The Ruling of the Turck Disc. Explanation of the Squares. In the first place, 

 we have the square which encloses the entire ruled surface. This is made up of 

 nine squares, each i mm. square. These are the squares to use in connection with 

 leukocyte counts with the white pipette. They may be termed the large squares. 

 The very smallest squares which can be found are those made by the intersection of 

 the triple ruled lines in the center; they are 1/40 mm. or 25 microns square and are 

 never used for any purpose, except possibly in connection with the counting of bacteria 

 in a vaccine. It will be observed that it requires four of these very small squares to 

 make one of the squares usually designated as the small square. 



There are 400 small squares in each large square, consequently, as there are nine 

 large squares, the entire ruled surface consists of 3600 small squares. There are 

 4000 small squares, in i cubic mrn. 



The unit in estimating the leukocyte or red cell content of blood is the cubic 

 millimeter. The unit is i/iooo of a cubic centimeter. 



In making a leukocyte count we usually take the white pipette, which has the 

 mark II just above the bulb, and draw up the blood to 0.5 and then with suction we 

 fill the pipette to the II mark with the diluting fluid for which a 1/2% solution of 

 glacial acetic acid in water is most satisfactory. This gives a dilution of 1-20. 



Counting with the 2/3 inch objective all of the highly refractile dots representing 

 leukocytes in one of the i mm. squares at either of the four corners we note the num- 

 ber and mentally multiply by 20 (the number of times the blood was diluted). As 

 the depth of the diluted blood between the ruled surface of the haemacytometer 

 slide and the under surface of the cover-glass is only i/io of a millimeter, we multiply 

 the figure as above obtained by 10 to get the number of cells in a 1-20 dilution of 

 blood in a space of one cubic millimeter. 



Example: Counted 90 leukocytes; 90X20=1800X10=18,000: equals number 

 of leukocytes in i cubic mm. of blood. 



For red counts we use the red count pipette which has the 101 mark just above the 

 bulb. Taking up blood to 0.5 we draw up the diluting fluid to 101. This gives a 

 dilution of 1-200. Counting the red cells in five of the aggregations of 16 small 

 squares (1/20 mm.) thus having counted 80 small squares we have counted 1/50 

 of the total number of small squares in a cubic mm., there being 4000 small squares 

 in a cubic mm. Consequently the number of red cells in 80 small squares multiplied 

 by 50 and then by the dilution of 200 gives the number of red cells in one cubic 

 mm. of the blood examined. 



It is well to make a second preparation and record the average of the two counts. 



