Mr. J. N. Lockyer on Recent Discoveries in Solar Physics. 151 



sulphide-of-carbon prism, and there you see on the screen the glo- 

 rious spectrum due to the dismemberment of the fine line of poly- 

 chromatic light. Mr. Pedler will now place a glass tube containing 

 metallic sodium, sealed up with hydrogen, in front of the slit, and 

 will heat it with a spirit-lamp. 



As the sodium-vapour rises you see the dark line of absorption 

 make its appearance as an extremely fine line, and finally you see 

 that the light which traverses the upper layer of the sodium scarcely 

 suffers any absorption — the line is thin ; while, on the contrary, the 

 light which has traversed the lower, denser layers has suffered tre- 

 mendous absorption : the line is inordinately thick, such as we see 

 it in the spectrum of a spot. 



So much, then, for the selective absorption. My recent observa- 

 tions, to which I will shortly draw attention, show, I think, that it 

 is of great importance, especially in connexion with the fact that the 

 passage from the penumbra to the umbra is generally less gradual 

 than that from the photosphere to the penumbra. You see now how 

 much is included in the assertion that the photosphere is gaseous. 



You are all, I know, familiar with that grand generalization of 

 KirchhofT's, by which he accounted for the Fraunhofer lines. 



If we have a gas or a vapour less luminous than another light- 

 source, and view that light-source through the gas or vapour, then 

 we shall observe absorption of those particular rays which the ga- 

 seous vapour would emit if incandescent. 



Let us confine our attention to the hydrogen Fraunhofer lines. 



When I observe the chromosphere on the sun's limb, with no 

 brighter light-source behind it, I observe its characteristic lines 

 bright. But when 1 observe them on the sun itself (that is, when 

 the brighter sun is on the other side of the hydrogen envelope), then, 

 as a rule, its function is reduced — is toned down ; the envelope acts 

 as an absorber, the lines are observed black. 



Now what must we conclude when I tell you that at the present 

 time it is almost impossible to observe the sun for an hour without 

 observing the hydrogen-lines, every now and then, bright upon the 

 sun itself} 



Not oniy are the lines observed bright, but it would appear that 

 the strongly luminous hydrogen is carried up by the tremendous 

 convection-currents at different pressures ; and under these circum- 

 stances the bright line is seen to be expanded on both sides of its 

 normal position. Moreover at times there is a dim light on both 

 sides of the black line, and the line itself is thinned out, showing that, 

 although there is an uprush of strongly luminous material, the co- 

 lumn is still surmounted by some less luminous hydrogen, possibly 

 separated from the other portion, which still performs the functions of 

 an absorber. This seems established by another fact, namely that 

 at times the lines, still black, expand on both sides, as if, in fact, in 

 these regions there was a depression in the chromosphere ; you 

 already know that the pressure is greater at the base of the chromo- 

 sphere than at the summit. 



For this reason it is best to observe these phenomena by means of 



