THE COUNTING OF WHITE CELLS 177 



a content equal to only i/io of a cubic millimeter. Then multiply 

 by the dilution. Example: First large square 50; second large square 

 70; third large square 60. Average 60. Then 60X10X20 = 12,000, 

 the number of leukocytes in i cubic millimeter of blood. The count 

 may be made with a low power (2/3 -inch objective) as the 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 

 nine large squares as with Zappert and Tiirck), 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 eight small squares. Now, remembering that the area of a circle equals 

 the square of the radius multiplied by TT, or 3.1416, we have the following calculation: 

 The diameter being eight small squares, the radius would be four small squares. 

 Squaring the radius, we have sixteen. This multiplied by 3.1416 gives us fifty. 

 This means that every field, with the microscope adjusted as stated, contains fifty 

 of the small squares, or 1/80 of the unit of one 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 fifty 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 forty 

 of the designated microscopic fields (this, of course, is only one-half the unit, hence 

 we should multiply by 2). Counted forty fields and noted fifty white cells. 50X2 

 = 100X200 (the dilution in red pipette) = 20,000. Consequently 20,000 would 

 represent the number of leukocytes in one cubic millimeter of the blood examined. 



After making a blood count, the hsemacytometer 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 allow air to pass through 

 to dry the interior. If the interior is stained, use i% HC1 in alcohol. If a vacuum 

 pump is not at hand, a bicycle pump or suction by mouth will answer. 



PREPARATIONS FOR THE STUDY OF FRESH BLOOD. 



Many authorities prefer a fresh-blood specimen to a stained dried 

 smear in the study of parasites of the blood. In malaria in particular 

 there is so much information as to species to be obtained from a fresh 

 specimen that the employment of this method should never be neglected. 



