Mr. J. N. Lockyer on Recent Discoveries in Solar Physics. 147 



and other gases and vapours, upon which we have for some time 

 been engaged. 



First, as to hydrogen, what could laboratory work tell us about 

 the chromosphere and the prominences ? 

 It was obviously of primary importance — 



(1) To determine the cause to which the widening of the F line 

 was due. 



(2) To study the hydrogen-spectrum very carefully under varying 

 conditions, with a view of detecting whether or not there existed a 

 line in the orange. 



We soon came to the conclusion that the principal, if not the only, 

 cause of the widening of the F line was pressure. 



Having determined, then, that the phenomena presented by the F 

 line were phenomena depending upon and indicating varying pres- 

 sures, we were in a position to determine the atmospheric pressure 

 operating in a prominence, in which the red and green lines are 

 nearly of equal width, and in the chromosphere, through which the 

 green line gradually expands as the sun is approached. 



With regard to the higher prominences, we have obtained evidence 

 that the gaseous medium of which they are composed exists in a con- 

 dition of excessive tenuity, and that even at the lower surface of the 

 chromosphere (that is, on the sun itself, in common parlance) the 

 pressure is very far below the pressure of the earth's atmosphere. 



Now I need hardly point out to you that the determination of the 

 above-mentioned facts leads us necessarily to several important mo- 

 difications of the received theory of the physical constitution of our 

 central luminary — the theory which we owe to Kirchhoff, who based 

 it upon his examination of the solar spectrum. According to his 

 hypothesis, the photosphere itself is either solid or liquid, and it is 

 surrounded by an extensive cool and non-luminous atmosphere com- 

 posed of gases and the vapours of the substances incandescent in the 

 photosphere. 



We find, however, instead of this compound cool and non-luminous 

 atmosphere outside the photosphere, one which is in a state of in- 

 candescence, is therefore luminous, and which gives us merely, or at 

 all events mainly, the spectrum of hydrogen ; and the tenuity of this 

 incandescent atmosphere is such that it is extremely improbable that 

 any considerable atmosphere, such as the corona has been imagined 

 to indicate, exists outside it. 



Here already, then, we find the " cool absorbing atmosphere " of 

 the theorists terribly reduced in height, and apparently much more 

 simple in its composition than had been imagined by Kirchhoff and 

 others. Dr. Frankland and myself have shown separately : — 



(1) That a gaseous condition of the photosphere is quite consist- 

 ent with its continuous spectrum, whether we regard the spectrum 

 of the general surface or of spots. The possibility of this condition 

 has also been suggested by Messrs. De La Rue, Stewart, and Loewy. 



(2) That a sun-spot is a region of greater absorption. 



(3) That when photospheric matter is injected into the chromo- 

 sphere we see bright lines. 



L2 



