194 PUBLIC HEALTH BACTERIOLOGY 



cover -slip, and incubated at 37 C. for 15 minutes. 

 Thereafter a film is made and stained by Leishman's 

 method, and the number of bacteria present in 50 poly- 

 morphonuclear leucocytes is observed. This number 

 divided by 50 gives the "opsonic index." Wright's 

 method is more elaborate and specific. He uses (i) 

 Leucocytes from the observer's blood repeatedly washed 

 with saline solution (0-85 per cent) ; (2) Bacterial emulsion 

 in salt solution ; (3) Serum from the blood to be tested, free 

 of clot and cells. These are now mixed, as above, in equal 

 amounts, in a capillary pipette, and the mixing is made 

 thorough by blowing out and sucking in, for ten times. 

 The mixture is then drawn into the tube of the pipette, 

 the end sealed, the rubber nipple removed, and the tube 

 put into the incubator for 15 minutes. The tube is then 

 removed, the end broken off, the contents are again mixed, 

 and films made, dried or fixed, and stained as desired 

 (Jenner, Leishman, or Giemsa), and bacteria counted in 

 50 to 100 cells. A control is done with normal serum, and 

 the ''Opsonic Index" taken is the quotient of that got 

 with the patient's serum divided by the index got with 

 the normal serum. The latter, to minimize variation, 

 may be made by mixing the serum of several healthy 

 individuals. The number of bacteria per leucocyte 

 (polymorphonuclear), in any one estimation, is also spoken 

 of as the " Phagocytic Index," and the opsonic index is 

 thus the proportion between the phagocytic index for the 

 patient's serum and that for the normal serum. A 

 modification of the method is to take a number of dilutions 

 of the patient's serum and of the normal serum and to 

 estimate in all these not the phagocytic index, but the 

 percentage of leucocytes which act as phagocytes, i.e., 

 the "Percentage Index," and the indices of the corres- 

 ponding dilutions can be compared. The bacterial 

 emulsion used is likewise thinner than in Wright's method. 

 By the same process, the dilution of the serum at which 

 phagocytosis is absent or very slight, can be determined. 

 This is called the " Opsonic Coefficient of Extinction." 

 Wright's Vaccine Therapy consists in injecting killed 

 bacterial cells into the infected individual, in order to 

 raise the phagocytic index to that particular cell. He 



