OPSONIC INDEX AND VACCINE THERAPY 353 



slides and stained with Jenner or any other suitable blood and bac- 

 terial stain. Under the field of the microscope the ratio between 

 the bacteria and blood cells is then determined, and from our 

 knowledge of the number of the red blood cells in this blood to each 

 c. mm. we can easily calculate the number of bacteria to the c. mm. 

 or c. c. 35 



A more accurate method of enumerating the bacteria in a sus- 

 pension to be used for vaccine is by direct count of an accurately 

 made dilution in a hemocytometer chamber, as was first suggested 

 by Malory and Wright in 1908. 36 The bacterial suspension is diluted 

 in blood-counting pipettes, 

 1-20 to 1-100 dilutions of 

 thick bacterial suspensions 

 being as a rule satisfactory. 

 As a diluent one may use 

 either salt solution or some 

 dilute anilin dye, such as one 

 made by mixing one part 

 alcoholic methylene blue 

 with 40 parts of 1 per cent, 

 carbolic acid. The dilute 

 suspension is then placed in 

 an ordinary Thoma-Zeiss 

 chamber, which was de- 

 signed for counting blood 

 platelets and has a depth of 

 0.02 mm. This enables one 

 to use an oil immersion lens 

 or high power dry system 

 with a short working dis- 

 tance. From such a count one may readily estimate the num- 

 ber of bacteria in the original suspension ; for example, if 20 

 squares in the Helber-Zeiss chamber are counted the result gives the 

 number of bacteria in 0.001 c. mm. 37 



Another method of standardization of vaccines w T hich is suffi- 

 ciently accurate for clinical purposes is that of Hopkins, which con- 

 sists in measuring the volume of the sediment 38 after centrifugaliz- 

 ing the preparation under standard conditions in a graduated tube. 

 The tubes may be made with a capacity of 10 to 15 c. c. with a capil- 



35 For such counts it is convenient to contract the field of the microscope 

 by using- a diaphragm or simply marking a circle on the eyepiece with a 

 grease pencil. 



36 Malory and Wright. "Pathological Technique," 4th Ed., New York, 

 1908. 



37 Glynn, Powell, Rees, and Cox. Jour, of Path, and Bact., Vol. 18, 1914, 

 p. 379. 



38 Hopkins. Jour. A. M. A., 1913, Vol. 60, p. 1615. 



MICROSCOPIC FIELD AS SEEN IN STANDARDIZA- 

 TION OF VACCINES BY WRIGHT'S METHOD. 



