FILM PREPARATIONS 93 



organism* by oblique illumination has been much practised, and 

 a number of substage condensers are in the market, by means of 

 which this is effected. The general principle involved in these 

 instruments is to stop out the rays passing directly towards the 

 tube of the microscope, and to arrange for light being thrown 

 obliquely on bacteria mounted in a drop of fluid between a slide 

 and cover-glass. The bacteria disperse these rays in all direc- 

 tions, and some passing up through the lens are 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 spirochcbte 

 pallida in secretions as a means of diagnosis. Generally speak- 

 ing, 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 of 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 

 <>t' the culture, and when this is rubbed up in the droplet of 

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



