Absorption Spectra. 231 



Here we have a continuous representation of what was repre- 

 sented in interrupted stages in Plate I. And this, I may 

 remark, is not a mere diagrammatic representation of what 

 is seen, but a representation carefully measured and drawn 

 to scale of the appearance as seen through one of Zeiss's 

 micro-spectroscopes. We notice that both absorption bands 

 are uniformly wedge-shaped, the broad ends of the wedges 

 corresponding to the darkest end of the slit. On tracing 

 each absorption band downwards, its edges are seen to con- 

 verge uniformly, until, when at a point in about the middle 

 of the figure, and long before the edges have come together, 

 the band becomes invisible. If, now, bearing in mind that 

 the convergence of the sides of the band is uniform, the 

 lower half of this band A is temporarily obscured from view, 

 we can correctly infer its position by continuing the two 

 sides of the upper, or unobscured, portion of the band in the 

 same lines, respectively, downwards : and the same may 

 be done in the case of the invisible portion, and so its 

 position be inferred, though we cannot see it. On doing 

 so we shall find that they converge in this instance to 

 a point at "578 on this scale of wave lengths, which is a 

 scale reflected from the surface of the spectroscopic prism. 

 This point of convergence is in the line of greatest intensity, 

 or the absorption line which first appears in the thinnest 

 film, and, therefore, a line essential to all thicknesses of this 

 medium. And this is what we were looking for. On 

 dealing with the other absorption band at B, the lines 

 drawn along the sides of the band converge to a point 

 below the scale, and on drawing a line at right angles to the 

 scale and through the point of convergence it is found to 

 cut the scale at "542. Constructing a spectrum with these 

 two primitive lines in position, we get such a one as is 

 depicted at (3) indicated as " the primitive spectrum of 

 human arterial blood." For purposes of comparison with 

 other and similar spectra, such a spectrum as this, assuming 



