BACTERIOLOGIC DIAGNOSIS. 279 



exceptional instances it has persisted for years. It is impor- 

 tant to ascertain whether or not the patient has had a previous 

 attack of typhoid fever, as in that event the reaction would 

 not hold good for the illness which may be present at that 

 particular time. 



The Widal test is performed as follows (Widal, when he 

 first described his method, advocated the use of fresh blood- 

 serum, but since then Wyatt Johnson, of Montreal, sug- 

 gested the use of dried serum) : The blood is obtained as 

 already described, and is mixed with from 5 to 10 times its 



FIG. 128. 



Widal reaction : bacilli gathered into one large and two small clumps, the few 

 isolated bacteria being motionless or almost so. (Park.) 



bulk of sterile water. If the blood-serum is dried, it is first 

 brought into solution with sterile water. To a drop of this 

 mixture placed on a clean cover-glass is added a platinum 

 needle loopful of an eighteen to twenty-four-hour-old bouillon 

 culture of typhoid bacillus, and the two are thoroughly mixed. 

 The drop is rimmed with vaselin and the cover-glass inverted 

 over a concave slide. The drop is examined with an ordi- 

 nary high power (-J-) lens. 



The first change noted in the drop is that the bacilli gradu- 



