ESTIMATION OF THE NUMBER OF LEUCOCYTES 



249 



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 num- 

 bers 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 



FIG. 50. SHOWING FIELD OF MICROSCOPE ADJUSTED so THAT ITS DIAMETER 

 is EQUAL TO THE WIDTH OF EIGHT SQUARES. 



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 milli- 

 metre, 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 num- 

 ber 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, 



