326 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



upper portion of the slit receives light from another 

 source ; and it passes to the lower part of the prism. 

 Two spectra are thus produced, one below the other, 

 and comparison can easily be made. 



The spectroscope in physiology. Hoppe 

 Seyler and Stokes were the first to show that blood 

 had a distinguishing spectrum of its own. If a layer 

 of blood be interposed between the source of light and 

 the slit of a spectroscope, the only rays that are per- 

 mitted to pass through the layer of blood are the red, 

 and only the red end of the spectrum is visible. As 

 the blood is diluted, more and more light is able to 

 pass through it, orange first, then yellow, and so on 

 till the whole spectrum is almost restored. But there 

 remain towards the red end two dark bands ; they 

 are situated between D and E of the solar spectrum. 

 One of them is on the violet side of D, is the thinner 

 of the two, but the more intense ; the other is much 

 broader, and lies to the red side of E, its edge coming 

 close up to E. These are the ABSORPTION BANDS OF 

 HAEMOGLOBIN, the red colouring matter of the red 

 blood corpuscles. More particularly this is the 

 spectrum of haemoglobin as it exists in normal blood 

 in loose chemical combination with oxygen. When 

 oxygen has been removed from the blood by increased 

 temperature and sufficiently low pressure, or by the 

 passage through the blood of some indifferent gas such 

 as hydrogen, or, still more rapidly and easily, by the 

 addition to the blood of reducing agents, such as 

 sulphide of ammonium, the spectrum gives a new 

 absorption band. The two bands disappear, and in 

 their place is one band, situated midway between the 

 positions of the bands of oxy haemoglobin. It is much 

 broader than either of the two, though not so dark, 

 and in its case less of the blue end of the solar 

 spectrum is absorbed. This band is distinguished 

 from the other as the ABSORPTION BAND OF REDUCED 



