204 MICROMETRY AND BLOOD PREPARATIONS 





depth of fluid from upper surface of shelf to lower surface of cover-glass is Mo mm. 

 Hence each space embraced by the small square and the depth of fluid is Mo 00 

 of the unit used in estimating number of corpuscles in blood, or i cu. mm. 

 (Mo X Mo X Mo = Mooo)- Count 100 of the small squares (this enables one 

 to use decimals). There are nine squares between triple-ruled lines, each containing 

 1 6 small squares. Count the number of corpuscles in the 16 small squares 

 contained in upper left-hand triple-ruled square. Put down this count. Next 

 count corpuscles in the adjoining 1 6 squares. Put down this count. Then in 

 third 1 6 squares. Put down the number. Now move down to next row of 

 three triple-ruled squares. Count the number of corpuscles in each of the three 

 i6-square spaces and set down the numbers for addition. We have now counted 

 96 small squares (6 X 16). Count at any place four additional small squares and 

 add number of blood-cells contained therein to those in the 96 small squares 

 already counted. Divide the sum by 100 or simply point off two decimals. 

 This gives the average for each small square. Multiply this by the dilution and then 

 (as the small square is only Mooo cu. mm.) by 4000. This will give the number of 

 corpuscles in i cu. mm. Example: 100 small squares contained 655 red cells. 

 Pointing off, 6.55 equals average number of red cells per small square. Multiply 

 by dilution (200) and then by 4000 (the small square is 4000 times smaller than the 

 unit: i cu. mm.) 6.55 X 200 = 1310 X 4000 = 5,240,000. 



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

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

 cells in five 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 cu. mm. (Eighty small squares is J-^Q 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 blood in the white pipette to 

 the mark 0.5. 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 0.5% 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 0.5 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 leukocytosis a i to 10 dilution 

 is not sufficient. In leukaemic blood it is better to use the red pipette with the 

 0.5% acetic acid solution. 



The blood having been drawn up to 0.5, we have a dilution of i to 20. 



