LEUCOCYTE COUNTS 205 



Making a preparation, exactly as was done in the case of the red count, we count 

 all of the white cells in one of the large squares (i sq. mm.). The cross ruling greatly 

 facilitates this. Note the number. Then count a second and a third large square. 

 Strike an average for the large squares counted and multiply this by 10, as the depth 

 of the fluid gives a content equal to only ^ cu. mm. Then multiply by the 

 dilution. Example: First large square 50; second large square 70; third large 

 square 60. Average 60. Then 60 X 10 X 20 = 12,000, the number of leukocytes in 

 i cu. mm. of blood. The count may be made with a low power (%-inch objec- 

 tive) as the leukocytes stand out like pearls. It is more accurate, however, to use a 

 higher power, so that pieces of foreign material may be recognized and not enumer- 

 ated as white cells. If one will accustom himself to comparing the distribution of 

 the leukocytes in a well-made stained dried blood film, prepared according to Ehrlich's 

 cover-glass method, with that in a haemacytometer preparation, he can readily 

 acquire an experience which will enable him to determine with considerable accuracy 

 the degree of leukocytosis by the examination of a stained cover-glass preparation 

 alone. Furthermore, one can identify the leukocytes in a Giemsa-stained smear 

 with the %-inch objective. This is specially true of the large mononuclears and 

 transitionals whose increase has such significance in the tropics. 



Special diaphragms for the ocular, with a square opening, which just covers one 

 of the large squares of the haemacytometer (400 small squares) may be purchased 

 or one may cut a square hole from a round piece of stiff paper which rests on the ocular 

 micrometer supports. This is easily measured by noting the number of spaces on 

 the ocular micrometer which equals one side of the square. With such an opening 

 one can count the leukocytes in unruled areas equal to a large square, when the ruling 

 is less convenient than is present in the Ttirck ruling. 



Combined Red and White Counts. In the absence of a white pipette or when 

 it is desired to make a white count with the same preparation as is used for the red 

 one, especially if the ruling is of the old style (only central ruling and not in nine 

 large squares as with Zappert and Tiirck), it is advisable to make use of the method 

 of counting by fields. With a Leitz No. 4 ocular and a No. 6 objective, with a tube 

 length of 120 mm., it will be observed that the field so obtained has a diameter 

 of eight small squares. Now, remembering that the area of a circle equals the square 

 of the radius multiplied by TT, or 3.1416, we have the following calculation: The 

 diameter being eight small squares, the radius would be four small squares. Squar- 

 ing the radius, we have 16. This multiplied by 3.1416 gives us 50. This 

 means that every field, with the microscope adjusted as stated, contains 50 of 

 the small squares, or Ho of the unit of i cu. mm. of the diluted blood. 



By keeping a single red cell in view while moving the mechanical stage from right 

 to left or from above downward, we know that a new field of 50 small squares is 

 brought into view when the circumference of the field cuts this individual cell. 

 Example: As 2000 small squares would ordinarily be a sufl&cient number to count 

 for a white count, this would require us to count the number of leukocytes in 40 

 of the designated microscopic fields (this, of course, is only one-half the unit, hence 

 we should multiply by 2). Counted 40 fields and noted 50 white cells. 50 X 2 

 = 100 X 200 (the dilution in red pipette) = 20,000. Consequently 20,000 would 

 represent the number of leukocytes in i cu. mm. of the blood examined. 



Cleaning Apparatus. After making a blood count, the haemacytometer slide 

 should be cleaned with soap and water and then rubbed dry, preferably with an old 



