132 THE PKEPARATION OF VACCINES 



with, a high eye-piece, e.g., a Zeiss No. 18 compensating. A 

 grating micrometer aids the enumeration. 



Wright's method consists in making a mixture of blood (whose 

 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 and the total of each added 

 up. As the number of red corpuscles per c.mm. is known the 

 number of bacteria can be readily calculated. In the case of 

 certain bacteria, e.g., the members of the coli-typhoid and 

 cholera groups, 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. In such cases direct enumeration, as above described, 

 should be adopted. 



Fairly accurate results, as regards number of organisms, may 

 with practice be obtained by taking a standard opacity of 

 emulsion, representing a known number of organisms, and 

 diluting down with saline the emulsion of organisms to be tested 

 till this opacity is reached. Tubes of the same diameter must 

 of course be used, and it may be said that in a tube half an inch 

 in diameter the lowest visible opacity in good daylight repre- 

 sents, in the case of cocci, approximately 100 millions per c.c. 

 Each observer must, however, work out such standards for 

 himself. 



General Bacteriological Diagnosis. 



Under this heading we have to consider the general routine 

 which is to be observed by the bacteriologist when any material 

 is submitted to him for examination. The object of such 

 examination may be to determine whether any organisms are 

 present, and if so, what organisms ; or the bacteriologist may 

 simply be asked whether a particular organism is or is not 

 present. In any case, his inquiry must consist (1) of a^micro- 



