ESTIMATION OF NUMBER OF LEUCOCYTES. 153 
permits of more squares being counted in a reasonable 
time, more accurate. 
Having focussed the rulings on the slide move the 
draw-tube of the microscope up and down until the 
upper and lower limits of the field of the microscope 
coincide exactly with two of the horizontal lines, and 
count the number of spaces (each enclosed between two 
horizontal lines) in the diameter of the field. Using a 
4 in. objective it will be found possible to arrange 
matters so that these are eight in number, and this will 
be found convenient, though any other number will do. 
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Fic. 25.—Showing field of microscope adjusted so that its diameter 
is equal to that of eight squares. 
The essential thing is that the upper and lower borders 
of the field shall coincide exactly with the rulings. We 
will suppose that the number is eight. Then the dia- 
meter of the field of the microscope is equal to eight 
times the length of a side of a square, and its radius is 
equal to four times the length of a side of a square. 
The total area of the field of the microscope is therefore 
22 ; 22 
4X 4X y? (? x w, where w is taken as 7) or 50 and 
a fraction. Practically, therefore, when we look down 
the microscope after it has been adjusted in this 
