274 



To find total number of leucocytes per mm.' — 

 Total number counted x loo X dilution (loo) 

 Number of microscope fields counted 

 Count one hundred if possible. 



The leucocytes may also be counted in the 

 special pipette for white cells, but here again the 

 method of counting by using the whole micro- 

 scope field should be used. If the ' white ' counter 

 is used, the diluting fluid should be acetic acid 

 o"3 per cent. Sufficient gentian violet or methyl, 

 violet is added to this to colour the nuclei. 



To Clean Pipettes 



For any accuracy of observation the pipettes 

 should be scrupulously clean. There should not 

 be the slightest tendency for the glass ball to stick 

 to the sides. After a count has been made, the 

 rubber tube is removed and the contents ejected 

 by blowing from the pointed end. 



1. Suck up dilute acetic acid so that all 

 traces of stain are removed. 



2. Suck up several lots of clean water to 

 remove the acid. 



3. Then absolute alcohol two to three times 

 to remove the water. 



4. Then ether two to three times to remove 

 the alcohol. 



5. Finally, blow hot air through with a 

 syringe, the glass barrel of which may be heated 

 in a flame (or simply suck air through). 



These procedures take a very short time, and 

 it is a satisfaction to know that the pipette has 

 been put away perfectly clean and ready for the 

 next observation. 



