136 THE PREPARATION OF VACCINES 



content in red blood corpuscles is known) with the bacterial 

 culture, and comparing the number of bacteria with the number 

 of corpuscles. The observer first estimates the red cells in his 

 blood ; a capillary pipette with a rubber nipple and with a mark 

 near its capillary extremity is then taken, blood is sucked up to 

 the mark, then an air-bubble, and then an equal volume of the 

 bacterial emulsion diluted according to the empirical estimate 

 the observer forms of its strength. The blood and bacterial 

 emulsion are then thoroughly mixed by being drawn backwards 

 and forwards in the wide part of the pipette, a drop is blown 

 out on to a slide, and a blood film is spread which may be 

 stained by Leishman's method. The bacteria and blood 

 corpuscles are now separately enumerated in a series of fields 

 in different parts of the preparation. If a dilution has been 

 taken in which a large number of bacteria are present, an 

 artificial field may be used, made by drawing with the oil pencil 

 a small square on a circular cover-glass, and dropping the latter 

 on to the diaphragm of the microscope eye-piece. Suppose, now, 

 that the observer's blood contained 5,000,000 red cells per c.mm., 

 that to tin bacterial emulsion three volumes of diluent had been 

 added, and that in the fields examined there were 500 red cells 

 and 600 bacteria. It is evident that in the undiluted culture 

 for 500 red cells there would have been 2400 bacteria. Now 

 500 : 2400 : : 5,000,000 : 24,000,000, which last figure is the 

 number of bacteria per c.mm. of the emulsion. 



It has been found in the case of certain bacteria, e.g., the 

 members of the coli-typhoid and cholera groups, that when an 

 emulsion of these is mixed with whole blood, the serum of the 

 latter may have a bacteriolytic or an agglutinating action on the 

 organisms, which interferes with the counting. To obviate the 

 inaccuracies or difficulties thus introduced, Harrison has modified 

 Wright's method by substituting, in a given quantity of blood, 

 normal saline for the serum. The method is as follows : A 

 capillary pipette has a mark made upon it, to which blood is 

 sucked up and quickly expelled into a small tube containing a 

 little '75 percent, sodium citrate solution; any remaining blood is 

 washed out of the pipette with the same fluid. The tube is then 

 centrifuged to deposit the corpuscles, the supernatant fluid 

 carefully removed, and the corpuscles are washed by centrifuging 

 twice or thrice with normal saline, care being always taken not 

 to lose any of the corpuscles in the successive washings. After 

 the last washing the corpuscles are sucked up into the pipette, 

 and then saline up to the mark which indicated the volume of 

 the original blood. Such a mixture is taken, and, to prevent 



