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Soon after Secchi successfully applied the samne method, as did Dr. Huggins, whose spectroscope was, however, not altogether well adapted to the work, having been specially designed for stellar researches.

It was a peculiarity of the new mode of observing the prominence-spectrum that it enabled the observer to determine at his leisure, and in a much more satisfactory manner than had been possible during the eclipse of 1868, the true position of the prominencelines, their characteristics as respects shape (the significance of which feature has been already referred to, p. 146), and also the existence of lines which had escaped observation while the eclipse was in progress. It is

to the observer. If this were so, the wisest (though most selfish) course which the students of science could follow would be to conceal all original ideas as to modes of observation or processes of research until they could apply those ideas on their own account. Now, the true lover of science cannot do this. He feels bound to publish any such ideas at once, let who will adopt and utilise them (and let who will obtain credit for the result). There may be cleverness in a more diplomatic course, but there is no true love of science.

I may add that as regards the question of priority in this matter more importance might be attached to it were it not well known that after Dr. Huggins's observations of T Coronæ, the principle on which the new mode of observation rests was recognised by all who understand spectroscopic analysis. Like the discovery of Sun-spots, therefore, this recognition of the prominence-lines in full daylight was an inevitable sequel of the construction of an adequately dispersive spectroscopic battery. This was, however, a work requiring a degree of skill on the part of the optician who was to devise the instrument which should cause us to assign no inconsiderable share of the credit to him. M. Janssen, I believe, devised the principle of the battery he employed. The battery employed by Mr. Lockyer was constructed, as already mentioned, by Mr. Browning, the inventor of the automatic spectroscope. Mr. Lockyer tells us that it so perfectly fulfils the required conditions that the least experienced observer can see the prominence-lines.

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obvious that the spectrum of the Sun's limb seen at the same time (or that of the illuminated terrestrial atmosphere) affords the most satisfactory means of determining the position of the bright prominence-lines; for there in the solar spectrum are those very linesthe dark lines C and D and F, and so on-with which the eclipse observers had associated the bright lines. It is only necessary to see whether those dark lines coincide in position with the bright ones (see figs. 36, 37, and 38, in which, however, but a few dark lines are shown) in order to tell whether the bright promi

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nence-lines are or are not due to the presence of those particular elements to which the solar dark lines belong.

Janssen found in this way that the orange line of the prominences does not correspond, as had been thought, with the D or sodium (double) line of the solar spectrum. He found, however, that the red and blue prominencelines do actually coincide with the c and F lines of hydrogen. So that these enormous objects, extending in some instances to a height of more than 80,000 miles above the Sun's surface, consist in part, at least, of the glowing vapour of hydrogen.

But it was possible to recognise other lines besides

these three by the new method, when an instrument of adequate dispersive power was employed. The accompanying picture (fig. 72), for example, exhibits the spectrum of a prominence, and of the adjacent portion of the Sun's limb, as shown by Mr. Lockyer's spectroscope. It will be seen that the double line of sodium as well as three lines of the magnesium spectrum are shown. It is only necessary to suppose that M. Rayet saw the two close double lines as single ones in order to account exactly for the nine lines seen by him.*

*The following is Fr. Secchi's description of the principal lines seen in the prominence-spectrum under ordinary conditions:

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The line c of hydrogen,' he says, 'is the most easily seen of all. It sometimes reaches the enormous height of three minutes, indicating the presence of such colossal prominences as are seen during eclipses. On the limb of the Sun generally the height of this line is very irregular, and on the average attains to from ten to fifteen seconds. This line also extends in a well-marked manner within the limb, overlapping the disc by ten seconds and more. Yet further on the dise there is a region where the line cannot be seen, being neither bright nor dark, but of the same tint exactly as the neighbouring part of the spectrum, which is thus in this part of its length continuous and uniformly bright. With a slit placed parallel to the limb the space throughout which the line disappears is often considerably extended. Outside the disc the line is much brighter near its base than at the summit, and the line is dilated at the base, and seems to terminate in a point where its light fades off, until, as I have already said, it becomes of the same brightness as the neighbouring part of the spectrum before becoming a dark line. Outside the Sun the line is bounded by two dark lines, which appear at first sight to be the effect of contrast, but may probably have another and a real cause. The bright line is often formed of knots and separate pieces, which are evidently so many fragments of different prominences, placed one beyond the other and unequally bright. If the aperture of the slit be enlarged as much as is possible without rendering the light unbearable, the bright line of the rose-coloured fringe is seen to be marked by irregularities which are due to the roughness of texture (scabrosità) of the prominences themselves.'

Mr. Lockyer was presently able to confirm the views of Grant, Secchi, and others, who had, as

After mentioning that he has been able to apply Janssen's method of determining the shape of the prominences by taking line-sections, Secchi remarks that he has often tried to determine whether there is any fixed law as regards the direction in which the prominences are bent, but has hitherto not succeeded in tracing any.

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The line in the yellow, near the sodium lines, is about twice and a half as far from the nearest of these as these are from each other. This line is sensibly the prolongation of a bright line in the solar spectrum. In height and brightness it corresponds closely to the c-line, but I have noticed that it will not bear high magnifying power so well. While the F and c lines remain brilliant under such powers, this line becomes weaker, so that only the practised eye can detect it. The line ends in a point, and it often extends itself brilliantly upon the disc of the Sun. Outside the dise it is not bordered by dark lines; on the contrary, with high powers it becomes diffused, and does not stand out sharply like the c-line.

The F-line is in general not so high as the c-line, and grows faint at the extremity, where it takes the form of a lance. Outside the disc it is accompanied by a narrow dark zone on the more refrangible side. Sometimes I have seen it prolonged beyond the edge of the solar dise, as in the case of the c-line; at other times a very fine black thread shows itself on the more refrangible side. This seems to show that the F-line is not due to hydrogen alone.' (This conclusion, however, has been negatived.)

'The third line of hydrogen near G I have seen as a bright line, but it is necessary to reduce the dispersive power of the prismatic battery in order to obtain sufficient light.'

Secchi is of opinion that where there are faculæ on or close to the limb, there prominences exist; but he is unable to say with equal confidence that wherever prominences exist there are also faculæ.

He was surprised to find on one occasion, when several lines besides the above-mentioned were visible, that of the three lines forming the group of magnesium one and one only was visible,-a second line holding a position midway between the other two lines of the same group. He remarks that Rayet had observed during the eclipse of August 1868 only two lines of this group, and that these two doubtless corresponded with those seen by himself, of which, as we have seen, one does not accord with either of the two remaining magnesium lines. He was so surprised at this peculiarity that he searched dili

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already mentioned, enunciated the theory that the Sun is wholly surrounded by a layer or envelope of the coloured matter whence as it would seem the prominences spring. To whatever part of the Sun's edge he directed his spectroscope the bright lines belonging to the prominence-spectrum could be seen, though reaching but to a short distance from the edge of the Sun, save where there is a prominence. Unaware that this gently for two hours to detect traces of the other lines of magnesium, but could find none whatever. He considers the objections which have been urged against this observation as 'inconclusive-to use no more severe expression-since it is unreasonable to suggest that he would have been careless when observing so remarkable a phenomenon.'

* In Mr. Lockyer's detailed account of his work (Report of the Astronomical Society, February 1869) he states that at the time the spectroscope revealed to him that the prominence-spectrum was never absent, and that, in fact, the prominence-matter formed a continuous envelope round the Sun, he was not aware that such an envelope had been suggested by previous observers.' The account goes on to speak of the physicists who had propounded this theory-Swann, Grant, Von Littrow, Leverrier, and Secchi-describing their views as ideas,' 'suggestions,' and 'surmises,' and adding that the experimental proof of the truth of these surmises is due to Mr. Lockyer.' It is bare justice to remark, however, that with some at least of the five physicists abovenamed the theory was much more than a surmise or conjecture. Thus Professor Grant writes, in 1858, 'The zone of a deep red colour observed at Toulon toward the part of the Moon's limb where the Sun was about to emerge, clearly indicates the accumulation of nebulous matter in the lower regions of the solar atmosphere, as well as the condensation of the circumambient fluid of which the latter is composed towards the surface of the Sun, arising from the pressure of the superincumbent strata.' Leverrier wrote in 1860, The existence of a bed of rose-coloured matter, partially transparent, covering the whole surface of the Sun, is a fact established by the observations made during the time of totality in the eclipse of this year.' Secchi was equally convinced of the existence of this rose-coloured region. The observation of eclipses,' he wrote, 'furnishes indisputable evidence that the Sun is really surrounded by a layer of this red matter, of which we commonly see no more than the most elevated points.' If these be surmises, how shall men of science express conviction?

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