140 



MICROMETRY AND BLOOD PREPARATIONS. 



blood on the filter-paper has lost its humid gloss, the comparison 

 should be made. This is best done by shifting the blood stained piece 

 of filter-paper suddenly from one to the other of the holes cut in each 

 shade the piece of filter-paper being underneath the color plate. 

 The error with this method is probably not over 10% after a little 

 experience. If the colored plate is not kept in the dark, the tints tend 

 to fade. The use of this method requires neither time nor trouble. 



To COUNT BLOOD-CORPUSCLES. 



The instrument almost universally used is the Thoma-Zeiss 



haemacytometer. 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 i to 100 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 circular disk of glass of 

 less diameter, so that an encircling 

 channel is left. The square and the 

 FIG. 50. Thoma-Zeiss blood circle of glass are cemented to a heavy 



counter, showing pipette count- lags s i ide The surfaces of each are 



mg chamber and ruled field. 



(Greene.} absolutely level and highly polished. 



That of the circular disk 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 between its under 

 surface and the ruled disk of .1 millimeter. The channel around the 

 disk 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 9 large squares, each i millimeter square. These are 

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



