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). In the centre of the hole thus left there 

 is a circular disc of glass (B) ; this inner disc is made 

 of glass which is exactly T \y 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. Now clip 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. 



L 2 



