METHOD OF COUNTING BACTERIA 67 



pipette, then other 25 c.mm. of fluid, then another bubble, and 

 so on till nine volumes each of 25 c.mm. have been sucked up. 

 A mark is then made on the tube at the upper level of this 

 amount, other 25 c.mm. are sucked up, and another mark made. 

 The fluid is expelled, the tube dried, and that part containing 

 the 225 and 250 marks is drawn out into an almost capillary 

 diameter, the manipulation by which the marks were originally 

 arrived at is repeated, and thus in the new marks made a more 

 accurate calibration for these amounts is attained. In order to 

 form a safety chamber a second bulb is formed by drawing out 

 the tube a little higher up as in the figure, and finally the upper 

 inch or two are bent at right angles to the calibrated limb. In 

 doing this a loop may be thrown on the plastic melted capillary 

 tube exactly in the way in which a similar loop may be thrown 

 on a piece of cord. With such a pipette any required dilution 

 of a culture can be made on the principles already described. 



Wright's Method of counting the Bacteria in Dead 

 Cultures. In the making of vaccines for use in Wright's pro- 

 cedures it is necessary to know the total number of bacterial 

 cells, whether dead or living, present in a culture, for the dead 

 as well as the living contain the toxins which may stimulate 

 the therapeutic capacities of the body. The 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, then (according to the empirical estimate the 

 observer forms of the strength of his bacterial emulsion) either 

 one volume of culture and three volumes of diluting fluid 

 (e.g. ! 85 per cent sodium chloride) or two of culture and two 

 of fluid, and so' on ; the five volumes are thoroughly mixed by 

 being drawn backwards and forwards in the wide part of the 

 pipette, a drop is then blown out on to a slide, and a blood 

 film is spread which may be stained by Irishman'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 one volume of bacterial 



