NORMAL SERUM. 39 



about as convenient to hold the capsule in the hand as to hold 

 the punctured finger near the table on which the slide box rests. 



Walker has also advocated that small quantities of full 

 strength or diluted sera and leucocytic cream be put into separ- 

 ate small test tubes in order to avoid carrying bacterial emulsion 

 to the lencocytic cream from which other preparations are to be 

 made later. This method is certainly to be recommended. 



Walker has also modified Wright's methods concerning the 

 capillary pipette containing the mixture. In making opsonic 

 index determinations, Walker breaks off the large end of the cap- 

 illary pipette and seals both ends of the capillary tube. When 

 this is done it becomes necessary to again fit the capillary part 

 of the pipette to a larger tube to which a rubber teat can be fas- 

 tened. This Walker accomplishes by fastening a thin rubber 

 sheet over the open end of the large pipette. Thisworks as a dia- 

 phragm and can be easily punctured by the capillary tube. 



SERUM. 



Immunity to and recovery from numerous infections accord- 

 ing to Wright's theory is due to the presence of opsonin. Opsonin 

 prepares bacteria for ingestion by the leucocytes and is a sub- 

 stance contained in the serum. In the determination of the 

 opsonic index the ratio of bacteria ingested by the leucocytes is 

 established by mixing bacteria and leucocytes in one case with 

 patient's serum and in another case with serum from healthy 

 individuals. It is thus evident that the serum is the important 

 and valuable factor. 



Normal Serum. 



In the determination of the opsonic index numerous in- 

 vestigators have been struck by the apparently unexplainable 

 indices found. This has lead to the investigation of the 

 opsonic index obtained with serum from the normal individual, 

 to determine whether the index does not vary in the normal, as 

 well as in the infected, individual. 



If reference is made to the technique, as proposed to obtain 

 the normal phagocytic index, it will be observed that Wright 

 recognized that there is a difference in the amount of opsonin 

 present in serum from the normal individual. He states that 



