38 



BOOK IL APPLICATION OF THE INQUIRY TO THE SUN 



AND STARS. 



CHAP. IV. THE SUN'S CHROMOSPHERE. 



I STATED in the previous chapter (p. 33), that in order to utilize the 

 information placed at our disposal by the discovery of the new lines 

 seen in the spectra of metals exposed to high temperatures, I had 

 brought the enhanced lines of the chief metallic elements together, and 

 thus formed a " test-spectrum " to use as a new engine of research in 

 regions of work where help might be expected from it. 



In this chapter I shall deal with the application of this test-spectrum 

 to a study of the sun. 



It is obvious that the general spectrum of the sun, like that of -stars 

 generally, is built up of all the absorptions which can make themselvc* 

 felt in every layer of its atmosphere from bottom to top, that is from 

 the photosphere to the outermost part of the corona. It is important 

 to note that this spectrum is cJiangeless from year to year. 



Now sun spots are disturbances produced in the photosphere ; and 

 the chromosphere, with its disturbances, called prominences, lie directly 

 above it. Here, then, we are dealing with the lowest part of the sun's 

 atmosphere. We find first of all that in opposition to the changeless 

 general spectrum, great changes occur with the sun-spot period, both 

 in the spots and chromosphere. 



The spot spectrum is indicated, as was found in 1866, by the widen- 

 ing of certain lines; the chromospheric spectrum, as was found in 1868, 

 by the appearance at the sun's limb of certain bright lines. In both 

 cases the lines affected seen at any one time are almost always rela- 

 tively few in number. 



Since 1868 we have been enabled to observe not only the spectrum 

 of the sun's spots, but that of the chromosphere as well, every day 

 when the sun shines. The chromosphere is full of marvels. At first, 

 when our knowledge of spectra was very much more restricted than 

 now, almost all the lines observed were unknown. In 1868 I saw a 

 line in the yellow, which I found behaved very much like hydrogen, 

 though I could prove that it was not due to hydrogen ; for laboratory 

 use the substance which gave rise to it I named helium. Next year, 

 as I stated in the last chapter, I saw a line in the green at 1474 of 

 Kirchhoff's scale. That was an unknown line, but in some subsequent 



