160 IMMUNE SERA 



One volume of this emulsion is mixed with one 

 volume of the bacterial suspension to be tested and 

 with one volume of the serum. This is best accom- 

 plished by means of a pipette whose end has been 

 drawn out into a capillary tube several inches in 

 length. With a mark made about three-quarters 

 of an inch from the end it is easy to suck up one such 

 volume of each of the fluids, allowing a small air 

 bubble to intervene between each volume. All 

 three are now expelled on a slide and thoroughly 

 mixed by drawing back and forth into the pipette. 

 Then the mixture is sucked into the pipette, the end 

 sealed and the whole put into the incubator at 37 C. 

 The identical test is made using a normal serum in 

 place of the serum to be tested. Both tubes are 

 allowed to incubate fifteen minutes and then ex- 

 amined by means of smear preparations on slides 

 spread and stained in the usual way. The degree of 

 phagocytosis is then determined in each by count- 

 ing a consecutive series of fifty leucocytes and find- 

 ing the average number of bacteria ingested per 

 leucocyte. This number for the serum to be tested 

 is divided by the number obtained with the normal 

 serum and the result regarded as the opsonic index 

 of the serum in question. The presence of a high 

 opsonic index Wright regards as indicative of in- 

 creased resistance. He further states that the fluc- 

 tuation of the opsonic index in normal healthy 

 individuals is not more than from .8 to 1.2, and that 



