CHAP. IV.] THE BLOOD. METHODS OF RESEARCH. 185 



The apparatus consists of two glass tubes of exactly the same size. 

 One contains a standard of the tint of a dilution of 20 cubic mm. of 

 blood in 2 cubic centimetres of water (1 in 100). 



The second tube is graduated, 100 degrees = two centimetres (100 

 times twenty cubic millimetres). 



The twenty cubic millimetres of blood are measured by a capillary 

 pipette (similar to, but larger than that used for the haemacytometer). 

 The quantity of the blood to be tested is ejected into the bottom of 

 the tube, a few drops of distilled water being first placed in the 

 latter. The mixture is rapidly agitated to prevent the coagulation 

 of the blood. The distilled water is then added drop by drop (from 

 the pipette-stopper of a bottle supplied for that purpose) until the tint 

 of the dilution is the same as that of the standard, and the amount of 

 water which has been added (i.e. the degrees of dilution) indicates the 

 amount of haemoglobin. 



Since average normal blood yields the tint of the standard at 100 

 degrees of dilution, the number of degrees of dilution necessary to 

 obtain the same tint with a given specimen of blood is the percentage 

 proportion of the haemoglobin contained in it, compared to the 

 normal. 



For instance, the twenty cubic millimetres of blood from a patient 

 with anaemia gave the standard tint at 30 degrees of dilution. Hence 

 it contained only thirty per cent, of the normal quantity of haemo- 

 globin. 



By ascertaining with the haemacytometer (p. 77) the corpuscular 

 richness of the blood, we are able to compare the relation between 

 the number of corpuscles and the amount of haemoglobin. A frac- 

 tion, of which the numerator is the percentage of haemoglobin, and 

 the denominator the percentage of corpuscles, gives at once the 

 average value per corpuscle. Thus the blood mentioned above, 

 containing thirty per cent, of haemoglobin, contained sixty per 

 cent, of corpuscles ; hence the average value of each corpuscle 

 was fj or i of the normal. Variations in the amount of haemo- 

 globin may be recorded on the same chart as that employed for 

 the corpuscles. 



In using the instrument the tint may be estimated by holding the 

 tubes between the eye and a window, or by placing a piece of white 

 paper behind the tubes ; the former is perhaps the best. Care must 

 be taken that the tubes are always held in the line of light, not below 

 it. In the latter case some light is reflected from the suspended 

 corpuscles from which the haemoglobin has been dissolved. If the 

 value of the corpuscles is small, then a perceptibly paler tint is seen 

 when the tubes are held below the line of illumination. If all the 

 light is transmitted directly through the tubes the corpuscles do not 

 interfere with the tint. 



In using the instrument it will be found that between six or 

 eight degrees of dilution it is difficult to distinguish a difference 

 between the tint of the tubes. It is therefore necessary to note the 



