COUNTING RED CELLS. 141 



squares used for averaging the red cells. These small squares are 

 1/20 of a millimeter square and are arranged in 9 groups of 16 small 

 squares by bordering double-ruled lines. As the unit in blood counting 

 is the cubic millimeter, if one counted all the white cells lying within 

 one of the large squares (i millimeter square), he would have only 

 counted the cells in a layer i/io of the required depth, so that it would 

 be necessary to multiply the number obtained by 10. This product, 

 multiplied by the dilution of the blood, would give the number of 

 white cells in a cubic millimeter of undiluted blood. 



To make a red count : Having a fairly large drop of blood, apply the 

 tip of the 10 1 pipette to it and, holding the pipette horizontally, care- 

 fully and slowly draw up with suction on the rubber tube a column of 

 blood to exactly .5 or i. The variation of 1/25 of an inch from the 

 mark would make a difference of almost 3 percent. If the column 

 goes above .5, it can be gently tapped down on a piece of filter-paper 

 until the .5 line is cut. Now insert the tip of the pipette into some 

 diluting fluid and, revolving the pipette on its long axis while filling it 

 by suction, you continue until the mark 101 is reached. A variation of 

 1/25 of an inch at this mark would only give an error of about 1/30 of 

 i%. After mixing thoroughly, by shaking for one or two minutes, the 

 fluid in the pipette below the bulb is expelled (this, of coarse, is only 

 diluting fluid). A drop of the diluted blood of a size just sufficient to 

 cover the disk when the cover-glass is adjusted, is then deposited on the 

 disk and the cover-glass applied by a sort of sliding movement, best 

 obtained by using a forceps in one hand assisted by the thumb and 

 index-finger of the other. 



Among diluting fluids Toisson's is probably the best : 



Sodium chloride, i gram. 



Sodium sulphate, 8 grams. 



Glycerin, 30 c.c. 



Distilled water, 160 c.c. 



Dissolve the sodium chloride and the sodium sulphate in the 

 glycerin water and add sufficient methyl or gentian violet to give a 

 rich violet tint. 



A salt solution of about 2% strength, tinged with about i drop of a 



