1 5 2 THE SUNS L UMINO US A TMOSPHERE. PART i. 



even to shade, the deepest dark lines corresponding to 

 the most brilliant bright ones, and he computed that 

 the chances are as 1 to the ninth power of 10, that 

 the coincidence of these seventy lines is not fortuitous, 

 but owing to a definite cause, whence he concluded 

 that the presence of iron vapour in the solar atmo- 

 sphere is proved with as much certainty as can be 

 attained in any question of natural science. 



In a later publication, M. Angstrom observes that, 

 although the coincident iron lines between D and F are 

 not so numerous as M. Kirchhoff affirmed, they are quite 

 sufficient to establish beyond a doubt the presence of 

 iron in the solar atmosphere. The iron lines are the 

 most characteristic in the whole solar spectrum, and if 

 a magnifying power be used, or if the light be refracted 

 through several prisms, these lines, or at any rate the 

 stronger ones among them, appear to be perfectly black. 

 M. Angstrom noticed that on a .careful examination of 

 the solar spectrum, certain lines can be discovered, im- 

 bedded in a mass of fainter ones, which, with increased 

 illumination, seem to withdraw themselves and disap- 

 pear, while the first mentioned lines, on the contrary, 

 only stand out in a stronger relief. These are metallic 

 lines of high fusion temperature ; the most remarkable 

 among them almost invariably belong to iron. 



The substances common on earth that have their 

 vapours in the atmosphere of the sun, though they 

 have fewer bright lines in their spectra than that of 

 iron, are quite as characteristic, and quite as distinctly 

 coincident with their reverses, whether they be single, 

 in groups, or double, as the sodium line, which is brighter 

 and its reverses darker than that of any other sub- 

 stance, because volatilized sodium gives out a greater 

 quantity of light, and consequently absorbs a greater 

 quantity. 



M, Angstrom has added aluminium and manganese 



