BLOOD. 



339 



(p) with its rays parallel. In passing through the prism the rays are dispersed by 

 unequal refraction, giving a spectrum. The spectrum thus produced is examined by 

 the observer with the aid of the telescope (B). When the telescope is properly focussed 

 for the rays entering it from the prism (p), a clear picture of the spectrum is seen. The 

 length of the spectrum will depend upon the nature and the number of prisms through 

 which the light is made to pass. For ordinary purposes a short spectrum is preferable 

 for haemoglobin bands, and a spectroscope with one prism is generally used. If the 

 source of light is a lamp-flame of some kind, the spectrum is continuous, the colors 

 gradually merging one into 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 " 



FIG. 87. 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. 



(see PI. I. , Frontispiece, for an illustration in colors of the solar spectrum). 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 in the charts below. If while using solar 

 light or an artificial light a solution of any substance which gives absorption bands is 

 so placed in front of the slit that the light is obliged to traverse it, the spectrum as 

 observed through the telescope will show one or more narrow or broad black bands, 

 which are characteristic of the substance used and which constitute its absorption spec- 

 trum. The positions of these bands may be designated by describing their relations to 

 the Fraunhofer lines, or more directly by stating the wave-lengths of the portions of the 

 spectrum between which absorption takes place. Some spectroscopes are provided with 

 a scale of wave-lengths superimposed on the spectrum, and when properly adjusted 

 this scale enables one to read off directly the wave-length of any part of the spectrum. 



When very dilute solutions of oxyhsemoglobin 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 "/9-band" (Fig. 88, and 

 also PI. I. spectrum 4). With a solution containing 0.09 per cent, of oxy- 

 hsemoglobin, and examined in layers one centimeter thick, the a-band extends 

 over the part of the spectrum included between the wave-lengths ^ 583 



