ESTIMATION OF RED CORPUSCLES. 149 
Some ¢ in. lenses focus too near the object to be of 
any use. If this is the case you must either get an 
objective specially for the purpose, or a cover-glass 
which is hollowed out in the centre. These can be 
bought from the same place as the hemocytometer. 
If the rulings of the slide are indistinct they may be 
darkened by rubbing them with a very soft lead pencil. 
5. Counting the corpuscles —Move the slide about until 
you have come to one corner (preferably the left upper 
corner) of the ruled area. You will see that each 
fifth space is marked off by a line running down its 
centre; this is to guide the eye and facilitate counting. 
If you exclude the spaces which are thus marked with a 
double line the whole area will be marked out into 
a series of large squares, each consisting of 4 x 4 = 16 
smaller squares (fig. 24). It is convenient to count the 
smaller squares in these groups of 16. A? Jeast a hun- 
dred of the smaller squares, 7.¢., six of the large groups 
and four small squares, should be counted. 
In counting one of the smaller squares it is convenient 
to begin with the corpuscles which are lying in the 
middle of the square, and then to count those which are 
lying on the lines. In dealing with these you count 
those which are lying on the upper and left hand lines 
as being within the square, and those that are on the 
lowey and vight hand lines as being without it; if you like 
you may reverse this, but you must keep to the same 
method throughout (see fig. 24). 
A few white corpuscles will be met with in every 
case, while if the blood was taken from a patient with 
leucocytosis or leucocythemia there will be many. 
‘They may be distinguished from the red corpuscles by 
their greater refractivity, or, if a stain has been used in 
the diluting fluid, by their being faintly tinged. It is 
