OF THE HUMAK SUBJECT. 57 



If the blood to be used has been stored in a pipette, the two 

 ends of this are broken off, and the contents blown gently on to 

 a slide. A hollow-ground slide having been prepared with a 

 ring of vaseline, and a clean cover-glass (before commencing the 

 proceedings just described), a single ose of the admixed broth 

 and sernm is placed on the center of the cover glass and gently 

 spread out so as to cover an area about 4 millimeters in diameter; 

 the cover is then inverted, placed over the hollow of the slide, and 

 gently pressed at the margin so as to render the enclosed space 

 quite air tight. The preparation is now placed beneath the 

 microscope and and examined, with a ^ homogeneous immer- 

 sion, or a ^ objective, which answers perfectly well for a study of 

 the result. As the preparation is unstained, much of the light 

 (if -y^- is used) must be cut off by means of the diaphragm the 

 bacilli are otherwise scarcely visible. 



If WidaFs reaction ensue, it is seen in the movements of the 

 microbes becoming sluggish and ultimately ceasing, whilst they 

 become at the same time aggregated into clumps of the kind 

 represented in Fig. 1, Plate XVIII. 



The time allowed for the observation should be half an 

 hour. If no reaction has ensued within this time, the result is 

 to be reckoned negative, and the existence of typhoid may be 

 excluded, not with absolute certainty, but with very high proba- 

 bility. In the case of such a negative result, similar examina- 

 tions must be repeated during the course of 'the disease, as the 

 reaction, for causes not known, is in some cases delayed. If, 

 however, the bacilli become motionless, even without any 

 marked clumping, or if they become motionless, and clump, the 

 result is to be reckoned as positive. 



Before deducing the existence of typhoid in these circum- 



