MIXTURE OF BACTERIA, LEUCOCYTES AND SERUM. 31 



culture, which is now rubbed off into the salt solution, the clumps 

 being broken up with a platinum needle. The suspension is then 

 filtered first through a loosely packed cotton filter and later through 

 a filter made with scraped filter paper. Filtration is repeated 

 until all clumps are removed, after which the clump-free suspen- 

 sion is heated to 75 C. for twenty to thirty minutes. 



STRENGTH OF BACTERIAL EMULSION. 



The strength of bacterial emulsion most commonly used has 

 a slightly opalescent appearance. It has been found that cloudi- 

 ness may be absent or be only very slight and still the emulsion 

 may contain too many bacteria. According to Wright's instruc- 

 tions the strength of the emulsion to be used is one which gives 

 an average count of from five to eight bacteria for leucocytes 

 when normal serum, leucocytes and the suspension of bacteria 

 are incubated for the proper length of time. Walker has obtained 

 better results with heavier suspensions of bacteria and diluted 

 serum. It is not very difficult to make an emulsion of staphy- 

 lococci, for after a bit of experience it can be determined by the 

 naked eye whether the suspension of this species is heavy enough 

 or not. The tubercle bacillus, however, presents greater diffi- 

 culties, it frequently being necessary to actually make a trial 

 test for the bacterial emulsion. Simon and others have proposed 

 the numerical determination of the number of bacteria per c. c. 

 in the bacterial emulsion. 



MIXTURE OF BACTERIA, LEUCOCYTES AND SERUM. 



From the definition of the opsonic index, it is evident that 

 similar quantities of the same factors must be taken to make com- 

 parative mixtures. Furthermore, according to the methods de- 

 vised by Wright, equal amounts of each factor are mixed to- 

 gether. When it is desired to use diluted serum equal volumes 

 may still be used if the serum be diluted properly before taking 

 the volume. 



In order to get equal volumes Wright has made a capillary 

 pipette, the walls of the capillary part of which are thick. The 

 end of the pipette is broken off squarely. About a quarter or 

 one-half inch from the open capillary end a mark is made with a 

 soft wax pencil. This pipette is shown in Fig. 8. By means of 



