336 HAEMOGLOBIN. [BOOK n. 



strongly marked absorption bands, lying between the solar lines D 

 and E. (See Fig. 58.) Of these the one towards the red side, 

 sometimes spoken of as the band a, is the thinnest, but the 

 most intense, and in extremely dilute solutions (Fig. 58 1) is the 

 only one visible; its middle lies at some little distance to the blue 

 side of D. Its position may be more exactly defined by expressing 

 it in wave-lengths. As is well known the rays of light which 

 make up the spectrum differ in the length of their waves, diminish- 

 ing from the red end where the waves are longest to the blue end 

 where they are shortest. Thus Frauenhofer's line D corresponds 

 to rays having a wave-length of 589*4 millionths of a millimetre. 

 Using the same unit, the centre of this absorption band a of hsemo- 



flobin corresponds to the wave-length 578; as may be seen in 

 ig. 58, where however the numbers of the divisions of the scale 

 indicate only 100,000 of a millimetre. The other, sometimes called 

 ft, much broader, lies a little to the red side of E, its blue-ward 

 edge, even in moderately dilute solutions (Fig. 58 2) coming close 

 up to that line; its centre corresponds to about wave-length 539. 

 Each band is thickest in the middle, and gradually thins away at 

 the edges. These two absorption bands are extremely characteristic 

 of a solution of haemoglobin. Even in very dilute solutions both 

 bands are visible (they may be seen in a thickness of 1 c.m. in a solu- 

 tion containing 1 grm. of haemoglobin in 10 litres of water), and 

 that when scarcely any of the extreme red end, and very little of the 

 blue end, is cut off. They then appear not only faint but narrow. 

 As the strength of the solution is increased, the bands broaden, and 

 become more intense ; at the same time both the red end, and still 

 more the blue end, of the whole spectrum, are encroached upon 

 (Fig. 58 3). This may go on until the two absorption bands become 

 fused together into one broad band (Fig. 58 4). The only rays 

 of light which then pass through the haemoglobin solution are 

 those in the green between the blueward edge of the united bands 

 and the general absorption which is now rapidly advancing from 

 the blue end, and those in the red between the united bands 

 and the general absorption at the red end. If the solution be still 

 further increased in strength, the interval on the blue side of the 

 united bands becomes absorbed also, so that the only rays which 

 pass through are the red rays lying to the red side of D; these are 

 the last to disappear, and hence the natural red colour of the solu- 

 tion as seen by transmitted light. Exactly the same appearances 

 are seen when crystals of haemoglobin are examined with a micro- 

 spectroscope. They are also seen when arterial blood itself 

 (diluted with saline solutions so that the corpuscles remain in as 

 natural condition as possible) is examined with the spectroscope, as 

 well as when a drop of blood, which from the necessary exposure to 

 air is always arterial, is examined with the micro-spectroscope. In 

 fact, the spectrum of haemoglobin is the spectrum of normal 

 arterial blood. 



