of fluid lying over the squares is ttt mm. (= o*i as 
marked on the slide). Therefore, the volume of fluid 
lying over each square is iuo x irr = iom mm. 5 . 
Suppose, now, that on counting we find ten red cells on 
a square. Then in voVo mm. * 2 3 4 5 there are ten red cells. 
Therefore in i mm 3 , there are io x 4000 = 40,000 red 
cells. But the blood has been diluted 100 times. 
Therefore, normally, there would be 40,000 x 100 = 
4,000,000 in 1 mm 3 . 
In practice it is not sufficient to count one square 
but it is necessary to count many. If then we add up 
all the red cells {e.g. 1000) counted and divide by the 
number of squares counted, we get the number for one 
square. So that we get this formula for the number of 
corpuscles per mm. 3 
4000 x dilution x total number of red cells counted 
Number of squares counted. 
The normal average values are for man 5,000,000 
and for woman 4,500,000. 
Orientation Lines .—Every fifth row of squares has 
a line drawn through its middle dividing it into two 
oblongs. These lines are simply to prevent the eye 
losing itself among so many small squares, and for 
purposes of counting or calculation are disregarded. 
Procedure. —1. The pipette must be quite dry 
and the glass ball move quite freely. 
2. Prick the finger ; the blood must flow freely. 
Pressure on the finger must not be used as plasma is 
squeezed out of the tissues. 
3. Place the tube in the mouth and hold the 
pipette so that the scale is completely in view. 
4. Suck up blood exactly half-way, i.e. y 0*5 for 
bloods fairly normal, but to 1 for anaemic bloods. 
5. Wipe off adherent blood, and placing the point 
