individual germs. In the latter case the material must before being stained be 

 obtained in a thin film ("smear") upon the slide or cover. 



