THE MOVEMENTS OF THE STARS. 289 



orchestra is absent and the strings are set in vibration by 

 fiddle bows or otherwise, they shed forth a volume of 

 sound all of that one particular note, which alone they are 

 competent to produce. In other words, we see that when 

 the strings are used as a radiating source of music, the 

 character of the music they emit is absolutely identical 

 with those particular notes which they refuse to allow to 

 pass when they are interposed between the orchestra and 

 auditorium. In other words, the particular notes which 

 are absorbed in the one case are identically the notes which 

 are radiated in the other. 



This illustration from sound will facilitate our ex- 

 planation of the dark lines in the solar spectrum. Hy- 

 drogen in the sun's atmosphere, being cold compared 

 with the deeper seated portions of the great luminary, 

 stops certain light on its way outwards from the heated 

 regions, and the light which the hydrogen stops is exactly 

 the same which that gas will itself give out if heated 

 to incandescence. We might make the illustration a 

 little more complete by supposing that the forest of strings 

 comprised not merely a single note, but that two, three, 

 or even more notes each have many strings belong- 

 ing to it. In that case the filtration from the music 

 produced by the orchestra would remove from it every 

 note of music to which there was a group of strings 

 oorresponding ; the auditors might therefore find not only 

 one note, but perhaps two, three, or even more wanting in 

 different parts. Now we come very close indeed to our 

 explanation of a system of dark lines corresponding to any 

 particular element in the solar spectrum. The molecules 

 of hydrogen, for instance, respond not merely to the 



