1869.] 



Observations of the Sun. 



351 



Dr. Frankland and myself have pointed out that, although the chromo- 

 sphere and the prominences give out the spectrum of hydrogen, it does not 

 follow that they are composed merely of that suhstance : supposing others 

 to be mixed up with hydrogen, we might presume that they would be indi- 

 cated by their selective absorption near the sun's limb. In this case the 

 spectrum of the limb would contain additional Fraunhofer lines. I have 

 pursued this investigation to some extent, with, at present, negative results ; 

 but I find that special instrumental appliances are necessary to settle the 

 question, and these are now being constructed. 



If we assume, as already suggested by Dr. Frankland and myself, that 

 no other extensive atmosphere besides the chromosphere overlies the pho- 

 tosphere, the darkening of the limb being due to the general absorption of 

 the chromosphere, it will follow : — 



I. That an additional selective absorption near the limb is extremely 

 probable. 



II. That the hydrogen Fraunhofer lines indicating the absorption of the 

 outer shell of the chromosphere will vary somewhat in thickness : 

 this I find to be the case to a certain extent. 



III. That it is not probable that the prominences will be visible on the 

 sun's disk. 



In connexion with the probable chromospheric darkening of the limb, 

 an observation of a spot on February 20th is of importance. The spot 

 observed was near the limb, and the absorption was much greater than 

 anything I had seen before ; so great, in fact, was the general absorption, 

 that the several lines could only be distinguished with difficulty, except in 

 the very brightest region. I ascribe this to the greater length of the 

 absorbing medium in the spot itself in the line of sight, when the spot is 

 observed near the limb, than when it is observed in the centre of the disk — 

 another indication of the great general absorbing power of a comparatively 

 thin layer, on rays passing through it obliquely. 



I now come to the selective absorption in a spot. I have commenced a 

 map of the spot-spectrum, which, however, will require some time to com- 

 plete. In the interim, I may state that the result of my work up to the 

 present time in this direction has been to add magnesium and barium to 

 the material (sodium) to which I referred in my paper in 1866, No. I. of 

 the present series ; and I no longer regard a spot simply as a cavity, but as 

 a place in which principally the vapours of sodium, barium, and magne- 

 sium (owing to a downrush) occupy a lower position than they do ordi- 

 narily in the photosphere. 



I do not make this assertion merely on the strength of the lines observed 

 to be thickest in the spot-spectrum, but also upon the following observa- 

 tions on the chromosphere made on the 21st and 28th ultimo. 



On both these days the brilliancy of the F line taught me that something 

 unusual was going on ; so I swept along the spectrum to see if any materials - 

 were being injected into the chromosphere. 



