Q2 BACTERIOLOGICAL DIAGNOSIS. 
(fig. 17, b). This is the complete positive reaction; it 
consists of two parts, clumping and paralysis, and is 
given only (in the dilution used) by the blood of a 
patient who is suffering or who has suffered from 
‘typhoid fever. If this is not the case the bacilli will 
continue to move about just as before, and will not 
collect into clumps. 
In the process which has been described above, the 
blood has been diluted to thirty times its volume, and 
this is the best dilution to use for diagnostic purposes. 
But the reaction is given earlier if a lesser dilution (one 
Fic. 17. 
a. Negative Widal’s reaction. b. Positive Widal’s reaction. 
in ten) is used, though there is then a greater chance of 
‘fallacy. 
Sometimes the reaction takes place almost as soon as 
‘the serum is added. At other times it is delayed, and 
‘for these it is necessary to fix a time limit. With a 
-dilution of one in thirty, one hour is a safe time limit to 
.adopt, and if the reaction takes place after this the re- 
sult should be looked upon with great suspicion, and the 
‘test reapplied after a day or two. 
Exactly similar processes are carried out if dead cul- 
‘tures of typhoid bacilli are used, but here the emulsion 
