430 



BLOOD AND LYMPH. 



the spectrum is continuous, the colors gradually merging one mto another 

 from red to violet. If sunlight is used, the spectrum will be crossed by a 

 number of narrow dark lines known as the " Fraunhofer lines " The position 

 of these lines in the solar spectrum is fixed, and the more distinct ones are 

 designated by letters of the alphabet, A,B,C, D, E, etc., as shown m the charts 

 below. If while using solar light or an artificial bght a solution of any sub- 

 stance which gives absorption bands is so placed in front of the slit that the 

 light is obUged to traverse it, the spectrum as observed through the telescope 

 mil show one or more narrow or broad black bands that are characteristic 

 of the substance used and constitute its absorption spectrum. The positions 

 of these bands may be designated by describing their relations to the Fraun- 

 hofer lines or more directly by stating the wave lengths of the portions ot 

 the spectrum between which absorption takes place. Some spectroscopes are 

 provided with a scale of wave lengths superposed on the spectrum, and when 

 properly adjusted this scale enables one to read off directly the wave lengths 

 of any part of the spectrum. 



Fig. 1 82.^Spectroscope : P, The glass prism ; A , the collimator tube, showing the slit, S, 

 through which the light is admitted; B, the telescope for observing the spectrum. 



When very dilute solutions of oxyhemoglobin are examined with 

 the spectroscope, two absorption bands appear, both occurring in 

 the portion of the spectrum included between the Fraunhofer lines 

 D and E. The band nearer the red end of the spectrum is known 

 as the "a-band"; it is narrower, darker, and more clearly defined 

 than the other, the "/?-band" (Fig. 183). The width and distinct- 

 ness of the bands vary naturally with the concentration of the solution 

 used (see Fig. 184) or, if the concentration remains the same 

 with the width of the stratum of liquid through which the light 

 passes. With a certain minimal percentage of oxyhemoglobin 

 (less than 0.01 per cent.) the /3-band is lost and the a-band is very 

 faint in layers 1 centimeter thick. With stronger solutions the 



