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 

 -| 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. 



FIG. 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 



4 x 4 X :, (r 2 X it, where if is taken as ) or 50 and 



a fraction. Practically, therefore, when we look down 

 the microscope after it has been adjusted in this 



