ESTIMATION OF THE RED CORPUSCLES 185 



is a circular disc of glass (5) ; this inner disc is made of glass 

 which is exactly J^ 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 j-\^ 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 hand- 

 kerchief moistened with water [not alcohol or xylol, which may 

 spoil the former), 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 contents 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 indiarubber 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 indiarubber 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. 



Cover the slide in this way : Place your finger at the side on 

 the glass square on the slide, and apply the cover-glass, letting it 

 rest against your finger ; lower it gently in place with a needle or 

 other suitable object. When it is in place press it gently with 

 the needle at each corner in succession, and look at it obliquely, 

 so as to see the light reflected from the surface. If the slide and 

 cover-glass are in sufficiently close contact, you will see Newton's 

 rings (looking like the eye of a peacock's feather) round the point 

 at which you are applying pressure. If you do not see this, the 

 inference is that there is some dust between the slide and cover- 

 glass ; you must clean both and begin again. 



It is a great advantage to clip the cover-glass to the counting- 

 chamber until the corpuscles have had time to settle. When this 

 has taken place the depth of the chamber is immaterial, and it 

 does not matter if it rises somewhat. The simplest method is to 



