THE MICROSCOPE AND MICROSCOPIC METHODS 35 



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 accomplished 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 length 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 preparation 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, 1 make use of a cube of nutrient agar 

 instead of a drop of fluid. Bacteria are distributed on the sur- 

 face of the agar, which is then applied to a cover-glass, and 

 mounted like a hanging-drop. The bacteria are thus kept in a 

 layer close to the glass, where growth may be studied. 



The Microscopic Preparation for Study by Dark-field Illumi- 

 nation. The central portion of a clean glass slide is encircled 

 with a ring of vaseline, and a drop of the fluid to be examined 

 is deposited on the clean surface in the center of the ring by means 

 of a capillary tube. It is then covered with a clean large cover- 

 glass so that the fluid spreads out in a moderately thin layer 

 beneath the cover-glass and is confined on all sides by the 



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



