LENCOCYTE COUNTING. 143 



small square. Multiply by dilution (200) and then by 4000 (the small 

 square is 4000 times smaller than the unit: icu. mm.) 6.55x200 = 

 1310 x 4000 = 5,240,000. 



At least 100 small squares, and preferably 200 should be counted. 

 If the blood appears normal, one may simply count the number of red 

 cells in 5 of the 16 small square spaces (80 small squares). Having 

 added the numbers and multiplying by 10,000, you obtain the number 

 of cells in i cubic millimeter. (Eighty small squares is 1/50 of the 

 unit of i cu. mm., or 4000 small squares. The blood dilution being 

 i to 200, we have 50 x 200 x number of cells in 80 small squares.) 



In counting, count corpuscles lying on the lines above and to the 

 right. Do not count those lying on lines below and to the left. 



In the small squares count only corpuscles lying in the space or 

 cutting the upper line. This prevents counting the same cell twice. 



To Count White Cells. Draw up the fluid in the white pipette to 

 the mark .5. Then, still holding the pipette as near the horizontal 

 as possible, because the column of blood tends to fall down in the 

 larger bore, draw up by suction a diluting fluid which will disintegrate 

 the red cells without injuring the whites. The best fluid is .3% of 

 glacial acetic acid in water. This makes the white cells stand out as 

 highly refractile bodies. Some prefer to tinge the fluid with gentian 

 violet. The .5 mark is preferred because it takes a very large drop of 

 blood to fill the tube up to the i mark and if there is much of a leukocy- 

 tosis a i to 10 dilution is not sufficient. In leukemic blood it is better 

 to use the red pipette with the .3% acetic acid solution 



The blood having been drawn up to .5, we have a dilution of i to 20. 

 Making a preparation, exactly as was done in the case of the red count, 

 we count all of the white cells in one of the large squares (i sq. mm.). 

 The cross ruling greatly facilitates this. Note the number. Then 

 count a second and a third large square. Strike an average for the 

 large squares counted and multiply this by 10, as the depth of the 

 fluid gives 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 60 x 10 x 

 20 = 12,000, the number of leukocytes in i cubic millimeter of blood. 

 The count may be made with a low power (2/3-in. objective) as the 



