1868.] 



Constitution of the Sun and Stais. 



57 



always turned towards their primary. All this seems in a very remarkable 

 degree to be what we see about us in the solar system. 



be due to such a moderate change of temperature as, for instance, would convert the 

 vapour of water into ice ? It should be borne in mind that if the earth was at any time 

 sufficiently hot, the ocean must have then formed an atmosphere of steam so vast, that it 

 may perhaps have even reached to a ring which afterwards became the moon. 



Possibly the giants of our system (Jupiter, Saturn, Uranus, and Neptune) owe their 

 small density to their great mass, by reason of which they retain enough of their pristine 

 heat to be still clothed in immense aqueous atmospheres. 



If these surmises should prove to have any foundation, water was probably the mate- 

 rial of rings thrown off originally hy the sun, and is therefore not improbably an in- 

 gredient of the atmosphere of those dilated stars which do not exhibit hydrogen lines. 

 (See footnote, p. 50.) 



POSTSCEIPT. 



[(Continuation of the note on p. 26.) I have during the present summer often re- 

 ceived the impression that I saw several other faint bright lines in other parts of the 

 spectrum, of which the principal is a line which is coincident with or very close to Kirch- 

 hofF's copper line of wave-length 52*23. It should be borne in mind that if such bright 

 lines exist, they are due to constituents of the solar atmosphere which are eminently 

 transparent to these rays, either from being intrinsically so, or from the excessive tenuity 

 of the gas. Hence the gas adds in these rays, but only adds a little, to whatever bright- 

 ness may be transmitted through it from beyond. It behaves like a faintish flame of 

 very high temperature placed between the eye and a more conspicuous but less hot coal. 

 Hence, if the background be the spectrum of the umbra of a spot, the bright line should 

 be a faint streak across it. On the only occasion on which I had an opportunity of ex- 

 amining the spectrum of a spot, one of the rays I suspect to be bright lines presented 

 this appearance to my eye. 



Mr. Lockyer and Mr. Huggins have observed that some dark lines appear broader in 

 the spectrum of a spot than in the spectrum of ordinary solar light. This is no doubt 

 because the wings of these lines lose brightness which had before shone through them 

 from beyond, and the duskier parts of them in consequence become dark enough to add 

 to the breadth of the central black stripe. Wings appear to be always (except in the 

 anomalous case of the iron line 49*61, which demands a careful experimental scrutiny) 

 fainter than the central band. This may arise in either of two ways, either, 1°, because 

 the gas is so rare, or else the perturbations which occasion the wings so evanescent, that 

 the wings are in a considerable degree transparent, and much light from the photosphere 

 streams through them; or 2°, because though opaque they come from a region hot 

 enough to render them less dark than the central stripe. It is in the case of wings of 

 the former kind only that the appearance recorded by Messrs. Lockyer and Huggins will 

 present itself. Lines of which the wings are quite opaque ought, on the other hand, to 

 appear narrowest when seen in the spectrum of the umbra of a spot, since the brighter 

 parts of the wings would be then undistinguishable from the faint background, which 

 would therefore seem to encroach upon them. — September 1868.] 



