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; (3) 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. 



