TYPHOID FEVER 



197 



FIG. 15 



cc 



again in solid column. Small drops are mounted on clean slides, 

 spread out like blood specimens, and, after drying, stained with 

 Jenner's stain or one of the numerous Romanowski modifications. 

 A small square diaphragm made of paper or cardboard is placed 

 in the ocular of the microscope, when the red cells and bacteria are 

 counted in successive fields until 1000 of the former have been gone 

 over. As the number of red cells in 1 c.c. of normal blood is about 

 5,000,000,000, and as the red cells and bacteria must be present 

 in the mixture in the same ratio to one another as in the original 

 units of volume, the number of bacteria 

 per cubic centimeter of the vaccine is 

 ascertained according to the equation: 

 Number of red cells counted : number of 

 bacteria counted : : 5,000,000,000 : x. 

 This method, of course, has no claims to 

 accuracy, but it is the one which is 

 usually employed in titrating out vaccines. 

 A more accurate method has of late been 

 suggested by Hopkins, which seems to 

 have many points in its favor. 



Hopkins' Method. 1 This is based upon 

 the concentration of a bacterial culture 

 by centrifugation and the preparation of 

 standard emulsions from the sediment. 

 To this end the washings from slant 

 cultures, after filtration through a small 

 cotton filter to remove larger clumps -of 

 bacteria and particles of agar, are placed in especially constructed 

 centrifugation tubes (prepared by the International Instrument 

 Company of Cambridge, Mass.; see Fig. 15), covered with rubber 

 caps, and centifugalized for half an hour at a speed of approximately 

 2800 revolutions per minute. The salt solution and bacteria above 

 the 0.05 mark are then removed and 5 c.c. of saline solution 

 measured into the tube, so as to make a 1 per cent, emulsion. If 

 the sediment does not reach the 0.05 mark its volume is read on the 

 scale and a corresponding quantity of saline added to make the emul- 

 sion 1 per cent, in strength. By means of a capillary pipette armed 



Special centrifuge tube with 

 graduated tip. 



1 Jour. Amer. Med. Assoc., vol. xl, No. 21, p. 1615. 



