But observations made by Fr. Secchi at Rome pointed to a view so different from Mr. Lockyer's, as to lead to a controversy which filled many pages of the Comptes Rendus, of the Philosopical Magazine, and of other publications-a controversy qonducted, as too many philosophical discussions have been, with a somewhat unphilosophical acrimony.
Fr. Secchi had noticed that when the very edge of the sun's disc is examined with the spectroscope, the dark lines disappear from the spectrum, which thus becomes a simple rainbow-tinted streak. He judged, accordingly, that the absorbing atmosphere exists above the sun's real surface; for he believed that just at the edge the bright lines corresponding to the light from the vapours themselves so nearly equal in intensity the light of the solar spectrum, that no signs of difference can be detected; or, in other words, that the dark lines are obliterated. On the other hand, the glowing atmosphere cannot, he argued, reach much above the sun's surface, since otherwise the spectroscope would show the bright lines belonging to that atmosphere's light. Now, no such lines are visible. So far as the spectroscopic evidence is concerned, it would appear as though immediately above the sun's surface, as we see it, there came the sierra-that low range of prominence-matter, which, strangely enough, some have regarded as an atmospheric envelope. The spectrum of the sierra shows beyond all question that, like the prominences, this region consists of glowing hydrogen, mixed up with a few, and at times with several other gases, but certainly not capable of accounting for the thousands of dark lines in the solar spectrum. It seems quite clear, also, that the sierra is not of the nature of an envelope at all.
Over the narrow layer which Secchi supposed to exist between the sun's surface and the coloured sierra, began, and presently waxed warm, the controversy above referred to. Fr. Secchi was positive that he could see the narrow continuous spectrum on which he founded his view; Mr. Lockyer was equally positive that the worthy father could see nothing of the kind. Fr. Secchi urged that his telescope was better than Mr. Lockyer's, and that he worked in a better atmosphere; Mr. Lockyer retorted that his spectroscope was better than Fr. Secchi's, and that the imagined superiority of the Roman atmosphere was a myth. Something was said, too, by the London observer about a large speculum, which was to decide the question, though this mirror does not seem to have been actually brought into action. Both the disputants expressed full confidence that time would prove the justice of their several views.
Soon after, an observation was made by Mr. Lockyer, which seemed to prove the justice of Fr. Secchi's opinion; for, on a very favourable day for observations, Mr. Lockyer was able to detect, not the narrow rainbow-tinted spectrum seen by Secchi, but a narrow strip of spectrum belonging to the region just outside the sun's edge, which showed hundreds of bright lines. Here seemed to be conclusive evidence of that shallow atmosphere of glowing vapours in which Fr. Secchi had faith. But Mr. Lockyer interpreted his observation differently. The presence of these vapours on this particular occasion he regarded as wholly exceptional, and the cause of the exception he held to be the energetic injection of vapours from beneath the surface of the sun.
At about this stage of the controversy I had occasion to consider the problems associated with the physical condition of the sun and his surroundings; and although I took no part in the discussion between Fr. Secchi and Mr. Lockyer, I expressed (in papers which I wrote upon the subject) opinions which agreed with the views of the Italian astronomer. It is necessary for me to present in this place my own reasoning on the question at issue, because it not only serves to introduce the special observation made last December, by which the problem has been finally solved, but also presents certain considerations which must be attended to in interpreting that observation.
In the first place, I noted that the darkening of the sun's disc near the edge, or rather the marked nature of that darkening, instead of showing (as had been so often stated) that the sun has a very deep atmosphere, proves, on the contrary, that his atmosphere must be exceedingly shallow by comparison with the dimensions of his globe. It is easy to show why this is; and although the considerations on which the matter depends are exceedingly simple, yet the case is by no means the first in which exceedingly simple considerations have been lost sight of by students of science. Suppose we have a brightly-white globe encased symmetrically within a globe of some imperfectly transparent substance - as green glass. Now, if the white globe is an inch in diameter and the green glass globe a yard in diameter, the brightness of the white globe will be more or less impaired according to the transparency of the glass; but it will not be much more impaired at the edge of the inner globe's disc than near the middle. For clearly, when we look at the middle, we look through a foot and a half of glass (wanting only half an inch), and when we look at the edge of the inner globe's disc, we also look through a foot and a half of glass (wanting only a small fraction of an inch). Neither the half inch in the one case, nor the small fraction of an inch in the other, can make any appreciable difference, so that the enclosing globe of glass cuts off as much light when we look at the centre of the inner globe's disc as when we look at the edge. But now suppose that the enclosing globe forms a mere shell around the inner one. Suppose, for instance, that the inner globe is a yard in diameter, and the shell of glass only half an inch thick. Then in this case, as in the former, the brightness of the inner globe will be more or less impaired according to the transparency of the glass; but it will no longer be affected equally whether we look at the middle or at the edge of the inner globe's disc. In the former case we only look though half an inch of glass, in the latter we look through a much greater range of glass; as the reader will see at once if he draw two concentric circles nearly equal in size to represent the inner globe and its enclosing shell. It is easy to calculate how long the range of glass actually is in the latter case. I have just gone through the calculation, and find that when the eye is directed to the edge of the enclosed globe, its line of sight passes through rather more than four inches and a quarter, so that more than eight times as much light is absorbed as in the case where the eye looks at the middle of the inner globe's disc, or directly through half an inch of glass.