250 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



if there are 112 leucocytes in 40 fields, the number per cubic 

 millimetre is 22,400. 



It happens with some microscopes that there is no combina- 

 tion of lenses that will enable you to secure a field having a 

 diameter of eight small squares. You will probably be able 

 to get one of seven, and in this case you can count 104 fields 

 and multiply by 100, or 52 and multiply by 200. 



Where very great accuracy is required the special diluting 

 pipette should be used.* 



All the steps are similar to those just described at full 

 length, except that a different diluting fluid is used. 



The diluting fluid is one which destroys ("lakes") the red 

 corpuscles, but does not injure the leucocytes. It consists of 

 a 0-3 or 0*5 solution of acetic acid (glacial) in water; it is better 

 to add a small quantity of methyl violet or gentian violet, so 

 that the leucocytes are stained and thereby rendered more 

 prominent. This solution is best prepared fresh, or at any 

 rate kept in a well-stoppered bottle. 



The pipette is distinguished from that used for the red cor- 

 puscles by its having the number n above the bulb. This 

 indicates that if blood be sucked up to the mark i below the 

 bulb, and diluting fluid up to the transverse mark above the 

 bulb, the dilution will be I in 10, and so on. 



The blood should be sucked up to the mark i if a great 

 excess of leucocytes is not expected. If the case is one of 

 leucocytosis, a greater dilution is better; whilst if there is a 

 great excess of leucocytes (such as occurs in severe leucocy- 

 thaemia), it is best to use the red corpuscles pipette with a dilu- 

 tion of i in 100, but employing the acetic acid diluting fluid. 

 Then proceed to make the preparation, and count by the field 

 method as above. 



Cleaning the Hcemocytometer Pipettes. Immediately after 

 use the pipettes must be thoroughly cleaned. The fluid which 

 remains in the bulb must be blown out, and for this purpose, 

 as well as for the subsequent washing's, it is an advantage 

 to reverse the position of the indiarubber tube, so that the 

 fluid may be blown out through the upper part of the pipette, 

 this being the wider. The whole pipette must now be filled 



* I allow this statement to stand, but have now great doubts as to its 

 correctness, and believe that the results obtained by the field method are 

 even more accurate, especially in unpractised hands. 



