78 APPLIED IMMUNOLOGY 



in dilution 1 in 40 and probably also in 1 in 80 will 

 have lost their motility, and most of them will be 

 collected together in clumps throughout the field 

 (Fig, 5) . In the control slide, containing no serum, the 

 organisms at the end of this time \W11 still retain their 

 motility, and there will be no clumping ( Fig. 6 ) . In 

 case of a negative reaction, the bacteria in all the slides 

 remain motile. If the slide in which a positive reaction 



Fig. 5. — Widal's test, positive (aggluti- Fig. 6. — Widal'e test, negative, 



nation or clumping). 



is taking place be observed from time to time, at first 

 a few of the bacteria will be seen to lose their motility, 

 and one will become attached to another, giving a V- 

 or Y-shaped appearance. These groups will gradu- 

 ally be added to by other bacilli until clmnps are 

 formed and all motility is lost. Various grades of in- 

 tensity of reaction are seen, ranging from formation 

 of a few clumps and many organisms retaining their 



