38 VACCINE AND SERUM THERAPY. 



number of bacteria ingested by the leucocytes in a mixture of 

 the same quantity of serum from a normal individual, the same 

 bacterial emulsion and leucocytic cream, incubated for the same 

 length of time at the same temperature. In the process of the 

 determination of the opsonic index mechanical technique, serum, 

 bacterial emulsion, leucocytic cream, and calculation of tile aver- 

 age number of bacteria ingested are of importance. Inasmuch 

 as the results obtained by the different investigators are so 

 discordant and inconsistent, all of these factors have been 

 considered, emphasized and, in the minds of some at least, 

 improved. 



MECHANICAL TECHNIQUE. 



The mechanical part of the technique of Wright has prob- 

 ably been less criticised than any other part of his method. Var- 

 ious investigators have reported their technique in full and some 

 at least have shown that they have not interpreted Wright's ex- 

 planations of the same correctly, while others have modified the 

 technique in order to give greater accuracy to the method of the 

 determination of the index. 



Only some of the modifications suggested can be mentioned. 

 Barber, thinking that the mark on the pipette as made by the 

 wax pencil, is too wide and therefore leads to inaccuracies in the 

 amounts used in the mixtures, has suggested that the mark on 

 the capillary pipette be made with a glass hair dipped in Bismarck 

 black. This suggestion, however, is not generally followed as it 

 is possible for one experienced in handling the capillary pipette 

 to draw fluids to the upper or lower edge of the wax pencil mark 

 with as much accuracy as it is to take it to the mark made by the 

 glass hair. 



Walker has suggested the use of a water bath for the incu- 

 bation of the mixture of serum, leucocytes, and bacterial emul- 

 sion. Undoubtedly this has advantages over incubation for 

 short periods of time in the air in the thermostat. Wright's op- 

 sonizer, however, is more convenient and almost immediately 

 brings the pipette to the temperature of the metal of the opsonizer. 



For the collection of serum, Walker has suggested that the 

 capsule be held in position by inserting it into a slit in a slide 

 box. Unless one is taking his own blood, however, it is 



