TYPHOID FEVER 



95 



of i in 25, i in 50, i in 125, and i in 250 respectively : the last tube 

 contains no serum and is a control. Now incubate in the manner 

 described, and read off the result of transmitted artificial light against 

 a dark background. It may facilitate matters to pass your finger 

 up and down behind the tube, between it and the light. Standard 

 agglutination consists of marked agglutination, the fluid being filled 

 with fine flocculi, but no sedimentation (Fig. 22, b). When this is 

 found, lower dilutions will be found to give " Standard plus " 

 agglutination (i.e., with some sedimentation), or complete agglutina- 

 tion with sedimentation (Fig. 22, a). If this should occur in the 

 highest dilution, make a second dilution of serum from the first, 

 taking 57 drops of normal saline and 3 drops of the i in 10 dilution. 



FIG. 22. 

 a, Complete agglutination ; 6, standard ; c, negative 



The remaining quantities are just as before, and this will give you 

 dilutions of i in 500, i in 1,000, i in 2,500, and i in 5,000, and simi- 

 larly for higher dilutions. Lower dilutions are sometimes required, 

 and can be made according to the following table : 



o 



I or 5 

 I or 5 

 3 or 15 



i or 2 

 3 or 6 

 6 or 12 



(i in 10) (i in 15) 



i drops of serum 

 7 normal saline 



12 culture 



(i in 20) 



When the dilution in which standard agglutination occurs is found, 

 the strength of the serum may be recorded either in this form or in 



