174 



MICROMETRY AND BLOOD PREPARATIONS 



To' COUNT BLOOD-CORPUSCLES. 



The instrument almost universally used is the Thoma-Zeiss haemacy- 

 tometer. The apparatus consists of two pipettes, one for leukocytes, 

 graduated to give a dilution of i to 10 or greater; the other for red cells 

 to give a dilution of a i to i oo or greater. The white pipette has the mark 

 ii above the bulb and the red pipette the mark 101. In addition, there 

 is a counting chamber. This consists of a square of glass with a round 

 hole in the center. Occupying the center of this round hole is a circu- 

 lar disc of glass of less diameter, SD that an encircling channel is left. 



The square and the circle of glass are 

 cemented to a heavy glass slide. The sur- 

 faces of each are absolutely level and 

 highly polished. That of the circular disc 

 is ruled into squares of varying size and is 

 exactly i/io of a millimeter below the 

 level of the surface of the surrounding 

 glass square. 



When a polished piano-parallel cover- 

 glass rests on the shelf, as the outer square 

 glass is termed, there is a space left be- 

 tween its under surface and the ruled disc 

 of o. i millimeter. The channel around the 

 disc is termed the moat or ditch. The most 

 desirable rulings are those of Turck and of 

 Zappert. In these the entire ruled surface 



consists of nine large squares, each i millimeter square. These are sub- 

 divided, and in the central large square are to be found the small squares 

 used for averaging the red cells. These small squares are 1/20 of a milli- 

 meter square and are arranged in nine groups of sixteen small squares 

 by bordering triple-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. 



FIG. 5 2. Thomas- Zeiss blood 

 counter showing pipette, count- 

 ing chamber, and ruled field. 

 (Greene.) 



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, carefully and slowly draw up 



