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that atmosphere's light. Now, no such be the energetic injection of vapors from lines are visible. So far as the spectro- beneath the surface of the sun. scopic evidence is concerned, it would At about this stage of the controversy I appear as though immediately above the had occasion to consider the problems sun's surface as we see it, there came the associated with the physical condition sierra-that low range of prominence- of the sun and his surroundings; and matter, which, strangely enough, some although I took no part in the discussion have regarded as an atmospheric envelope. between Fr. Secchi and Mr. Lockyer, I The spectrum of the sierra shows beyond expressed (in papers which I wrote upon all question that, like the prominences, the subject) opinions which agreed with this region consists of glowing hydrogen, the views of the Italian astronomer. It * mixed up with a few, and at times with is necessary for me to present in this place several, other gases, but certainly not ca- my own reasoning on the question at pable of accounting for the thousands issue, because it not only serves to introof dark lines in the solar spectrum. It duce the special observation made last seems quite clear, also, that the sierra December, by which the problem has been is not of the nature of an envelope at finally solved, but also presents certain all.
considerations which must be attended to Over the narrow layer which Secchi in interpreting that observation. supposed to exist between the sun's sur- In the first place, I noted that the darkface and the colored sierra began, and ening of the sun's disc near the edge, or presently waxed warm, the controversy rather the marked nature of that darkenabove referred to. Fr. Secchi was positive ing, instead of showing (as had been so that he could see the narrow continuous often stated) that the sun has a very deep spectrum on which he founded his view; atmosphere, proves, on 'the contrary, that Mr. Lockyer was equally positive that his atmosphere must be exceedingly the worthy father could see nothing of the shallow by comparison with the dimenkind. Fr. Secchi urged that his telescope sions of his globe. It is easy to show why was better than Mr. Lockyer's, and that this is; and although the considerations he worked in a better atmosphere ; Mr. on which the matter depends are exceedLockyer retorted that his spectroscope ingly simple, yet the case is by no means was better than Fr. Secchi's, and that the the first in which exceedingly simple conimagined superiority of the Roman atmos- siderations have been lost sight of by phere was a myth. Something was said, students of science. Suppose we have too, by the London observer about a large a brightly-white globe encased symmetspeculum, which was to decide the ques- rically within a globe of some imperfectly tion, though this mirror does not seem to transparent substance - as green glass. have been actually brought into action. Now, if the white globe is an inch in diBoth the disputants expressed full confi- ameter and the green glass globe a yard dence that time would prove the justice in diameter, the brightness of the white of their several views.
globe will be more or less impaired acSoon after, an observation was made by cording to the transparency of the glass; Mr. Lockyer, which seemed to prove the but it will not be much more impaired at justice of Fr. Secchi's opinion ; for, on a the edge of the inner globe's disc than very favorable day for observations, Mr. near the middle. For clearly, when Lockyer was able to detect, not the nar- we look at the middle, we look through a row rainbow-tinted spectrum seen by foot and a half of glass (wanting only half Secchi, but a narrow strip of spectrum an inch), and when we look at the edge of belonging to the region just outside the the inner globe's disc, we also look through sun's edge, which showed hundreds of a foot and a half of glass (wanting only a bright lines. Here seemed to be conclu- small fraction of an inch). Neither the sive evidence of that shallow atmosphere half inch in the one case, nor the small of glowing vapors in which Fr. Secchi fraction of an inch in the other, can make had faith. But Mr. Lockyer interpreted any appreciable difference, so that the his observation differently. The presence enclosing globe of glass cuts off as much of these vapors on this particular occa- light when we look at the centre of the inner sion he regarded as wholly exceptional, globe's disc as when we look at the edge. and the cause of the exception he held to But now suppose that the enclosing globe orms a mere shell around the inner one. Still it might be thought that patience only Suppose, for instance, that the inner would be needed to detect the signs of globe is a yard in diameter, and the shell such an atmosphere, shallow though it be. of glass only half an inch thick. Then But there is a peculiarity of telescopic obin this case, as in the former, the bright- servation which renders the recognition of ness of the inner globe will be more or such an atmosphere, if of less than a cerless impaired according to the transpar- tain depth, not difficult merely, but imposency of the glass; but it will no longer be sible. It may be well to exhibit the nature affected equally whether we look at the mid- of the peculiarity at length, because it is dle or at the edge of the inner globe's disc. of considerable interest to all who possess In the fornier case we only look through or use telescopes. I take an illustrative half an inch of glass, in the latter we look case which seems, at first, to have little through a much greater range of glass; as connection with my subject. the reader will see at once if he draw two Every reader of this serial has heard of concentric circles nearly equal in size to the double stars, and I dare say most of represent the inner globe and its enclos- those who read this particular article have ing shell. It is easy to calculate how seen many of these beautiful objects. It is long the range of glass actually is in the known that some double stars are much latter case.
I have just gone through the closer than others, and we commonly hear calculation, and find that when the eye is it mentioned as a proof of the excellence directed to the edge of the enclosed globe, of a telescope that it will divide such and its line of sight passes through rather such a double star. But it might seem more than four inches and a quarter, so that if a telescope of a certain size were that more than eight times as much light constructed with extreme care, it should is absorbed as in the case where the eye be capable of dividing any double star, looks at the middle of the inner globe's because we might use an eye-piece of any disc, or directly through half an inch of magnifying power we pleased, and so, as glass.
it were, force apart the two star-images Now, we cannot tell what proportion formed by the object-glass. Instead of holds in the case of the sun's disc, because this being the case, however, there is a we do not know how much light has been limit for every object-glass, beyond which absorbed where we look at the middle of no separation is possible ; for this reason the disc. All we know is that whatever simply, that the star-images formed by the remains after such absorption is about object-glass are not points of light, as they twice as much as we receive from near would be if they correctly represented the the edge of the disc. It is easily seen that stars of which they are the optical images. this knowledge is insufficient for our re- The larger the object-glass (assumed to be quirements. But there can be no question perfect in construction) the smaller is the whatever that the total absorption near the star-image ; * but it has always a definite edge exceeds many times that near the size, and if this size is such that the two middle of the disc; and on very reason- images of the stars forming a pair actually able assumptions as to this excess, it may touch or overlap, we cannot separate them readily be shown that the absorbing at by using highly-magnifying eye-pieces. mosphere cannot exceed some five or six Now, what is true of a star is true of hundred miles in depth. Probably it is every point of any object we examine with even shallower.
a telescope. The image of the point is alNow, there is a circumstance which per- ways a circle of light, which, though mifectly accounts for the non-recognition by nute, has yet appreciable dimensions. The spectroscopists of an atmosphere relatively so shallow as this. Let it be remembered, in passing, that the average height of the
* A curious illustration of this is given by the be set at about five thousand fact that a certain astronomer of old, having re
duced the aperture of his telescope to a mere pinmiles ; so that the atmosphere we are hole, announced that he was thus enabled to meas. dealing with would be at the outside but ure the real globes of the stars, for instead of seeone-fifth as high as that fine rim of red ing the stars through his telescope as minute points light with saw-like edge which astronomers
of light, he now saw them with discs like the plandetected around the eclipsed sun in the qualities of his telescope, instead of altogether de
ets. He thought he was improving the defining total eclipses of 1842, 1851, and 1860. stroying them.
image of the object is made up of all these gins is preparing to apply the powers of circles, which necessarily overlap. Nor a much larger telescope than either Mr. let the reader suppose that on this account Lockyer's or Fr. Seechi's, we may possibly telescopic observation is untrustworthy. still hope to hear that the relatively shalPrecisely the same peculiarity affects ordi- low atmosphere can be studied when the
There is no such thing as a sun is not eclipsed. For we may now perfect optical image of an object; though speak of the existence of this atmosphere neither eyesight nor telescopic vision need as a demonstrated fact. The difficulty be regarded as deceptive on this account which seemed to present insuperable obOur power of seeing minute details are stacles to the observers who study the unlimited by this peculiarity, but we are not eclipsed sun, has been overcome by the actually deceived. If microscopic writing ingenuity of one of the most skilful of be shown us, for instance, we may find those very observers—Professor Young, of ourselves, after poring over it for some America—when studying the solar eclipse time, unable to make out its meaning, the of last December. letters seeming all blended together; but If during any total eclipse of the sun, we know what our failure really means, the moon just concealed the whole of the and by no means fall into the mistake of sun's disc (as may well happen), and if our concluding that there are no details be- satellite were only complaisant enough to
the actual details are inscruta- stay still for a few minutes in such a poble.
sition so that one of these exact total Let us apply this consideration to the eclipses could be studied as readily as one sun, and more particularly to the appear- of greater extent (which never can hapance presented by the edge of the sun's pen), then the shallow atmosphere I have disc. The image of every point of this been speaking of could be recognized. edge is a small circle; the combination of The difficulty above considered would all these small circles must produce a ring no longer exist. For the ring of light of light all round the true outline of the which actually hides the shallow atmosdisc. If the sun's atmosphere did not phere when the sun is not eclipsed, is an reach beyond this ring, then no contriv- extension of the bright rim of the disc ance whatever could render the atmos- outwards : if the disc is completely hidphere discernible, let the telescope be den, there is no bright rim to be extendnever so perfect and the observer never so ed, and anything existing close by the clear-sighted or skilful. Now, the actual sun's globe can be recognized. extension of this ring will be greater or But then, unfortunately, no total eclipse less according as the object-glass of the can present these desirable features. If telescope is less or greater. It may readily a total eslipse is to be worth seeing at all, be shown that neither Mr. Lockyer's tel- the moon's disc as seen at the time must escope nor Fr. Secchi's could possibly be appreciably larger than the sun's. show any signs of a solar atmosphere under When totality begins the outlines of the two hundred miles in depth, while in all two discs just touch at a single point, and probability an atmosphere four or five when totality ends the two discs just touch times as deep would escape their scrutiny. at another point; but during all the rest
Are we then to remain altogether in of the totality the two outlines do not ignorance of such an atmosphere, suppos- touch at all, that of the moon surrounding that it actually exists, and that the ing without touching that of the sun. dark lines in the solar spectrum are due The outlines of the two discs do twice to its absorptive power ? Is there no way touch, however, in each case for one moof obviating the difficulty which has just ment and at one point. What Professor been dealt with ?
Young determined to do, therefore, was So far as the method of observing the to bring under special examination that sun when uneclipsed is concerned, the one point where the outlines touch at the answer to these questions must be nega- exact moment when totality begins. In tive; or, rather, it must be answered that other words, he directed his special attenour only hope of meeting the difficulty tion to the point where the last trace of consists in increasing the size of the tele- the sun's disc was about to disappear. It is scopes with which the sun is spectroscopi- perhaps scarcely necessary to say that he cally studied. And inasmuch as Dr. Hug- did not trust to the powers of his telescope, but that he employed a powerful spectro- colored lines which two men only have scope. And further, he did not depend as yet beheld. We may increase the dion his own observation alone, but had ad- mensions and power of our telescopes unjusted a spectroscope for the use of Mr. til the existence of these lines can be Pye, an English gentleman residing in the recognized without the aid of eclipsepart of Spain where the eclipse-observing darkness, but the lines can never be seen, parties were stationed, so that that gentle save during eclipse, as Young and his colman also might make the required obser- leagues saw them last December. And vations.
these observers tell us that in a second or In his account, Professor Young does two the lines vanished, the advancing not mention what he expected to see. It moon hiding the shallow solar atmosphere. is probable that he had in his thoughts the If it should ever be given to any man to observations of Fr. Secchi, and hoped to see six total eclipses (which has never yet obtain evidence respecting that shallow happened to any), and to successfully apply atmospheric envelope which Secchi be- in each instance the method employed by lieved in and Lockyer rejected; though Professor Young, then in all, during his it is quite possible he merely desired to life, that man would have seen the beautiascertain whether the constitution of the ful line-spectrum to perfection for some lower part of the sierra differed in any ten or twelve seconds ; but no otherwise marked respect from that of the upper can even so long a total period of obserportion. As the moment approached vation be secured. No single observer, when the last fine sickle of sunlight was then, can hope to learn much about to be obscured, the solar spectrum which the thousands of lines which have still was visible in the spectroscopic field of to be mapped during eclipse opportuniview grew rapidly fainter. The region tỉes. actually examined by Professor Young But now let us consider the import of was in reality a narrow, almost linear the observation. What are these myriads space, touching the edge of the sun's of colored lines ? Every dark line of the disc; so that before totality had com- solar spectrum, says Professor Young, menced he had the light from our own il- seemed to have its representative in this luminated atmosphere, and not direct sun- bright-line spectrum. Many of the groups light, to deal with. Thus he had just such of lines which had flashed so quickly into a solar spectrum as is seen when a spec- view and endured but so brief a period, troscope is directed to the sky in the day- were familiar to him; in other words, his time. But as the moment of totality study of the solar spectrum had made him drew near, the illumination of the atmos- conversant with the corresponding groups phere, and with it the brightness of the of dark lines. It follows, then, beyond all rainbow-tinted streak, rapidly diminished. possibility of question, that the source of At last the solar spectrum vanished; and light was a highly complex atmosphere, then-What was it replaced by ? What formed of those very vapors which, by was found to be the spectrum of the solar their absorptive power, produce the dark atmosphere close by the sun's surface ? lines—formed, that is, of the vapors of In place of the rainbow-tinted riband iron and of copper, of zinc, sodium, magcrossed by thousands and thousands of nesium, and of all those elements whose dark lines, there appeared a new and presence in the sun's substance had been most beautiful spectrum—a riband of inferred from the study of the solar specrainbow-tinted lines, thousands in nuni- trum. ber and of all degrees of thickness,-hun- Here, then, at length we have the true dreds of red lines, and then, in order, solar atmosphere an atmosphere of a hundreds of orange lines, hundreds of yel- highly complex nature, and doubtless exlow, green, indigo, and violet lines, like ceedingly dense near the visible surface colored cross-threads on a black riband, of the sun, because subject to a pressure only infinitely more beautiful. A charm- so enormous. The upper limit of this ing spectacle, truly, but so short-lived that atmosphere cartnot lie very far above the no man can ever hope, though he lived to sun's surface, at least not very far comfourscore years and ten, to let his eyes pared with the sun's dimensions. Suprest in all his life for more than ten or posing the actual time during which the twelve seconds on the beautiful array of line-spectrum was visible to have been two seconds, then it is easy to tell how neath as well as above the cloud-layer deep the atmosphere is. For in two which forms the sun's visible surface, and seconds the moon must have traversed a beneath and between the other cloudspace corresponding to about three hun- layers revealed by telescopic observadred miles at the sun's distance. An tions. atmosphere three hundred miles deep is, But passing from the very difficult
questherefore, indicated by Professor Young's tions suggested by the consideration of observations. It need hardly be said, regions below the sun's visible surface, let however, that in the excitement of us discuss briefly the bearing of Professor eclipse observation, the estimate of min. Young's discovery upon our views reute intervals of time can scarcely be relied specting those outer regions--the colored upon, unless checked by instrumental prominences and sierra, the corona itself, arrangements, which was not the case in and, in fine, all the portions of space the present instance. We may fairly con- which lie above the true atmosphere. clude that the depth of the solar atmos- In the first place, it seems to me that phere lies between some such limits as a the observations made during the late hundred miles and five hundred miles. eclipse dispose finally of the theory that
In the above estimate I have supposed the colored sierra is an atmospheric en the measurement to be made from the velope, properly so-called. I had long sun's visible surface. But it is very un- since been led to question whether the likely that that surface is the true lower sierra could be so regarded. Let me relimit of the atmosphere. It seems far mind the reader that the sierra is nothing more probable that the surface we see is more nor less than the region which Lockmerely a layer of clouds (as Sir William yer rediscovered in 1868. It had, in fact, Herschel suggested so long ago) in the been recognized by telescopists since solar atmosphere, and that the actual 1806, the name sierra having been given depth of the atmosphere is more truly to it by the observers of the eclipse of indicated by the appearances seen when 1842. It is a red region, having (as its large sun spots are examined. That these name implies) a serrated upper surface, as spots are cavities has been abundantly seen in the telescope, and seemingly exestablished. That they are openings tending all round the sun's disc. The red through layers of solar clouds has not prominences appear to spring from its been indeed demonstrated, yet it is diffi- upper surface. Strangely enough, when cult to conceive how they can otherwise Lockyer made his ingenious observations be interpreted. As to the way in which of the colored prominences, he had not the spots are formed, theorists are at issue, heard of this discovery, or had forgotten it. some urging that there is an uprush from Accordingly, finding traces of prominencedepths beneath the solar surface; others, matter all round the sun, he concluded that there is a downrush of matter from that there was a continuous envelope of without. But neither of these views is in hydrogen (mixed with some other gases) any way incompatible with Herschel's surrounding the whole of the sun's globe. theory that the spots are openings in solar It was probably through being misled by cloud-layers.
this supposition that he gave to the sierra We might thus be led to compare the a new name-entitling it the chromosphere solar atniosphere with our own, though it -announcing at the same time that its will of course be obvious that there are
upper surface was smooth in outline. many marked points of difference. But Respighi, the eminent Italian spectroin our own atmosphere we have at least scopist--also working, it would seem, in two distinct cloud-levels, the region, name ignorance or forgetfulness of the prior ly, where the cumulus or wool-pack clouds recognition of the layer—announced presare formed, and that where the cirrus or ently that the upper surface of the sofeathery clouds make their appearance. called chromosphere* was altogether irThere is air above the cirrus clouds, air between the cirrus and cumulus layers, * It affords strange evidence of the caution with and air between the cumulus clouds' and which new names should be suggested, that this the earth. And precisely in the same
name, embodying, as we see, an erroneous theory,
and also perpetuating the remembrance of a misway we may conceive that there exists at
taken claim, is scarcely yet beginning to fall into all times a solar atmospheric region be- disuse. Perhaps its Greek origin and its length New SERIES - VOL. XIV., No. 1.