FILM PREPARATIONS 91 



arrange for light being thrown obliquely on objects, e.g., 

 bacteria, mounted in a drop of fluid between a slide and cover- 

 glass. The bacteria disperse these rays in all directions, and some 

 passing up through the lens can be focused by it. It is also 

 necessary to place a suitable stop within the objective of the 

 microscope to cut off the peripheral rays. The organisms 

 appear as brightly illumined objects on a dark background. 

 The method has been employed for bacteria in general, and 

 especially for the demonstration of spirochetes in secretions. 

 Generally speaking, the internal structure of the organisms 

 under observation is well brought out, and their movements can 

 be readily studied. 



2. Film Preparations. — (a) Dry Method. — This is the most 

 extensively applicable method for microscopically examining 

 bacteria. Fluids containing bacteria, such as blood, pus, 

 scrapings of organs, can be thus investigated, as also cultures 

 in fluid and solid media. The first requisite is a perfectly clean 

 cover-glass or slide. Many methods are recommended for ob- 

 taining such. The test of this being accomplished is that, when 

 the drop of fluid containing the bacteria is placed upon the glass, 

 it can be uniformly spread with the platinum needle all over the 

 surface without snowing any tendency to retract into droplets. 

 The best method is that recommended by Van Ermengem. The 

 cover-glasses or slides are placed for some time in a mixture 

 of concentrated sulphuric acid 6 parts, potassium bichromate 

 6 parts, water 100 parts, then washed thoroughly in water and 

 dried. If a fluid is to be examined a loopful may be placed on 

 the cover-glass and spread out over the surface with the needle. 

 When a culture on a solid medium is to be examined, a loopful 

 of water is placed on the cover-glass, and a minute particle 

 of growth rubbed up in it and spread over the glass. The 

 great mistake - made by beginners is to take too much of the 

 growth. The point of the straight needle should just touch the 

 surface of the culture, and when this is rubbed up in the droplet 

 of water and the film dried, there should be an opaque cloud just 

 visible on the cover-glass. When a film has been spread, it 

 must next be dried by being waved backwards and forwards at 

 arm's-length above a Bunsen flame. The film must then be fixed 

 on the glass by being passed three or four times slowly through 

 the flame. In doing this a good plan is to hold the glass 

 between the right forefinger and thumb ; if the fingers just 

 escape being burned no harm will accrue to the bacteria in 

 the film. 



In ordinary routine examinations' it is more convenient to 



