1869.] 



Observations of the Sun. 



77 



regards the possible results of future laboratory work. The same remark 

 applies to magnesium and barium. 



Dr. Frankland and myself have determined that the widening out of the 

 sodium-line in the spectrum of a spot which I pointed out in 1866, and 

 then stated to be possibly an evidence of greater absorption, indicates a 

 greater absorption due to greater pressure. 



The continuous widening out of the sodium-line in a spot must there- 

 fore be regarded as furnishing an additional argument (if one were now 

 needed) in favour of the theory of the physical constitution of the sun first 

 put forward by Dr. Frankland and myself — namely, that the chromosphere 

 and the photosphere form the true atmosphere of the sun, and that under 

 ordinary circumstances the absorption is continuous from the top of the 

 chromosphere to the bottom of the photosphere, at whatever depth from 

 the bottom of the spot that bottom may be assumed to be. 



This theory was based upon all our observations made from 1866 up to 

 the time at which it was communicated to the Royal Society and the Paris 

 Academy of Sciences, and has been strengthened by all our subsequent 

 work ; but several announcements made by Father Secchi to the Paris 

 Academy of Sciences and other learned bodies are so opposed to it, and 

 differ so much from my own observations, that it is necessary that I should 

 refer to them, and give my reasons for still thinking that the theory above 

 referred to is not in disaccord with facts. At the same time I must state 

 that Father Secchi does not combat this theory ; indeed it is not to be 

 gathered from any of his communications that he has seen any of the papers 

 communicated by myself to the Royal Society. 



Father Secchi states that the chromosphere is often separated from the 

 photosphere, and that between the chromosphere and the photosphere 

 there exists a stratum giving a continuous spectrum, which he considers to 

 be the base of the solar atmosphere, and in which he thinks that the in- 

 version of the spectrum takes place. 



With regard to the first assertion, I may first state that all the observa- 

 tions I have made have led me to a contrary conclusion. Secondly, in an 

 instrument of comparatively small dispersive power, such as that employed 

 by Father Secchi, in which the widening out of the F line at the base of 

 the chromosphere is not clearly indicated, it is almost impossible to deter- 

 mine, by means of the spectroscope, whether the chromosphere rests on the 

 sun or not, as the chromosphere is an envelope and we are not dealing merely 

 with a section. But an instrument of great dispersive power can at once 

 settle the question ; for since the F line widens out with pressure, and as 

 the pressure increases as the sun is approached, the continuous curvature 

 of the F line must indicate really the spectrum of a section ; and if the 

 chromosphere were suspended merely at a certain height above the photo- 

 sphere, we should not get a widening due to pressure : but we always do 

 get such a widening. 



