EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 35 



down upon this drop of oil. The beginner often experi- 

 ences difficulty in focusing upon a hanging-drop. It is 

 well to shut off most of the light by means of the iris dia- 

 phragm. Often it is well to secure the focus roughly upon 

 the extreme outer edge of the chamber, or to find the edge 

 of the drop of fluid with the low power and then to focus 

 upon this edge with the oil-immersion objective. Above all 

 things guard against breaking the cover-glass by forcing 

 the objective down upon it. The motility of certain bacteria 

 is one of the most striking phenomena to be observed in 

 the hanging-drop. It is not to be confused with the so- 

 called " Brownian movement " which is exhibited by fine 

 particles suspended in a watery fluid. It is well for the 

 beginner to observe the character of the Brownian move- 

 ment by rubbing up some carmine in a little water, and with 

 the microscope to study the trembling motion exhibited by 

 these particles of carmine. It will be noticed that, although 

 the particles oscillate, no progress in any direction is ac- 

 complished unless there are currents in the fluid. Such 

 currents might give rise to the impression that certain 

 bacteria possessed motility when they were, in fact, power- 

 less to move of themselves. In the hanging-drop the 

 multiplication of bacteria can be studied, the formation of 

 spores and the development of spores into fully formed 

 bacteria. The hanging-drop has recently been put into 

 service for the demonstration of the so-called serum- 

 reaction with the bacillus of typhoid fever. Sometimes 

 bacteria must be watched in the hanging-drop for hours, 

 or even days, and it may be necessary to keep it at the 

 temperature of the human body for this length of time. 

 Various complicated kinds of apparatus have been devised 

 for this purpose, but they are needful only with special 

 kinds of work. When the hanging-drop preparation is no 

 longer required, the slide and cover-glass should be 

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