THE TYPHOID BACILLUS 273 



the concavity having been carefully smeared with vaselin, so as to make 

 a closed chamber), and the hanging drop then examined under the 

 microscope by either daylight or artificial light, a high-power dry lens 

 being used, or, somewhat less serviceably, a 1/12 oil-immersion lens. 

 Ordinarily the dried blood is not weighed, but the measure of dilution 

 is estimated by the color of the drop. To judge this the beginner 

 must carefully make dilutions of fluid blood and notice the depth of 

 color in 1:10 and 1 : 20 dilutions. Besides the faulty judgment of the 

 dilution color by the examiner, the variation in depth of color of 

 different specimens of blood makes the estimation of dilutions more 

 or less inaccurate, but fortunately this does not greatly interfere with 

 the value of the test. 



The Reaction. If the reaction takes place rapidly the first glance 

 through the microscope reveals the reaction almost completed, most of 

 the bacilli being in loose clumps and nearly or altogether motionless 



FIG. 93 



Gruber-Widal reaction. Bacilli gathered into one large and two small clumps, the few isolated 

 bacteria being motionless or almost so. 



(Fig. 93). Between the clumps are clear spaces containing few or no 

 isolated bacilli. If the reaction is a little less complete a few bacilli 

 may be found moving slowly between the clumps in an aimless way, 

 while others attached to the clumps by one end are apparently trying 

 to pull away, much as a fly caught on fly-paper struggles for freedom. 

 If the agglutinating substances are present, but still less abundant, the 

 reaction may be watched through the whole course of its development. 

 Immediately after mixing the blood and culture together it will be 

 noticed that the bacilli move more slowly than before the addition of 

 serum. Some of these soon cease all progressive movement, and it will 

 be seen that they are gathering together in small groups of two or more, 

 the individual bacilli being still somewhat separated from each other. 

 Gradually they close up the spaces between them, and clumps are 



18 



