ESTIMATION OF THE OPSONIC POWER OF THE BLOOD 205 



narrower than a slide. To use it, take a clean slide and rub 

 the surface gently with very fine emery paper (Hubert's oo), 

 which will greatly facilitate the preparation of a good film by 

 this method. Place a drop of the mixture at one end of the 

 slide : touch this with the concave end of the spreader (see 

 Fig. 39), and move the latter gently from side to side, so that 



FIG. 38. 



the blood fills the whole concavity, and then, holding it at an 

 angle as shown in the figure, draw the spreader steadily to 

 the other end of the slide. A little practice is required to 

 enable you to take the right amount of blood for the purpose. 

 In a film spread in this way the leucocytes, especially the 

 larger forms, are mainly collected at the free edg'e of the film 



FIG. 39. 



(x x in Fig. 40), where the drop of blood " petered out," and 

 this greatly facilitates counting. 



The films have next to be stained. When the organism in 

 use is the staphylococcus, pneumococcus, etc., Jenner's stain 

 is as good as any; or the film may be fixed with formalin 

 (p. 259) or perchloride of mercury (p. 258), and stained with 



