TYPHOID FEVER 79 



powerful serum is used the whole process may take place in 

 a few minutes, and as a matter of fact it is unusual to meet with 

 a positive reaction in which there is not marked clumping in the 

 I in lo pipette in ten minutes or a quarter of an hour. If it is 

 not practicable to watch the actual clumping, it is possible to 

 distinguish between the agglutinated bacilli and those that have 

 merely settled without being clumped, the deposit in the former 

 case being so much looser and more voluminous than in the 

 latter. 



Sera are often met with which clump in much higher dilutions 

 than are here described, but it is not necessary to go higher 

 in practical diagnosis. A positive reaction at i in 50 is 

 conclusive, at i in 30 practically so, and it is extremely rare to 

 find a serum which clumps in half an hour at i in 10 which does 

 not indicate a positive reaction. 



Method of Performing Widal's Reaction by the 

 Microscopic Method. 



Requisites. — i. A young culture (not more than eighteen hours 

 old) of typhoid bacilli on agar. 



Where dead bacilli are to be used, this is to be replaced by a 

 culture obtained from the laboratory and prepared in the manner 

 described. 



2. A small funnel provided with a double thickness of white 

 filter-paper. This is unnecessary if dead cultures are to be used. 



3. Three clean watch-glasses. 



4. A platinum loop. This should be made of fine wire and 

 have a loop (which must be completely closed) about yV inch in 



diameter. 



5. A hollow-ground slide. This is an ordinary slide having a 

 well about J inch in diameter sunk in its centre. If it is not at 

 hand a cell may be built up on an ordinary slide. Take a piece 

 of thin card i inch square and cut out a square ^ inch in diameter 

 from its centre. Fix this perforated square down on to the slide 

 with vaseline or immersion oil. 



6. Thin cover-glasses. 



7. The microscope. The test can be carried out quite well 

 with a ^-inch lens. 



Process— I. Making the Emulsion.— Pour a small quantity of 

 tap-water into the culture tube, or, better, scrape off some of the 



