ESTIMATION OF THE NUMBER OF LEUCOCYTES 



213 



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; 



Fig. 45. Showing Field of Microscope adjusted so that its Diameter 



IS EgnAL TO the Width of Eight Squares. 



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. 



It happens with some microscopes that there is no combination 

 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 



