THE BLOOD 117 



substance contained in a vessel with parallel sides (the haematoscope 

 of Hermann, F in fig. 33), the spectrum is found to be no longer 

 continuous, but is interrupted by a number of dark shadows, or 

 absorption bands, corresponding to the light absorbed by the coloured 

 medium. Thus a solution of oxyhaemoglobin of a certain strength 

 gives two bands between the D and E lines ; haemoglobin gives only 

 one ; and other red solutions, though to the naked eye similar to 

 oxyhsemoglobin, will give characteristic bands in other positions. 



A convenient form of small spectroscope is the direct-vision 

 spectroscope, in which, by an arrangement of alternating prisms 

 of crown and flint glass (see fig. 34), the spectrum is observed 

 by the eye in the same line as the tube furnished with the slit. 

 Such small spectroscopes may be used for class purposes, and 

 may for convenience be mounted on a stand provided with a gas- 

 burner and a receptacle for the test-tube (see fig. 35). In the 

 examination of the spectrum of small coloured objects, a combina- 

 tion of the microscope and direct-vision spectroscope, called the 

 micro-spectroscope, is used. 



Fig. 36 illustrates a method of representing absorption spectra 



FIG. 34. Arrangement of prisms in direct-vision spectroscope. 



diagrammatically. The" solution' was examined in a layer 1 cen- 

 timetre thick. The base line has on it at the proper distances 

 the chief Fraunhofer lines, and along the right-hand edges are 

 the percentage amounts of oxyhaemoglobin present in I, of haemo- 

 globin in II. The width of the shadings at each level represents the 

 position and amount of absorption corresponding to the percentages. 

 The characteristic spectrum of oxyhaemoglobin, as it actually 

 appears through the spectroscope, is seen in the next figure (fig. 37, 

 spectrum 2). There are two distinct absorption bands between the 

 D and E lines ; the one nearest to D (the a band) is narrower, 

 darker, and has better defined edges than the other (the /3 band). 

 As will be seen on looking at fig. 36, a solution of oxyhaemoglobin of 

 concentration greater than 0'65 per cent, and less than O85 per cent. k 

 (examined in a cell of the usual thickness of 1 centimetre) gives one 



