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many others, exhibits a general approach to the trapezoidal figure. The slightly curved streak of white light crossing the longest of the coronal beams is a very remarkable feature. This streak is described as of an intensely white and uniform light. It appeared (or was at least first noticed) some two minutes after the beginning of the totality, and remained visible until the Sun began to reappear.

The polariscopic observations made during this eclipse on the light of the corona were not successful. The observers agreed, indeed, that the light of the corona was polarised in a plane through the Sun's centre; a circumstance which, if confirmed, would go far to prove that the corona shines by reflecting the Sun's light; but the Astronomer Royal, who has carefully examined their accounts, considers that no dependence can be placed on their conclusions. I may as well add that, during the eclipse of 1869, the American observers obtained a different result, and that, in the opinion of those best competent to judge, the question of the polarisation of the corona's light in a plane through the Sun's centre remains still in abeyance.

Lieutenant Tennant examined the light of the corona with the same spectroscope which, as we saw in the last chapter, has given information of such interest respecting the coloured prominences. He saw a faint continuous spectrum. Thinking that want of light prevented my seeing the bright lines which I had fully expected to see on the lower strata of the corona, I opened the jaws of the slit.' He still failed to

recognise any signs of bright lines. What I saw,'

he writes (the italics are his) tinuous spectrum, and I saw

was undoubtedly a conno lines. There may

have been dark lines, of course, but with so faint a spectrum, and the jaws of the slit wide apart, they might escape notice.'*

Before discussing this result, I proceed to mention other evidence bearing on the same point.

During the eclipse of August, 1869, several of the American observers renewed the attempt to determine the exact nature of the corona-spectrum. Their results were not accordant. Professor Pickering obtained a faint continuous spectrum crossed by three bright lines. Professor Harkness recognised only one bright line on a continuous background. After observing the spectrum of the prominences, he asked Professor Eastman, who was directing the telescope, to bring the corona into the field. A bright part of the corona was thus brought under examination, but no spectrum appeared. I asked him to try another place. Still nothing was visible; and, raising my head from the instrument for the first time since the commencement of the totality, I remarked, "Can't see any spectrum; don't believe we will get any." "Oh! yes, we will," said he.

It is strange that, notwithstanding the very plain account given by Lieut.-Col. Tennant, it should continue to be asserted that, according to his observations, the corona gave a solar spectrum; that is, a spectrum crossed by the Fräunhofer lines. The American observers were so misled by this assertion as to search specially for the dark lines which Colonel Tennant was supposed to have seen. The obvious meaning of his narrative is, that he saw a continuous spectrum, without either dark lines or bright lines.

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At that instant it struck me that perhaps the slit was too narrow; so I opened it a little, and then again placed my eye at the instrument. In the meantime Eastman had put the needle at a very bright part of the corona, and I at once saw a continuous spectrum. about as bright as that given by the full Moon on a clear night. Remembering that the observers in India, in August 1868, had said that the corona gives a continuous spectrum with absorption lines' (a mistaken idea, as mentioned in the last note), I looked very carefully for them; but, to my great surprise, I could see none, and I am perfectly satisfied that none were visible in my instrument. On the contrary, I saw an absolutely continuous spectrum crossed by a single bright line, whose position was recorded.' This line was in the green, and, if actually in the place assigned by Harkness, would correspond to a line belonging to the spectrum of copper. But as he makes the line coincident with one of the prominence-lines, it seems certain that it can be no other than a line of iron, close by the E lines, which has been seen by several observers in the spectrum of the prominences. Professor

This iron line appears also as a bright line in the spectrum of the aurora, according to the best observations hitherto made. As I write, I receive from Mr. Browning an account of his observations on the aurora of October 25, with one of his miniature spectroscopes. He saw a bright line near E, and another not far from B. Mr. Birmingham of Tuam, with a similar instrument, saw the usual bright line in the green, one not far from it to the left, very faint, and one of medium brightness near F. Professor Wenlock notes four lines in the yellow-green part of the spectrum, and one somewhat more refrangible than the F line. All these accounts are reconcilable when we remember the extreme faintness of the auroral light, and the fact that no exact determination by the

Young paid particular attention to the spectroscopic observation of the corona. He also had been misled by erroneous accounts respecting the Indian observations, and so expected to see a faint solar spectrum. He found, on the contrary, that the light of the corona gave a spectrum of bright lines. He saw three such lines, and he considers it certain, from their close agreement with those shown in Professor Winlock's picture of the aurora-spectrum, that the corona is simply an electric discharge, no doubt varying with great rapidity, as we see in the case of the aurora; in fact, that the corona is a permanent solar aurora.'

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Now, although these accounts seem at first sight discordant, it appears to me that they can be brought into agreement, not only with each other, but with Lieutenant-Colonel Tennant's, by a consideration of the circumstances under which they were severally made. Tennant, seeing only a continuous spectrum, opened the slit somewhat widely: the jaws of the slit were wide apart,' he says; too wide, I imagine, to show the bright lines. For, from what is shown at p. 144, it will be seen, that the brightness of the coronal bands

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method of coincidences has ever yet been attempted. I had written further to the effect that, even when the auroral light is only ruddy to the eye, no red lines are seen, so that we may conclude that the excess of red is due to a peculiarity in the light of mixed refrangibility forming the continuous spectrum,' when I learned that, on the evening of October 24, Mr. J. R. Capron, with one of Browning's small direct-vision spectroscopes (adapted to star observation), had succeeded in observing a line in the red, 'very much like the lithium line, but rather more dusky. It was only well seen in the rosy patches of the aurora, but could be faintly traced wherever the rose-tint at all extended.'

or lines could not be increased in this way, though their breadth, and so the total amount of light from them, would be increased in precisely the same proportion as the opening of the slit. But the brightness of the continuous background would be increased in this same proportion. Hence the bright lines which Tennant could not see, on account of their fineness, were changed by opening the slit into broad bands of no greater brightness, and rendered invisible by reason of the increased brightness of the background. An intermediate amount of opening would in all probability have shown the lines. Now we see that Professor Harkness failed even to see a continuous spectrum when he used a narrow slit; and the fineness of the lines (not nearly so brilliant as the prominence-lines) caused them to escape his notice precisely as had happened with Tennant. But when he opened the slit a little,' he saw the continuous spectrum and one bright line. Had he opened it somewhat more, he would not have seen that bright line, but would have failed as Tennant had, and for the same reason. Had he opened it a little less, he would probably have seen the continuous spectrum and the three bright lines, as Professor Pickering did. With a somewhat smaller opening the continuous spectrum would disappear through excessive faintness; but the three bright lines seen by Professor Young would be even more distinctly visible. We see in fact that Professor Young, who succeeded readily in seeing three bright lines, failed to recognise the continuous spectrum.

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