ESTIMATION OF RED CORPUSCLES. 147 
. 3. Preparation of the specimen.—The slide which is 
supplied with the instrument consists of a thick and 
perfectly flat slip of glass (fig. 23, 0) on which is 
cemented a glass square having a round hole in its 
centre (W). Inthe centre of the hole thus left there 
is a circular disc of glass (B); this inner disc is made 
of glass which is exactly #, of a millimetre thinner than 
that of which the outer glass is constructed. When 
the whole cell is covered with a perfectly flat cover- 
glass (D) there will, therefore, be a space exactly =, of 
a millimetre between the lower surface of this cover- 
glass and the upper surface of the central disc; this 
space is to be filled with the diluted blood. 
Slide and cover-glass are to be wiped clean with 
a soft handkerchief moistened with water and then 
thoroughly dried; there must not be the minutest 
particle of dust on any part of the surface. 
The slide and cover-glass being ready, mix the con- 
tents of the pipette as you did before (this must always 
be done immediately before making the specimen, no 
matter how carefully it had been done a short time 
previously) and blow out about half of the fluid in the 
bulb; this is to wash the diluting fluid out of the lower 
part of the stem. Nowclip the india-rubber tube firmly 
between your finger and thumb so as to prevent the 
access of air, and, therefore, the escape of fluid, and wipe 
the tip of the pipette from all fluid; this may be done 
with the forefinger. Place the tip of the pipette on the 
centre of the central disc of the slide and relax your 
pressure on the india-rubber tube so as to allow a small 
drop of fluid to escape; this is perhaps the most difficult 
part of the process, and the exact amount which must 
be allowed to fall on to the slide can only be learnt by 
experience. 
L2 
