90 MICROSCOPIC METHODS. 



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 four or five times quickly 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 

 deposit a small quantity centrally on the cover-slip, then to place 

 another cover on top and draw the two apart. The result will 

 be a film of uniform depth throughout, available at almost 

 all parts for examination. Scrapings of organs are very con- 

 venient if only the presence or absence of organisms is inquired 

 after. Such scrapings 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 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 chamber at 120 C. for half an hour; () in 

 a mixture of equal parts of alcohol and ether for half an hour, 

 then washed and dried ; (c) in formol-alcohol (Gulland) (formalin 

 i part, absolute alcohol 9 parts) for five minutes, then washed 

 and dried ; or (d) in a saturated solution of corrosive sublimate 

 for two or three minutes, then washed well in running water 

 and dried. (Fig. 78 shows a film prepared by the last method.) 

 In the case of urine, the specimen must be allowed to stand, 

 and films made from any deposit which occurs ; or, what is 

 still better, the urine is centrifugalised, and films made from 

 the deposit which forms. After dried films are thus made 

 from urine it is an advantage to place a drop of distilled water 

 on the film and heat gently to dissolve the deposit of salts ; then 

 wash in water and dry. In this way a much clearer picture is 

 obtained when the preparation is stained. Preparations of broth 

 or milk-cultures can be rendered free of stain-retaining material 

 by allowing glacial acetic acid to act upon the film, after fixation, 

 for five seconds, and then washing thoroughly in water. 



