igS CLINICAL BACTERIOLOGY AND HyEMATOLOGY 



completely fill the tube, being drawn up by capillary attraction. 

 When quite full, wipe both ends of the tube with the fingers,^'and 

 apply the end of the glass pipette (previously filled with water) to 

 the pointed end of the capillary tube. Now squeeze the nipple 

 gently, so as to force the blood and (subsequently) the water drop 

 by drop into the cell. Interrupt the process occasionally, and stir 

 the contents of the cell with the metal handle of the measuring 

 tube. Continue to add water until the cell is exactly full : this is 

 the first step which presents the slightest difficulty. Apply the 

 cover-glass ; this must not enclose any air under it, nor cause any 

 of the diluted blood to flow into the moat round the cell. 



The specimen is now ready for comparison with the standards. 

 It is to be taken into a dark room and examined by the light of 

 one of the candles. This is to be placed in front of the observer 

 at a short distance from the specimen and standards, which must 

 lie side by side. 



The viewing is best done by means of a camera-tube which 

 folds into the box containing the whole apparatus. It terminates 

 in a diaphragm which is perforated by two small holes, one of 

 which is to be placed over the centre of the specimen and the 

 other over the centre of the standard. The latter is to be moved 

 about until a disc is found which nearly or quite corresponds in 

 colour with the diluted blood in the cell. If the correspondence 

 is exact, the process is at an end ; the number against the disc in 

 question represents the percentage amount of haemoglobin. If 

 there is no disc which exactly matches the specimen, the latter is 

 placed against the disc which is nearest to it, but not so deep in 

 colour. For example, if we found that the specimen was darker 

 than the disc numbered 50, but paler than that numbered 60, then 

 it would be placed opposite to 50. A slip of colourless glass is 

 then applied over the specimen, and riders over the standard disc, 

 until an exact match is obtained. If, in the case mentioned above, 

 we had to add a rider marked 5 to the standard to bring about an 

 exact match, the percentage amount of haemoglobin in the blood 

 would be 55. 



It is an advantage to place cell and standards side by side 

 rather than one above the other, for the upper and lower portions 

 of the retina differ in sensitiveness to colour, whilst the sides 

 do not. 



