1868.] 



Constitution of the Sun and Stars, 



27 



atmosphere, the effect of obliquity will be very much greater ; so that we 

 may expect to find these rays most conspicuous in spectra of light from 

 very near the edge of the disk. This appears to account for observations* 

 lately made by Angstrom. 



38. Let us now consider the information given to us by the lines of the 

 spectrum which are due to hydrogen, sodium, and magnesium. In the 

 first place the sodium lines are narrow and sharply defined. In both 

 respects they differ from the lines of hydrogen and magnesium, which are 

 broad and winged, that is, shaded off on one or both sides into dusky 

 bands less dark than themselves. Now at and up to the temperature of 

 the flame of a spirit-lamp sodium vapour can give rise to such lines ; but 

 at the temperature of a Bunsen's burner the sodium lines have begun to 

 expand and be ill defined. Hence we learn that in those upper regions of 

 the sun's sodium atmosphere in which these lines originate, the temperature 

 is lower than that of the flame of a Bunsen's burner. Nor need we be 

 astonished that this or a much lower temperature can prevail so close to 

 the fierce heat of the photosphere, when we take into account how effectu- 

 ally the outer parts of the sun's atmosphere are screened from the glare 

 beneath by the stoppage in the intermediate regions of almost every ray 

 that could act upon them. 



39. The absence of wings to the lines D indicates f to us that there is not 

 in the sun's atmosphere enough of sodium vapour of temperatures inter- 

 mediate between the temperature of a Bunsen's burner and the temperature 

 of the photosphere to be in a sensible degree opake to the wings of the rays 

 which it emits. This both shows what a mere trace of sodium is diffused 

 through the solar atmosphere, and also to what a vast height it rises as com- 

 pared with the thickness of that part of the solar atmosphere which ranges 

 in temperature between a temperature below that of a Bunsen's flame, and 

 a temperature comparable with the intense heat of the photosphere. In 

 fact, the atmosphere of sodium, owing to the small mass of its molecules, 

 which is less than half the mass of molecules of iron, must spread to a 

 vast distance beyond the iron atmosphere ; and through this immense 

 space the temperature appears to vary very slowly, and to be nowhere 

 high. 



40. The outward stream of heat which reaches the upper layer of the 

 iron atmosphere for the most part escapes into space from that neighbour- 

 hood through the numberless lines of iron, calcium, chroniufn, manganese, 

 and through the darker of the lines of nickel and cobalt, all of which 



* See Comptes Eendus of October 15, 1866, or Philosophical Magazine of January 

 1867. It would be very desirable to have observations made upon spectra of light 

 taken from different parts of the sun's disk, brought one over the other into the same 

 field. 



f [I remain unsatisfied with part of this discussion of the absence of sodium wings. 

 There is something in the limitation of the wings of the rays of this and of some other 

 gases, especially of hydrogen, of which I do not see the explanation. — September 1868.] 



