148 PATHOGENIC BACTERIA. 
all together, and mount as a hanging-drop prepara- 
tion.! 
(6) Examine with the 4 inch or with the oil- 
Fic. 83.—The Chester and Robin pipet for delivering 
uniform drops of blood for the Widal test. A glass and 
rubber pipet, the bulb of which is enclosed between two 
strips of metal held in place by a Hoffman clamp. 
& 
immersion lens. If the case is one of typhoid, after 
some time, varying between a few seconds and a 
tions may be made with a fair degree of accuracy from 
dried blood. It consists of an ordinary medicine-dropper 
of a given size, the bulb of which is enclosed on either side 
by two narrow strips of metal (Fig. 83, ¢, ¢.), and both placed 
in a medium-sized Hoffman clamp. The inward movement 
of the clamp by means of the screw a compresses the bulb, 
while a slight turn in the opposite direction dilates it a 
little and permits a small drop of blood to enter. In expel- 
ling the blood the dropper is held vertically over a strip of 
thick filter-paper, and the clamp is slowly compressed until 
a single drop falls of its own weight. This drop is then 
dried, and when the test is to be applied the blood-spot is 
cut from the paper and drops of the diluting fluid are 
added from an exactly similar pipet until the required 
dilution is reached. 
‘It is convenient to use a slide on which two glass rings 
have been cemented, so that one may be used as a control, 
containing the culture of bacilli alone. 
