ESTIMATION OF THE OPSONIC POWER OF THE BLOOD 173 



Then place the pipette in the incubator at 35° to 37° C, noting 

 the time exactly, and proceed to prepare a second pipette in 

 exactly the same way, using the same emulsions of bacteria and 

 leucocytes, but the control serum instead of the patient's. Place 

 this in the incubator by the side of the other, noting the time at 

 which you do so. When no incubator is at hand the tubes may 

 be placed in a vessel of water, which can be kept at blood-heat for 

 the necessary time (fifteen minutes) with very little trouble, or the 

 Dewar flask previously mentioned may be used. 



When each pipette has been incubated for a quarter of an 

 hour, remove it from the incubator, break off the end, fit the 

 nipple to the thick end, and expel the contents on to a clean 

 slide. Next mix them thoroughly together. Then prepare suit- 

 able films in the usual manner. The best method for this is 

 the one given on p. 219, in which the film is spread on two cover- 

 glasses. A small drop of the mixture is placed on one of the 

 cover-glasses held in the left hand in the manner described, the 

 second glass applied, and the two slid apart. The process is very 

 easy, and if the cover-glasses have been properly cleaned two 

 excellent films will result. 



Most bacteriologists spread their films on slides, using a second 

 slide as a spreader. I am convinced that this method is not so 

 good as that given above. It is true that it renders the counting 

 somewhat easier, since the leucocytes are all collected together at 

 the sides of the film, whereas with the cover-glass method they are 

 scattered all over the surface. But the counts by the latter 

 method are much more uniform, the leucocytes more defined, and 

 there is less danger of bacilli being ground mechanically into the 

 cells ; there is always a considerable margin of error in the pro- 

 cess, and I believe that the extra amount of accuracy obtainable 

 in this way is quite worth the additional trouble involved. 



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. 223) or 

 perchloride of mercury (p. 222), and stained with carbol thionin. 

 In the case of tubercle bacilli it is best to fix with saturated solu- 

 tion of perchloride of mercury (one or two minutes), wash, stain 

 in the ordinary way with hot carbol fuchsin, decolorize for half to 

 one minute in 2^ per cent, sulphuric acid in methylated spirit, and 

 to counterstain for about five minutes in borax methylene blue, 

 or in Delafield's haematoxylin. It is necessary to get the proto- 



