ESTIMATION OF THE RED CORPUSCLES 203 



2. Filling the Pipette. — The degree of dilution is determined by 

 the number of corpuscles per cubic millimetre which you expect 

 to find. If the patient is anaemic, use i in loo ; if he has approxi- 

 mately the normal number of corpuscles, or if you have reason 

 to think that they are present in increased quantities, use a 

 dilution of i in 150 or i in 200. 



In most cases you will find it advisable to count the red and 

 the white corpuscles in the same specimen, and if this is the case, 

 use a dilution of i in 100, whether you expect the patient to be 

 anaemic or not. It is less easy to count the reds (if numerous) 

 with this low degree of dilution than with a higher one, but it is 

 not really difficult, and if you use a higher degree of dilution 

 considerable error will be introduced into the leucocyte count. 



Having decided upon the degree of dilution, insert the tip of 

 the pipette into the drop of blood lying on the skin, take the bone 

 mouth-piece attached to the indiarubber tube in your mouth, and 

 suck the blood up to the appropriate mark. If air-bubbles gain 

 access, blow the blood out and begin again quickly. If you over- 

 shoot the mark, remove some of the blood by touching the tip 

 of the pipette against some lint or absorbent cotton-wool. Be 

 careful, also, to wipe off any blood there may be on the outside 

 of the tip. Place the tip of the pipette in the diluting fluid ; a 

 small quantity should be poured out into a watch-glass or other 

 suitable vessel, so as to avoid any possibility of allowing some 

 blood to escape into the stock bottle, and invalidating a sub- 

 sequent observation. Suck the diluting fluid slowly into the 

 pipette until it reaches the single mark above the bulb; rotate 

 the pipette between the finger and thumb as you do so. 



Now remove the pipette from the diluting fluid, place the tip 

 of the finger over the aperture of the pipette (Fig. 41, 5), and 

 proceed to mix the contents by rotating the pipette and by turning 

 it over and over. 



If the examination is to be made at a distance, remove the 

 indiarubber tube and stretch an indiarubber band over it, so as 

 to close both apertures of the pipette. It is advisable to make 

 the examination in a few hours, otherwise considerable errors 

 may creep in. 



3. Preparation of the Specimen. — The slide which is supplied with 

 the instrument consists of a thick and perfectly flat slip of glass 

 (Fig. 41, 0), on which is cemented a glass square having a round 

 hole in its centre (PK). In the centre of the hole thus left there 



