96 MICROSCOPIC METHODS, 



distilled water is placed on the cover-glass and a minute 

 particle of growth rubbed up in it and spread all 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 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 are 

 very convenient if only the presence or absence of organisms 

 is inquired after. Such scrapings may be smeared directly 

 on the cover-glasses with or without the addition of sterile 

 distilled water. 



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

 allowed to spread itself between two 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 must be fixed by one of the following methods : 

 by being placed (a) in- a hot-air chamber at 120 C. for 

 half an hour, (b) in a mixture of equal parts of alcohol and 

 ether for half an hour, then washed and dried, or (c) in a 

 saturated solution of corrosive sublimate for two or three 

 minutes, then washed for half an hour in running water 

 and dried. (Fig. 61 shows a film prepared by the latter 

 method.) In the case of urine, the specimen must be 



