TYPHOID AND COLON BACILLI. 149 
half-hour, the bacilli will be seen to become less 
motile, and finally to cease all movement and 
Fic, 84.—Slide with two cells for observing Widal reaction 
with a control. 
appear in clumps here and there throughout the 
field. 
a@ 
Fic, 85.—Widal reaction: @, bouillon culture of Ba- 
cillus typhosus ; 4, the same after the addition of typhoid 
serum. 
This reaction can be reversed and made to serve 
as a means of identifying the typhoid bacillus if 
the serum of a person known to have typhoid fever 
is at hand. 
10 
