144 MICROMETRY AND BLOOD PREPARATIONS. 



leukocytes stand out like pearls. It is better, however, to use a higher 

 power, so that pieces of foreign material may be recognized and not 

 enumerated as white cells. 



When it is desired to make a white count with the same preparation 

 as is used for the red one, especially if the ruling is of the old style 

 (only central ruling and not in 9 large squares as with Zappert and 

 Turck), it is advisable to make use of the method of counting by fields. 

 With a Leitz No. 4 ocular and a No. 6 objective, with a tube length of 

 120 millimeters, it will be observed that the field so obtained has a 

 diameter of 8 small squares. Now, remembering that the area of a 

 circle equals the square of the radius multiplied by *-, or 3.1416, we 

 have the following calculation: The diameter being 8 small squares, 

 the radius would be 4 small squares. Squaring the radius, we have 

 1 6. This multiplied by 3.1416 gives us 50. This means that every 

 field, with the microscope adjusted as stated, contains 50 of the 

 small squares, or 1/80 of the unit of i cubic millimeter of the diluted 

 blood. 



By keeping a single red cell in view while moving the mechanical 

 stage from right to left or from above downward, we know that a new 

 field of 50 small squares is brought into view when the circumference 

 of the field cuts this individual cell. Example: As 2000 small squares 

 would ordinarily be a sufficient number to count for a white count, 

 this would require us to count the number of leukocytes in 40 of the 

 designated microscopic fields (this, of course, is only 1/2 the unit, 

 hence we should multiply by 2). Counted 40 fields and noted 50 

 white cells. 50 x 2 = 100 x 200 (the dilution in red pipette) = 20,000. 

 Consequently 20,000 would represent the number of leukocytes in i 

 cubic millimeter of the blood examined. 



After making a blood count, the haemacytometer slide should be 

 cleaned with soap and water and then rubbed dry, preferably with an 

 old piece of linen. As the accuracy of the counting chamber depends 

 upon the integrity of the cement, any reagent such as alcohol, xylol, 

 etc., and in particular heat, will ruin the instrument. The pipettes 

 should be cleaned by inserting the ends into the tube from a vacuum 

 pump, as a Chapman pump. First draw water or i% sod. carbonate 

 solution through the pipette, then alcohol, then ether, and finally 



