15 



finger from the tip ; the diluting fluid then enters. 

 Keep the pipette nearly horizontal. Fill to the 

 mark n. 



6. The rest is the same as for the red cells. 



7. Count a thousand leucocytes, if possible. 

 The use of a special counting chamber may, 



however, be obviated by the method of counting fields. 



1. Draw the tube of the microscope out so that 

 the diameter of the microscope field is exactly equal 

 in length to that of eight squares of the counter. Mark 

 this point on the tube for future use. The area of the 

 field is then almost equal to that of fifty squares. 



2. Now count the leucocytes in say eighty fields. 

 This would give 4,000 squares (and normally a possible 

 total of 1,000 leucocytes). 



3. The calculation is the same as before. 



THE ESTIMATION OF THE HAEMOGLOBIN 



Tallqvist. The blood drop is absorbed by a strip 

 of blotting paper and compared directly with a series 

 of coloured standards. Clinically useful and accurate 

 to within 5 or 10 per cent. 



Gowers. The standard of comparison is here a 

 tube of picrocarmine gelatine equivalent in colour to 

 one per cent, solution of normal blood. The height of 

 this solution is the same as that marked 100 in- the 

 second tube, and is the height occupied by 2 c.c. of fluid. 

 The blood pipette holds 20 mm. 3 Hence 20 mm. 3 in 

 2,000 mm. 3 (2 c.c.) = I in a 100, i.e., one per cent, 

 solution of normal blood. If now the colours corre- 

 spond the blood examined is normal. If less water is 

 required the blood is so much per cent, below the 

 normal; the readings being made directly. Daylight 

 is used. 



