92 MICROSCOPIC METHODS 



make films on clean glass slides. After these are fixed and 

 stained they may be examined without a cover-glass, a drop 

 of cedar-oil being placed on the film. If such a preparation is 

 to be preserved, the oil should be removed by xylol, which is 

 then dried off, and the slide may then be kept in a box free from 

 dust. In making films of a thick fluid such as pus it is best to 



spread it out with the needle. 

 The result will be a film of 

 irregular thickness, but suffi- 

 ciently thin at many parts for 

 Fig. 36.-Cor M f s forceps for holding proper examination It is often 

 cover-glasses. advisable to dilute the material 



by emulsifying with a loop of 

 water. Scrapings of organs may be smeared directly on a cover- 

 glass or slide. 



In the case of blood, a small drop is placed near one end of a 

 clean slide, the edge of a second slide is lowered through the 

 drop on to the surface of the glass on which the blood has been- 

 placed. This second slide is held at an angle to the first, and 

 the droplet of blood by capillarity spreads itself in the angle 

 between the two slides. The edge of the second slide is then 

 stroked along the surface of the first slide, and the blood is spread 

 out in a film whose thickness can be regulated by the angle 

 formed by the second slide. Large-sized films can thus be 

 obtained. A film prepared in this way may be too thick at one 

 edge, but at the other is beautifully thin. Another method is 

 to allow a drop of blood 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. If it is desired to pre- 

 serve the red blood corpuscles in 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; (b) in a mixture of equal 

 parts of alcohol and ether for half an hour, then washed and 

 dried ; (c) in formol-alcohol (Gulland) (formalin 1 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. 61 

 shows a film prepared by the last method.) In using the 

 Romanowsky stains no previous fixation is necessary (vide infra). 

 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 



