150 Royal Institution : — 



I think the recent work has shown to be the last layer of the true 

 atmosphere of the sun. I shall now invite your attention to spots. 



Now, as a rule, precisely those lines which are injected into the 

 photosphere by convection-currents are most thickened in the spec- 

 trum of a spot, and the thickening increases with the depth of the 

 spot ; so that I no longer regard a spot simply as a cavity, but as a 

 place in which principally the vapours of sodium, barium, iron, and 

 magnesium occupy a lower level than they do ordinarily in the 

 atmosphere. 



I have told you before, that when these lines are observed in the 

 chromosphere, they usually are thinner than their usual Fraunhofer 

 lines. 



I will now show a photograph of a spot-spectrum on the screen. 

 You will see a black band running across the ordinary spectrum ; 

 that black band indicates the general absorption which takes place 

 in a sun-spot. Now mark the behaviour of the Fraunhofer lines ; 

 see how they widen as they cross the spot, putting on a sudden 

 blackness and width in the case of a spot with steep sides, expanding 

 gradually in a shelving one. The behaviour of these lines is due to 

 selective absorption. 



We have, then, the following facts : mark them well : — 



(1) The lines of sodium, magnesium, and barium, when observed 

 in the chromosphere, are among those which are thinner than their 

 usual Fraunhofer lines. 



(2) The lines of sodium, magnesium, and barium, when observed 

 in a spot, are among those which are thicker than their usual Fraun- 

 hofer lines. 



They show, I think, that a spot is the seat of a downrush or 

 downsinking. 



Messrs. De La Rue, Stewart, and Loewy, who brought forward 

 the theory of a downrush before my observations of an actual down- 

 rush were made in 1865, at once suggested as one advantage of this 

 explanation that all the gradations of darkness, from the faculoe to 

 the central umbra, may be supposed to be due to the same cause, 

 namely, the presence to a greater or less extent of a relatively cooler 

 absorbing atmosphere — thus suggesting as one cause of the darken- 

 ing of a spot 



(1) The general absorption of the atmosphere, thicker here than 

 elsewhere, as the spot is a cavity. 



To which the spectroscope added in 1866, as you know, 



(2) Greater selective absorption. 



I have Dr. Frankland's permission to exhibit an experiment con- 

 nected with our researches on absorption which will show you that 

 this increased selective absorption can be fairly grappled with in our 

 laboratories. I will show you on the screen the absorption-line due 

 to sodium-vapour, in one part as thin as it is in the ordinary solar 

 spectrum, in another almost if not quite as thick as it appears in a 

 spot ; and I accomplish this result in the following way : — Here I 

 have an electric lamp, and by means of this slit I only permit a fine 

 line of light to emerge from it ; here the beam passes through a bi- 



