TECHNIC. 327 



The slide is then stained with an appropriate 

 stain (for most bacteria one of the eosinates of 

 methylene blue) and examined with the immer- 

 sion lens of a microscope. 



The number of bacteria in successive leucocytes 

 is counted and an average made. Various figures 

 are given as the necessary number of leucocytes 

 which should be counted to give accurate results. 

 The number, however, should be governed by the 

 uniformity of the numbers of bacteria in succes- 

 sive leucocytes. If, for instance, the average 

 number of bacteria in three successive counts of 

 ten leucocytes is nearly the same, it is more ac- 

 curate to take such an average than if more leuco- 

 cytes are counted with no uniformity of numbers. 



The ratio of the average number of bacteria to 

 the leucocyte taken up in the presence of a given 

 serum to the average number taken up in the 

 presence of a serum taken as normal is the op sonic 

 index. 



By the immunization of animals by various immune 



. . ", . Opsonins. 



bacteria and other cells a serum 01 nigh, opsonic 

 power may be produced. The opsonic action of 

 such serums, in contrast to that of normal serum, 

 is not destroyed by heating to 56 C. and differs 

 from normal opsonin in other respects which will 

 be discussed later. In immunizing animals with 

 typhoid bacilli, it was noticed that the estimation 

 of the concentration of opsonin in the typhoid im- 

 mune serum by the Wright method of comparison 

 did not show results that would be expected. That 

 is, a highly immune animal would show little 

 difference from one with low immunity. Klien, 

 therefore, estimated the opsonic power by determ- 



