CALCULATION OF OPSONIC INDEX. 35 



isfactory results for organisms, other than the bacillus of tuber- 

 culosis, by fixing the spread with methyl alcohol for one minute, 

 washing off with water and then staining with Loeffler's methy- 

 lene blue for three to five minutes. In staining slides on which 

 the index for the tubercle bacillus is to be obtained, a modification 

 of Wright's method has given most satisfactory results. The smears 

 are first fixed in a saturated solution of bichloride of mercury which 

 is then washed off with water. After this the slide is immersed 

 in a jar containing Ziehl's carbol-fuchsin . The stain is heated 

 by placing the jar in a heated water bath. After five minutes 

 of staining in hot carbol-fuchsin, the stain is washed off and the 

 smear is decolorized in a mixture of 97 parts of alcohol and :> 

 parts of concentrated hydrochloric acid. The smear is counter 

 stained in a solution of one-half grams each of sodium carbon- 

 ate and methylene blue in 100 c. c. of water. This counter-stain 

 acts rapidly and has the advantage that if the slide be over 

 stained in the process it can be easily decolorized with warm water. 



EXAMINATION OF SMEARS. 



The spreads are examined by first going over the slide with 

 the low power of the microscope to determine whether the slide 

 has been properly stained and also to find the part of the slide 

 where the polynuclear leucocytes are most numerous. Usually 

 it is found that these are most abundant near the margins and 

 at the end of the smear. Wright has recommended that those 

 at the end of the smear be examined. 



Under the high power of the microscope the number of bac- 

 teria engulfed by a definite number of polynuclear leucocytes is 

 determined. The number of leucocytes examined varies. Wright 

 based many of his determinations on examinations of twenty 

 leucocytes. Most investigators, however, have counted the num- 

 ber of bacteria in fifty or more polynuclear leucocytes. 



The average number of bacteria per leucocytes is spoken 

 of as the phagocytic index. 



CALCULATION OF THE OPSONIC INDEX. 



The opsonic index of Wright is determined by dividing the 

 phagocytic index obtained when patient's serum is used with 



