IMPORTANT DISCOVERY DURING THE LATE ECLIPSE.
PROFESSOR RESPIGHI, the eminent Italian spectroscopist, succeeded in making a noteworthy advance in our knowledge of Solar physics during the progress of the Eclipse of last December; and, what is of even greater importance, he employed a totally novel method of observing the eclipsed sun, his actual discovery being probably but the first-fruits of this method. We propose to give a brief account of the results obtained by Professor Respighi, and a description of those features of the method which constitute its importance. But for obvious reasons, this is not the place for a description of the instrument employed by Respighi.
It will be in the remembrance of many of our readers that during the eclipse of December, 1870, considerable attention was directed to the circumstance that the sun's Corona appeared to consist of two distinct portions. Close to the black disc of the moon there was seen a bright ring of pearly light, not uniformly wide, but nowhere extending to a distance much exceeding a fourth or fifth part (for accounts differed) of the moon's apparent diameter. Outside this ring-formed corona ap- peared a much more extensive, but much fainter halo, radiated in its general structure,—or rather cloven in places by certain well-marked dark rifts or gaps. There was so sudden a de- gradation of luminosity near the borders of the bright inner corona, as to suggest very strongly the idea that the two coronas are totally distinct solar envelopes. Indeed, it was proposed to give the inner corona a name specially invented for the occasion—the name Leucosphere —which fortunately was not received with favour. Indeed, it presently appeared that the supposed discovery was no discovery at all,—the twofold nature of the corona having been recognized 165 years ago, and having since been repeatedly confirmed during total eclipses. So far back as 1852 our own Astronomer Royal had suggested for the two envelopes the names of the " Ringformed " and the " Radiated" coronas. It must be added, furthermore, that when favourably seen, the inner or ring- formed corona is not pearly white in hue, but marked by a distinct tinge of rose-colour.
Now, astronomers have been endeavouring since the great eclipse of 1868 to determine the real nature of the light emitted by the various parts of the corona. They have sought, in fact, to apply the modern method of observation called spectrum analysis to the corona, as they had already applied it to the prominences ; and thus to learn whether the light of the corona comes from glowing vapour or from incandescent particles, or is merely sunlight reflected• from opaque matter spread in a sort of cosmical dust around the solar orb.
But important difficulties stood in their way. They did obtain on turning their spectroscopes towards the corona a spectrum which, in itself, indicated that the source of light was glowing vapour. A certain green line appeared, which, if it really were the spectrum of the corona, could bear no other interpretation. But it was not clear that this green-line spectrum belonged to the corona at all,—the doubt arising from the fact that the green line still made its appearance when the spectroscope was turned to parts of the sky to which the corona could not be supposed to extend. This is easily explained. In these days every one knows that the sun's light, when dealt with by a spectroscope, presents a rainbow- tinted streak crossed by dark lines, and that these dark lines indi- cate the presence, in the sun, of the vapours of many familiar elements,—as iron, copper, zinc, and so forth. But if we turn a spectroscope towards the sky, or even towards a sheet of white paper illuminated by the sun, we see the same dark lines ; yet we know that there is no glowing iron-vapour in the sky or in the paper. The fact really is, that we receive from the sky, and from the paper, reflected sunlight, and so naturally find in such light the qualities of sunlight. How, if in like manner, when the obser- vers of recent eclipses have seen a bright-line spectrum, while ex- amining the corona, they have been in reality examining only reflected light, and so had still to determine the true source of the light? Nay, rather, it was clear that from some parts of the sky they did get this green line from reflected light ; how, then, were they to distinguish where the inherent luminosity ceased and the reflected luminosity began?
The problem seemed intractable ; but, as a matter of fact, Professor Young, of America, solved it very satisfactorily by care- fully considering the amount of this green luminosity received under differentcircumstances. We need not examine his reasoning, but the result may be very simply stated. He inferred that in all proba- bility a region so mewhat more extensive than the ring-formed corona shines with this green-line-forming light. We believe that no one who examines and understands Professor Young's reasoning can doubt that he legitimately established this conclusion. It follows that the ring-formed corona, or a somewhat larger region around the sun, is due to a true atmospheric envelope. The interest of this discovery is enhanced by the circumstance that the green line of the coronal spectrum is a conspicuous feature of the spec- trum of our own auroras. Professor Respighi has confirmed Young's discovery. In confirming it, however, he has added another equally important.
Thus far we have been speaking of a green line of the inner corona. But it occurred to Respighi that he would endeavour to see a green image of this solar envelope. There were two ways in which he might try to effect this. The first is a method devised inde- pendently by Huggins and Dinner, and first successfully applied by the former, though occasionally called the Janssen-Lockyer method (being confounded, apparently, with a perfectly distinct method of observation). The other was proposed by Fraiinhofer, in the very infancy of the science of spectroscopic analysis,—and has lately been revived by the Italian astronomers Secchi and Respighi. Neither method need be described, but each has this effect,—that when the source of light is a glowing gas, then, instead of a spectrum of such and such coloured lines, there is formed a series of correspondingly coloured images of the source of light. Thus when one of the solar prominences is observed in this way, instead of a red, an orange, a green, and an indigo line (and other faint lines), the methods referred to show a red image of the prominence, an orange image, a green image, and an indigo image (the images corresponding to the fainter lines being too faint to be discernible under ordinary circumstances).
Now let the reader carefully note the importance of this method as applied to the corona. As applied during eclipse to the coloured prominences, it could teach nothing new—for it would merely re- solve the prominences, already visible as rose-coloured objects, into four several pictures—alike in figure, but differing in colour. The effect might be exceedingly beautiful—or rather was so, for Respighi has seen such hn iges—but it taught nothing new. As applied to the corona, however, the value of the method was far greater. So soon as totality began, Respighi saw, instead of the bright green line which Professor Young had proved to belong to the corona, a beautiful green image of the inner ring-formed corona. Here, indeed, was proof patent to the eye that the green light is inherent in the inner corona, and not merely due to re- flection in our own atmosphere. For just as our sky in the day- time, when we examine it with a spectroscope, is found to give the solar dark lines, but cannot possibly give an image of the sun, so the sky in eclipse could give the green line by reflection, but not an image of the source whence the green light comes. Only be- cause the inner corona is itself that source, could its image be rendered visible by applying Respighi's method.
So far, however, Respighi's work only confirms a result already established. But another result, and one altogether new, was at the same time obtained. The green image of the inner corona was not alone ; two others—one red, and the other blue-green— made their appearance, in or near the places corresponding to the two bright lines of hydrogen called by spectroscopists the C-line (red) and the F-line (blue-green). The three images were not strictly alike, and we may infer from the brief telegraphic account sent in the first instance that the hydrogen images were not quite so extensive as the green image. But into minutiae of this sort we need not at present enter. The great facts rendered patent by Respighi's late observation are these,—that surrounding the sun to a depth of nearly 200,000 miles, reaching, therefore, above the summits of the loftiest prominences, there is an atmosphere con- sisting of glowing hydrogen, and of some other vapour, distinct in condition and composition from the chromatosphere, whose average height is but about 4,000 miles. This enormous external atmospheric shell must be of exceeding rarity, or the pressure on the chromato- sphere would enormously exceed the actual observed pressure. It is outside this atmosphere that the radiated corona projects into the sun-surrounding space to distances often exceeding a million miles. We may confidently expect that the news which we have received respecting the inner ring-formed corona will be so supplemented by the photographic records now on their way to Europe, that we shall obtain much clearer ideas than we have hitherto had respecting the outer and radiated corona. Truly, a remark with which the spectroscopist Janssen closes his letter to the Paris Academy res- pecting his own observations seems abundantly justified :—" The question whether the corona is due to the earth's atmosphere is now disposed of [trencher], and we may look forward to a series of researches into the matter surrounding the sun which cannot fail to be extremely interesting and fruitful."
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