of Bright Lines in the Solar Spectrum. 51 



shown with considerable certainty to be present as dark lines 

 in the solar spectrum. 



Since the publication of Professor Draper's discovery, I 

 have given much attention to the consideration of a cause for 

 the apparently anomalous brightness of the oxygen-lines ; and 

 in the present paper I venture to advance an explanation 

 which has recommended itself as -being worthy of notice, not 

 only because it offers a reconciliation of the known solar spec- 

 trum with the generally accepted views of the constitution of 

 the sun's atmosphere, but likewise because it furnishes a sug- 

 gestive hypothesis for the attack of many other obscure prob- 

 lems in solar physics. 



1. I shall throughout this paper consider it to be established 

 that the gaseous envelopes surrounding the sun succeed each 

 other in the following order, commencing with the lowest :— > 



1. Photosphere. 



2. Reversing layer. 



3. Chromosphere. 



4. Coronal atmosphere. 



I also assume the truth of the hypothesis, first advanced by 

 Johnstone Stoney *, who showed, from purely theoretical con- 

 siderations, that in the sun's atmosphere the various elements 

 must extend to heights which are, broadly speaking, inversely 

 as their vapour-densities. This view has, in my belief, been 

 substantially confirmed by subsequent observation. Thus ni- 

 trogen and oxygen, having the respective densities 14 and 16 

 (H = l), would extend to a great height in the solar atmo- 

 sphere, rising above sodium, calcium, and magnesium, and 

 having exterior to them the unknown substance giving the D 3 

 line (helium), hydrogen, and the element giving the coronal 

 line " 1474." 



2. Two suppositions can be made concerning the sun's tem- 

 perature. In the first place, it may be assumed that the 

 temperature is so enormously elevated that no chemical com- 

 pound is anywhere capable of existing in his atmosphere ; in 

 other words, dissociation may be considered to be complete. 

 In the next place, it may be supposed that the temperature 

 falls off sufficiently at some region of the outer portion of the 

 sun's atmosphere for certain chemical combinations to take 

 place. 



3. Let us first assume that the temperature of the sun is so 



* Proc. Roy. Soc. xvi. p. 25, and xvii. p. 1 : Phil. Mag-. August 1808 ; 

 Monthly Notices Roy. Astr. Soc. Dec. 18(37 ; Lockycr, Phil. Trans. 1873, 

 vol. clxiii. p. 265". 



E2 



