The shining face of each one of the innumerable suns aggregated into the vast system of the galaxy is, as we have said, veiled by absorbing vapours; but in widely varying degrees. The light of Vega, though indelibly stamped with the characteristic lines of hydrogen, reaches our upper air without sensible general modification. Sunlight is not only charged with significant inscriptions, but throughout toned down and mellowed; while in Aldebaran, the process of stoppage in the blue has been carried so far as to leave the red rays visibly predominant. In Aldebaran, too, the first symptoms begin to appear of a generic change in the manner of absorption through the emergence of dark bands in addition to the dark lines of the spectrum.

This change is full of meaning. Isolated rays of definite wave-lengths, forming in the spectrum what we call ‘lines,’ bright or dark, are emitted only at very high temperatures. They represent perhaps the fundamental vibrations of the ‘atoms’ of each different substance. But these, at lower grades of heat, are not free to thrill separately. Bound together into ‘molecules,’ they give rise, by their associated vibrations, to complex systems of light-waves, dispersed into sets of prismatic ‘flutings,’ each with a sharp and a nebulous side. The fluted spectra thus constituted are derived from chemical compounds such as oxides and chlorides, as well as from ‘elementary’ substances, both metallic and non-metallic, at moderate degrees of heat.