194 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



tion. Practically, therefore, when we look down the microscope 

 after it has been adjusted in this way we are looking at fifty 

 squares ; and this fact enables us to dispense entirely with the 

 rulings, and count over the whole area of the disc with great 

 rapidity. The slide is placed in position, and all the cells which 

 are seen in the field counted and the result noted down, or, 

 preferably, dictated to someone else. The slide is then moved on 

 until a perfectly fresh portion of the field comes into view ; it is 

 advisable to go too far rather than not far enough. For this 

 purpose (as for a great deal of blood-work) a mechanical stage is 

 a great advantage. In this way 4,000 squares — i.e., eighty fields — 

 may be counted in a very short time. 



It is a very great advantage to be able to dictate these numbers to 

 an assistant, who will tell you when forty fields have been counted. 

 In most cases this will be enough, but if the numbers come out 

 irregularly — i.e., several in one field and none in others — it is best 

 to count eighty fields or to make a fresh preparation. 



With the arrangement recommended — that is, with a field eight 

 small squares in diameter — you can tell at a glance whether there 

 is or is not leucocytosis. // the leucocytes average one per field, 

 they are 8,000 per cubic millimetre, a ^ normal count ; if two per 

 field, they are 16,000 per cubic millimetre, a moderate leucocytosis ; 

 if three per field, they are 24,000 per cubic millimetre, a high 

 leucocytosis. 



The calculation in this case is very simple. If you have 

 counted eighty fields, the total number is the number of leucocytes 

 in 80 X 50 = 4,000 small squares. Now this is the number of 

 small squares in a cubic millimetre, so that the number only 

 requires to be multiplied by the dilution, in this case 100, to give 

 the number of leucocytes per cubic millimetre. If you have 

 counted eighty squares, therefore, add up the result and put on 

 two noughts; if you have counted forty squares, multiply the 

 result by two and then put on two noughts. Thus, if there are 112 

 leucocytes in 40 fields, the number per cubic millimetre is 22,400. 



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. 



* 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 h^nds, 



