94 MICROSCOPIC METHODS 



passing directly towards the tube of the microscope, and to 

 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 focussed by it. The organisms thus 

 appear as brightly illumined objects on a dark background. 

 The method has been employed for bacteria in general, and 

 especially for the demonstration of the spirochcete pallida in 

 secretions as a means of diagnosis. Generally speaking, the 

 internal structure of the organisms under observation is well 

 brought out. 



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. Many methods are recommended for obtaining 

 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 showing any tendency to retract into droplets. 

 The best method is that recommended by Van Ermengem. The 

 cover-glasses are placed for some time in a mixture of con- 

 centrated sulphuric acid 6 parts, potassium bichromate 6 parts, 

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

 absolute alcohol. For use, a cover-glass is either dried by 

 wiping with a clean duster or is simply allowed to dry. This 

 method will amply repay the trouble, and really saves time in 

 the end. A clean cover having been obtained, the film pre- 

 paration can now be made. If a fluid is to be examined a 

 loopful may be placed on the cover-glass, and either spread 

 out over the surface with the needle, or another clean cover 

 may be placed on the top of the first, the drop thus spread 

 out between them and the two then drawn apart. When 

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

 distilled 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 the 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 



