CHAP. IL] RESPIRATION. 561 



the same bright arterial colour. A tolerably dilute solution placed 

 before the spectroscope is found to absorb certain rays of light in 

 a peculiar and characteristic manner. A portion of the red end of 

 the spectrum is absorbed, as is also a much larger portion of the 

 blue end ; but what is most striking is the presence of two 

 strongly marked absorption bands, lying between the solar lines D 

 and E. (See Fig. 75.) 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. 75, 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 denned 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 millimeter. 

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

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

 Fig. 75, where however the numbers of the divisions of the scale 

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

 /?, much broader, lies a little to the red side of E, its blueward 

 edge, even in moderately dilute solutions (Fig. 75, 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 character- 

 istic 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 solution 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. 75, 3). This may go on until 

 the two absorption bands become fused together into one broad band 

 (Fig. 75, 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 solution as seen by transmitted light. Exactly 

 the same appearances are seen when crystals of haemoglobin are 

 examined with a microspectroscope. They are also seen when 

 arterial blood itself (diluted with saline solutions so that the 



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