288 INFECTION AND IMMUNITY 



and uniform a smear as possible should be made. They are then stained 

 either by Jenner's or Wright's blood stain. 



The preparations are examined with an oil-immersion lens. In 

 order to limit a definite microscopic field, it is convenient to use an 

 Ehrlich diaphragm, or else, in lieu of this, to mark a circle with a blue 

 pencil upon the lens of the eye-piece. The red blood cells and bacteria, 

 in a number of these fields, are counted and the ratio between them is 

 estimated. Knowing the number of red blood cells to the cubic milli- 

 meter in the particular blood employed, by previous blood count, 

 and knowing that equal volumes of blood and of bacterial emulsion 

 have been used in the mixture, it is easy from this ratio to ascertain 

 the number of bacteria contained in a cubic millimeter of the original 

 emulsion. Thus, for instance, if in an average of twenty fields bacteria 

 are to red blood cells as two is to one, and the blood employed con- 

 tains five million red blood cells to each cubic millimeter,, then a cubic 

 millimeter of our emulsion contained ten million bacteria, and a cubic 

 centimeter one thousand times as many. 



Special centrifuge tubes with graduated narrow tips at the bottom 

 have been suggested by Hopkins for vaccine standardization. Bacteria 

 centrifugalized up to a certain mark represent a definite number per 

 cu. mm. when taken up in a given volume of salt solution. 



The vaccine, thus standardized, is sterilized at 60 C. for one hour 

 for several days. Its sterility is then controlled by culture and animal 

 inoculation. 



From this stock emulsion small quantities may be drawn off and 

 diluted for therapeutic use. 



The initial dose given by Wright in staphylococcus infections, in 

 which the method has been most frequently employed, varies from 

 fifty to one hundred millions of bacteria. In working with the tubercle 

 bacillus, the ordinary tuberculin dosage is adhered to. 



Wright, in his work, makes use of the opsonic index in order to 

 estimate changes in the resistance of the patient against the given in- 

 fection. In other words, he bases his judgment as to whether the 

 patient is improving or not, upon the opsonic power of the patient's 

 serum. In following the opsonic index of a patient during systematic 

 treatment with vaccine, Wright has found definite changes upon the 

 basis of which he constructs a curve of opsonic power. Immediately 

 after the injection of vaccine, he finds that there is a brief period during 

 which the opsonic power of the patient is depressed below its original 

 state. This he calls the negative phase. The length of time occupied 

 by this negative phase depends both upon the condition of the patient 



