THE MICROSCOPE AND MICROSCOPIC METHODS 35 



already been pointed out (page 26) , colorless objects may be clearly 

 seen only when illuriiinated either by a narrow central beam or 

 by oblique illumination (dark-field). 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_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 movement 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 accompUshed unless there are currents in the fluid. Such cur- 

 rents might give rise to the impression that certain bacteria 

 possessed motility when they were, in fact, powerless 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 is also 

 used extensively for the demonstration of the agglutination 

 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 lerigth of time. Various complicated kinds of 

 apparatus have been devised for this purpose, but they are needful 

 only for special kinds of work. When the hanging-drop prepara- 

 tion is no longer required, the slide and cover-glass should be 

 dropped into a 5 per cent carbolic acid solution and afterward 

 sterilized by steam. 



The Hanging-block. — Hanging-block preparations, which 

 were introduced by Hill,^ make use of a cube of nutrient agar 



' Journal. of Medical Research, Vol. VII, March, 1902. 



