142 MICROMETRY AND BLOOD PREPARATIONS. 



saturated alcoholic solution of gentian violet to about 50 c.c., is a good 

 substitute, or the salt solution alone will answer when no white count is 

 to be made at the same time as the red one. 



It is important to work quickly in adjusting the cover-glass, or 

 there will be cells settling in the center of the drop from a greater depth 

 than the one which the apposition of the cover-glass makes (i/io 

 millimeter deep). 



A good preparation should show: 



1. Presence of Newton's rings. 



2. Absence of air bubbles. 



3. Entire surface of ruled disc covered. 



4. Equal distribution of cells. 



Before counting, about 5 minutes should be allowed for the settling 

 of the cells. 



It will be remembered that the small squares are 1/20 millimeter 

 square. The depth of fluid from upper surface of shelf to lower sur- 

 face of cover-glass is i/io mm. Hence each space embraced by the 

 small square and the depth of fluid is 1/4000 of the unit used in esti- 

 mating number of corpuscles in blood, or i cubic millimeter (1/20x1/20 

 x 1/10 = 1/4000). Count 100 of the small squares (this enables one 

 to use decimals). There are 9 squares between double -ruled lines, 

 each containing 16 small squares. Count the number of corpuscles in 

 the 1 6 small squares contained in upper left-hand double-ruled square. 

 Put down this count. Next count corpuscles in the adjoining 16 

 squares. Put down this count. Then in third, 16 squares. Put 

 down the number. Now move down to next row of three double- 

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

 1 6 square spaces and set down the numbers for addition. We have now 

 counted 96 small squares (6 x 16). Count at any place 4 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 1/4000 cu. mm.) by 4000. This will give the number of cor- 

 puscles in i cubic millimeter. Example: 100 small squares contained 

 655 red cells. Pointing off 6.55 equals average number of red cells per 



