FILM PREPARATIONS 87 



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 par- 

 ticle of growth rubbed up in it 

 and spread over the glass. The 

 great mistake made by begin- 

 ners is to take too much of the FlG ' 4 ^~~ C f r h ldillg 

 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 on the glass by being 

 passed three or four times slowly through the flame. In doing 

 this a good plan is to hold the cover-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 making films of a thick fluid such as pus it is best to 

 spread it out on one cover with the needle. The result will be 

 a film of irregular thickness, but sufficiently thin at many parts 

 for proper examination. Scrapings of organs may be smeared 

 directly on the cover-glasses. 



In the case of blood, a fairly large drop should be allowed to 

 spread itself between two clean cover-glasses, which are then to 

 be slipped apart, and being held between the forefinger and 

 thumb are to be dried by a rapid to-and-fro movement in the 

 air. A film prepared in this way may be too thick at one edge, 

 but at the other is beautifully thin. If it is desired to preserve 

 the red blood corpuscles in such a film it may be fixed by one 

 of the following methods : by being placed (a) in a hot-air 



