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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 prominencematter, 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 colored 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 favorable 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 vapors in which Fr. Secchi had faith. But Mr. Lockyer interpreted his observation differently. The presence of these vapors on this particular occasion he regarded as wholly exceptional, and the cause of the exception he held to
be the energetic injection of vapors 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 orms 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 through 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 tne 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.
Now, we cannot tell what proportion holds in the case of the sun's disc, because we do not know how much light has been absorbed where we look at the middle of the disc. All we know is that whatever remains after such absorption is about twice as much as we receive from near the edge of the disc. It is easily seen that this knowledge is insufficient for our requirements. But there can be no question whatever that the total absorption near the edge exceeds many times that near the middle of the disc; and on very reasonable assumptions as to this excess, it may readily be shown that the absorbing atmosphere cannot exceed some five or six hundred miles in depth. Probably it is even shallower.
Now, there is a circumstance which perfectly accounts for the non-recognition by spectroscopists of an atmosphere relatively so shallow as this. Let it be remembered, in passing, that the average height of the sierra may be set at about five thousand miles; so that the atmosphere we are dealing with would be at the outside but one-fifth as high as that fine rim of red light with saw-like edge which astronomers detected around the eclipsed sun in the total eclipses of 1842, 1851, and i860.
Still it might be thought that patience only would be needed to detect the signs of such an atmosphere, shallow though it be. But there is a peculiarity of telescopic observation which renders the recognition of such an atmosphere, if of less than a certain depth, not difficult merely, but impossible. It may be well to exhibit the nature of the peculiarity at length, because it is of considerable interest to all who possess or use telescopes. I take an illustrative case which seems, at first, to have little" connection with my subject.
Every reader of this serial has heard of the double stars, and I dare say most of those who read this particular article have seen many of these beautiful objects. It is known that some double stars are much closer than others, and we commonly hear it mentioned as a proof of the excellence of a telescope that it will divide such and such a double star. But it might seem that if a telescope of a certain size were constructed with extreme care, it should be capable of dividing any double star, because we might use an eye-piece of any magnifying power we pleased, and so, as it were, force apart the two star-images formed by the object-glass. Instead of this being the case, however, there is a limit for every object-glass, beyond which no separation is possible; for this reason simply, that the star-images formed by the object-glass are not points of light, as they would be if they correctly represented the stars of which they are the optical images. The larger the object-glass (assumed to be perfect in construction) the smaller is the star-image ; * but it has always a definite size, and if this size is such that the two images of the stars forming a pair actually touch or overlap, we cannot separate them by using highly-magnifying eye-pieces.
Now, what is true of a star is true of every point of any object we examine with a telescope. The image of the point is always a circle of light, which, though minute, has yet appreciable dimensions. The image of the object is made up of all these circles, which necessarily overlap. Nor let the reader suppose that on this account telescopic observation is untrustworthy. Precisely the same peculiarity affects ordinary vision. There is no such thing as a perfect optical image of an object; though neither eyesight nor telescopic vision need be regarded as deceptive on this account. Our power of seeing minute details are limited by this peculiarity, but we are not actually deceived. If microscopic writing be shown us, for instance, we may find ourselves, after poring over it for some time, unable to make out its meaning, the letters seeming all blended together; but we know what our failure really means, and by no means fall into the mistake of concluding that there are no details because the actual details are inscrutable.
* A curious illustration of this is given by the fact that a certain astronomer of old, having reduced the aperture of his telescope to a mere pinhole, announced that he was thus enabled to measure the real globes of the stars, for instead of seeing the stars through his telescope as minute points of light, he now saw them with discs like the planets. He thought he was improving the defining qualities of his telescope, instead of altogether destroying them.
Let us apply this consideration to the sun, and more particularly to the appearance presented by the edge of the sun's disc. The image of every point of this edge is a small circle; the combination of all these small circles must produce a ring of light all round the true outline of the disc. If the sun's atmosphere did not reach beyond this ring, then no contrivance whatever could render the atmosphere discernible, let the telescope be never so perfect and the observer never so clear-sighted or skilful. Now, the actual extension of this ring will be greater or less according as the object-glass of the telescope is less or greater. It may readily be shown that neither Mr. Lockyer's telescope nor Fr. Secchi's could possibly show any signs of a solar atmosphere under two hundred miles in depth, while in all probability an atmosphere four or five times as deep would escape their scrutiny.
Are we then to remain altogether in ignorance of such an atmosphere, supposing that it actually exists, and that the dark lines in the solar spectrum are due to its absorptive power? Is there no way of obviating the difficulty which has just been dealt with?
So far as the method of observing the sun when uneclipsed is concerned, the answer to these questions must be negative; or, rather, it must be answered that our only hope of meeting the difficulty consists in increasing the size of the telescopes with which the sun is spectroscopically studied. And inasmuch as Dr. Hug
gins is preparing to apply the powers of a much larger telescope than either Mr. Lockyer's or Fr. Seechi's, we may possibly still hope to hear that the relatively shallow atmosphere can be studied when the sun is not eclipsed. For we may now speak of the existence of this atmosphere as a demonstrated fact. The difficulty which seemed to present insuperable obstacles to the observers who study the uneclipsed sun, has been overcome by the ingenuity of one of the most skilful of those very observers—Professor Young, of America—when studying the solar eclipse of last December.
If during any total eclipse of the sun, the moon just concealed the whole of the sun's disc (as may well happen), and if our satellite were only complaisant enough to stay still for a few minutes in such a position so that one of these exact total eclipses could be studied as readily as one of greater extent (which never can happen), then the shallow atmosphere I have been speaking of could be recognized. The difficulty above considered would no longer exist. For the ring of light which actually hides the shallow atmosphere when the sun is not eclipsed, is an extension of the bright rim of the disc outwards: if the disc is completely hidden,4here is no bright rim to be extended, and anything existing close by the sun's globe can be recognized.
But then, unfortunately, no total eclipse can present these desirable features. If a total eslipse is to be worth seeing at all, the moon's disc as seen at the time must be appreciably larger than the sun's. When totality begins the outlines of the two discs just touch at a single point, and when totality ends the two discs just touch at another point; but during all the rest of the totality the two outlines do not touch at all, that of the moon surrounding without touching that of the sun. The outlines of the two discs do twice touch, however, in each case for one moment and at one point. What Professor Young determined to do, therefore, was to bring under special examination that one point where the outlines touch at the exact moment when totality begins. In other words, he directed his special attention to the point where the last trace of the sun's disc was about to disappear. It is perhaps scarcely necessary to say that he did not trust to the powers of his telescope, but that he employed a powerful spectroscope. And further, he did not depend on his own observation alone, but had adjusted a spectroscope for the use of Mr. Pye, an English gentleman residing in the part of Spain where the eclipse-observing parties were stationed, so that that gentleman also might make the required observations.
In his account, Professor Young does not mention what he expected to see. It is probable that he had in his thoughts the observations of Fr. Secchi, and hoped to obtain evidence respecting that shallow atmospheric envelope which Secchi believed in and Lockyer rejected; though it is quite possible he merely desired to ascertain whether the constitution of the lower part of the sierra differed in any marked respect from that of the upper portion. As the moment approached when the last fine sickle of sunlight was to be obscured, the solar spectrum which was visible in the spectroscopic field of view grew rapidly fainter. The region actually examined by Professor Young was in reality a narrow, almost linear space, touching the edge of the sun's disc; so that before totality had commenced he had the light from our own illuminated atmosphere, and not direct sunlight, to deal with. Thus he had just such a solar spectrum as is seen when a spectroscope is directed to the sky in the daytime. But as the moment of totality drew near, the illumination of the atmosphere, and with it the brightness of the rainbow-tinted streak, rapidly diminished. At last the solar spectrum vanished; and then— What was it replaced by? What was found to be the spectrum of the colar atmosphere close by the sun's surface? In place of the rainbow-tinted riband crossed by thousands and thousands of dark lines, there appeared a new and most beautiful spectrum—a riband of rainboiu-tinted lines, thousands in number and of all degrees of thickness,—hundreds of red lines, and then, in order, hundreds of orange lines, hundreds of yellow, green, indigo, and violet lines, like colored cross-threads on a black riband, only infinitely more beautiful. A charming spectacle, truly, but so short-lived that no man can ever hope, though he lived to fourscore years and ten, to let his eyes rest in all his life for more than ten or twelve seconds on the beautiful array of
colored lines which two men only have as yet beheld. We may increase the dimensions and power of our telescopes until the existence of these lines can be recognized without the aid of eclipsedarkness, but the lines can never be seen, save during eclipse, as Young and his colleagues saw them last December. And these observers tell us that in a second or two the lines vanished, the advancing moon hiding the shallow solar atmosphere. If it should ever be given to any man to see six total eclipses (which has never yet happened to any), and to successfully apply in each instance the method employed by Professor Young, then in all, during his life, that man would have seen the beautiful line-spectrum to perfection for some ten or twelve seconds; but no otherwise can even so long a total period of observation be secured. No single observer, then, can hope to learn much about the thousands of lines which have still to be mapped during eclipse opportunities.
But now let us consider"the import of the observation. What are these myriads of colored lines? Every dark line of the solar spectrum, says Professor Young, seemed to have its representative in this bright-line spectrum. Many of the groups of lines which had flashed so quickly into view and endured but so brief a period, were familiar to him; in other words, his study of the solar spectrum had made him conversant with the corresponding groups of dark lines. It follows, then, beyond all possibility of question, that the source of light was a highly complex atmosphere, formed of those very vapors which, by their absorptive power, produce the dark lines—formed, that is, of the vapors of iron and of copper, of zinc, sodium, magnesium, and of all those elements whose presence in the sun's substance had been inferred from the study of the solar spectrum.
Here, then, at length we have the true solar atmosphere an atmosphere of a highly complex nature, and doubtless exceedingly dense near the visible surface of the sun, because subject to a pressure so enormous. The upper limit of this atmosphere carmot lie very far above the sun's surface, at least not very far compared with the sun's dimensions. Supposing the actual time during which the line-spectrum was visible to have been two seconds, then it is easy to tell how deep the atmosphere is. For in two seconds the moon must have traversed a space corresponding to about three hundred miles at the sun's distance. An atmosphere three hundred miles deep is, therefore, indicated by Professor Young's observations. It need hardly be said, however, that in the excitement of eclipse observation, the estimate of minute intervals of time can scarcely be relied upon, unless checked by instrumental arrangements, which was not the case in the present instance. We may fairly conclude that the depth of the solar atmosphere lies between some such limits as a hundred miles and five hundred miles.
In the above estimate I have supposed the measurement to be made from the sun's visible surface. But it is very unlikely that that surface is the true lower limit of the atmosphere. It seems far more probable that the surface we see is merely a layer of clouds (as Sir William Herschel suggested so long ago) in the solar atmosphere, and that the actual depth of the atmosphere is more truly indicated by the appearances seen when large sun spots are examined. That these spots are cavities has been abundantly established. That they are openings through layers of solar clouds has not been indeed demonstrated, yet it is difficult to conceive how they can otherwise be interpreted. As to the way in which the spots are formed, theorists are at issue, some urging that there is an uprush from depths beneath the solar surface; others, that there is a downrush of matter from without. But neither of these views is in any way incompatible with Herschel's theory that the spots are openings in solar cloud-layers.
We might thus be led to compare the solar atmosphere with our own, though it will of course be obvious that there are many marked points of difference. But in our own atmosphere we have at least two distinct cloud-levels, the region, namely, where the cumulus or wool-pack clouds are formed, and that where the cirrus or feathery clouds make their appearance. There is air above the cirrus clouds, air between the cirrus and cumulus layers, and air between the cumulus clouds and the earth. And precisely in the same way we may conceive that there exists at all times a solar atmospheric region beNew Seriks.—Vol. XIV., No. 1.
neath as well as above the cloud-layer which forms the sun's visible surface, and beneath and between the other cloudlayers revealed by telescopic observations.
But passing from the very difficult questions suggested by the consideration of regions below the sun's visible surface, let us discuss briefly the bearing of Professor Young's discovery upon our views respecting those outer regions—the colored prominences and sierra, the corona itself, and, in fine, all the portions of space which lie above the true atmosphere.
In the first place, it seems to me that the observations made during the late eclipse dispose finally of the theory that the colored sierra is an atmospheric envelope, properly so-called. I had long since been led to question whether the sierra could be so regarded. Let me remind the reader that the sierra is nothing more nor less than the region which Lockyer rediscovered in 1868. It had, in fact, been recognized by telescopists since 1806, the name sierra having been given to it by the observers of the eclipse of 1842. It is a red region, having (as its name implies) a serrated upper surface, as seen in the telescope, and seemingly extending all round the sun's disc. The red prominences appear to spring from its upper surface. Strangely enough, when Lockyer made his ingenious observations of the colored prominences, he had not heard of this discovery, or had forgotten it. Accordingly, finding traces of prominencematter all round the sun, he concluded that there was a continuous envelope of hydrogen (mixed with some other gases) surrounding the whole of the sun's globe. It was probably through being misled by this supposition that he gave to the sierra a new name—entitling it the chromosphere —announcing at the same time that its upper surface was smooth in outline. Respighi, the eminent Italian spectroscopist—also working, it would seem, in ignorance or forgetfulness of the prior recognition of the layer—announced presently that the upper surface of the socalled chromosphere* was altogether ir
* It affords strange evidence of the caution with which new names should be suggested, that this name, embodying, as we see, an erroneous theory, and also perpetuating the remembrance of a mistaken claim, is scarcely yet beginning to fall into disuse. Perhaps its Greek origin and its length