PROGRESS OF KNOWLEDGE REGARDING THE SUN

The discovery of sun-spots in 1610 by Fabricius and Galileo first opened a way for inquiry into the solar constitution; but it was long before that way was followed with system or profit. The seeming irregularity of the phenomena discouraged continuous attention; casual observations were made the basis of arbitrary conjectures, and real knowledge received little or no increase. In 1620 we find Jean Tarde, Canon of Sarlat, arguing that because the sun is "the eye of the world," and the eye of the world cannot suffer from ophthalmia, therefore the appearances in question must be due, not to actual specks or stains on the bright solar disc, but to the transits of a number of small planets across it! To this new group of heavenly bodies he gave the name of "Borbonia Sidera," and they were claimed in 1633 for the House of Hapsburg, under the title of "Austriaca Sidera" by Father Malapertius, a Belgian Jesuit.[131] A similar view was temporarily maintained against Galileo by the justly celebrated Father Scheiner of Ingolstadt, and later by William Gascoigne, the inventor of the micrometer; but most of those who were capable of thinking at all on such subjects (and they were but few) adhered either to the cloud theory or to the slag theory of sun-spots. The first was championed by Galileo, the second by Simon Marius, "astronomer and physician" to the brother Margraves of Brandenburg. The latter opinion received a further notable development from the fact that in 1618, a year remarkable for the appearance of three bright comets, the sun was almost free from spots; whence it was inferred that the cindery refuse from the great solar conflagration, which usually appeared as dark blotches on its surface, was occasionally thrown off in the form of comets, leaving the sun, like a snuffed taper, to blaze with renewed brilliancy.[132]

In the following century, Derham gathered from observations carried on during the years 1703-11, "That the spots on the sun are caused by the eruption of some new volcano therein, which at first pouring out a prodigious quantity of smoke and other opacous matter, causeth the spots; and as that fuliginous matter decayeth and spendeth itself, and the volcano at last becomes more torrid and flaming, so the spots decay, and grow to umbrÆ, and at last to faculÆ."[133]

The view, confidently upheld by Lalande,[134] that spots were rocky elevations uncovered by the casual ebbing of a luminous ocean, the surrounding penumbrÆ representing shoals or sandbanks, had even less to recommend it than Derham's volcanic theory. Both were, however, significant of a growing tendency to bring solar phenomena within the compass of terrestrial analogies.

For 164 years, then, after Galileo first levelled his telescope at the setting sun, next to nothing was learned as to its nature; and the facts immediately ascertained, of its rotation on an axis nearly erect to the plane of the ecliptic, in a period of between twenty-five and twenty-six days, and of the virtual limitation of the spots to a so-called "royal" zone extending some thirty degrees north and south of the solar equator, gained little either in precision or development from five generations of astronomers.

But in November, 1769, a spot of extraordinary size engaged the attention of Alexander Wilson, professor of astronomy in the University of Glasgow. He watched it day by day, and to good purpose. As the great globe slowly revolved, carrying the spot towards its western edge, he was struck with the gradual contraction and final disappearance of the penumbra on the side next the centre of the disc; and when on the 6th of December the same spot re-emerged on the eastern limb, he perceived, as he had anticipated, that the shady zone was now deficient on the opposite side, and resumed its original completeness as it returned to a central position. In other spots subsequently examined by him, similar perspective effects were visible, and he proved in 1774,[135] by strict geometrical reasoning, that they could only arise in vast photospheric excavations. It was not, indeed, the first time that such a view had been suggested. Father Scheiner's later observations plainly foreshadowed it;[136] a conjecture to the same effect was emitted by Leonard Rost of Nuremburg early in the eighteenth century;[137] both by Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches on the solar limb; while in 1770 Pastor SchÜlen of Essingen, from the careful study of phenomena similar to those noted by Wilson, concluded their depressed nature.[138] Modern observations, nevertheless, prove those phenomena to be by no means universally present.

Wilson's general theory of the sun was avowedly tentative. It took the modest form of an interrogatory. "Is it not reasonable to think," he asks, "that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark, and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the sun would seem to derive the whole of his vivifying heat and energy?"[139] He further suggests that the excavations or spots may be occasioned "by the working of some sort of elastic vapour which is generated within the dark globe," and that the luminous matter, being in some degree fluid, and being acted upon by gravity, tends to flow down and cover the nucleus. From these hints, supplemented by his own diligent observations and sagacious reasonings, Herschel elaborated a scheme of solar constitution which held its ground until the physics of the sun were revolutionised by the spectroscope.

A cool, dark, solid globe, its surface diversified with mountains and valleys, clothed in luxuriant vegetation, and "richly stored with inhabitants," protected by a heavy cloud-canopy from the intolerable glare of the upper luminous region, where the dazzling coruscations of a solar aurora some thousands of miles in depth evolved the stores of light and heat which vivify our world—such was the central luminary which Herschel constructed with his wonted ingenuity, and described with his wonted eloquence.

"This way of considering the sun and its atmosphere," he says,[140] "removes the great dissimilarity we have hitherto been used to find between its condition and that of the rest of the great bodies of the solar system. The sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface, the rotation upon its axis, and the fall of heavy bodies, leads us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe."

We smile at conclusions which our present knowledge condemns as extravagant and impossible, but such incidental flights of fancy in no way derogate from the high value of Herschel's contributions to solar science. The cloud-like character which he attributed to the radiant shell of the sun (first named by SchrÖter the "photosphere") is borne out by all recent investigations; he observed its mottled or corrugated aspect, resembling, as he described it, the roughness on the rind of an orange; showed that "faculÆ" are elevations or heaped-up ridges of the disturbed photospheric matter; and threw out the idea that spots may ensue from an excess of the ordinary luminous emissions. A certain "empyreal" gas was, he supposed (very much as Wilson had done), generated in the body of the sun, and rising everywhere by reason of its lightness, made for itself, when in moderate quantities, small openings or "pores,"[141] abundantly visible as dark points on the solar disc. But should an uncommon quantity be formed, "it will," he maintained, "burst through the planetary[142] regions of clouds, and thus will produce great openings; then, spreading itself above them, it will occasion large shallows (penumbrÆ), and mixing afterwards gradually with other superior gases, it will promote the increase, and assist in the maintenance, of the general luminous phenomena."[143]

This partial anticipation of the modern view that the solar radiations are maintained by some process of circulation within the solar mass, was reached by Herschel through prolonged study of the phenomena in question. The novel and important idea contained in it, however, it was at that time premature to attempt to develop. But though many of the subtler suggestions of Herschel's genius passed unnoticed by his contemporaries, the main result of his solar researches was an unmistakable one. It was nothing less than the definitive introduction into astronomy of the paradoxical conception of the central fire and hearth of our system as a cold, dark, terrestrial mass, wrapt in a mantle of innocuous radiance—an earth, so to speak, within—a sun without.

Let us pause for a moment to consider the value of this remarkable innovation. It certainly was not a step in the direction of truth. On the contrary, the crude notions of Anaxagoras and Xeno approached more nearly to what we now know of the sun, than the complicated structure devised for the happiness of a nobler race of beings than our own by the benevolence of eighteenth-century astronomers. And yet it undoubtedly constituted a very important advance in science. It was the first earnest attempt to bring solar phenomena within the compass of a rational system; to put together into a consistent whole the facts ascertained; to fabricate, in short, a solar machine that would in some fashion work. It is true that the materials were inadequate and the design faulty. The resulting construction has not proved strong enough to stand the wear and tear of time and discovery, but has had to be taken to pieces and remodelled on a totally different plan. But the work was not therefore done in vain. None of Bacon's aphorisms show a clearer insight into the relations between the human mind and the external world than that which declares "Truth to emerge sooner from error than from confusion."[144] A definite theory (even if a false one) gives holding-ground to thought. Facts acquire a meaning with reference to it. It affords a motive for accumulating them and a means of co-ordinating them; it provides a framework for their arrangement, and a receptacle for their preservation, until they become too strong and numerous to be any longer included within arbitrary limits, and shatter the vessel originally framed to contain them.

Such was the purpose subserved by Herschel's theory of the sun. It helped to clarify ideas on the subject. The turbid sense of groping and viewless ignorance gave place to the lucidity of a possible scheme. The persuasion of knowledge is a keen incentive to its increase. Few men care to investigate what they are obliged to admit themselves entirely ignorant of; but once started on the road of knowledge, real or supposed, they are eager to pursue it. By the promulgation of a confident and consistent view regarding the nature of the sun, accordingly, research was encouraged, because it was rendered hopeful, and inquirers were shown a path leading indefinitely onwards where an impassable thicket had before seemed to bar the way.

We have called the "terrestrial" theory of the sun's nature an innovation, and so, as far as its general acceptance is concerned, it may justly be termed; but, like all successful innovations, it was a long time brewing. It is extremely curious to find that Herschel had a predecessor in its advocacy who never looked through a telescope (nor, indeed, imagined the possibility of such an instrument), who knew nothing of sun-spots, was still (mistaken assertions to the contrary notwithstanding) in the bondage of the geocentric system, and regarded nature from the lofty standpoint of an idealist philosophy. This was the learned and enlightened Cardinal Cusa, a fisherman's son from the banks of the Moselle, whose distinguished career in the Church and in literature extended over a considerable part of the fifteenth century (1401-64). In his singular treatise De Doct IgnorantiÂ, one of the most notable literary monuments of the early Renaissance, the following passage occurs:—"To a spectator on the surface of the sun, the splendour which appears to us would be invisible, since it contains, as it were, an earth for its central mass, with a circumferential envelope of light and heat, and between the two an atmosphere of water and clouds and translucent air." The luminary of Herschel's fancy could scarcely be more clearly portrayed; some added words, however, betray the origin of the Cardinal's idea. "The earth also," he says, "would appear as a shining star to any one outside the fiery element." It was, in fact, an extension to the sun of the ancient elemental doctrine; but an extension remarkable at that period, as premonitory of the tendency, so powerfully developed by subsequent discoveries, to assimilate the orbs of heaven to the model of our insignificant planet, and to extend the brotherhood of our system and our species to the farthest limit of the visible or imaginable universe.

In later times we find Flamsteed communicating to Newton, March 7, 1681, his opinion "that the substance of the sun is terrestrial matter, his light but the liquid menstruum encompassing him."[145] Bode in 1776 arrived independently at the conclusion that "the sun is neither burning nor glowing, but in its essence a dark planetary body, composed like our earth of land and water, varied by mountains and valleys, and enveloped in a vaporous atmosphere";[146] and the learned in general applauded and acquiesced. The view, however, was in 1787 still so far from popular, that the holding of it was alleged as a proof of insanity in Dr. Elliot when accused of a murderous assault on Miss Boydell. His friend Dr. Simmons stated on his behalf that he had received from him in the preceding January a letter giving evidence of a deranged mind, wherein he asserted "that the sun is not a body of fire, as hath been hitherto supposed, but that its light proceeds from a dense and universal aurora, which may afford ample light to the inhabitants of the surface beneath, and yet be at such a distance aloft as not to annoy them. No objection, he saith, ariseth to that great luminary's being inhabited; vegetation may obtain there as well as with us. There may be water and dry land, hills and dales, rain and fair weather; and as the light, so the season must be eternal, consequently it may easily be conceived to be by far the most blissful habitation of the whole system!" The Recorder, nevertheless, objected that if an extravagant hypothesis were to be adduced as proof of insanity, the same might hold good with regard to some other speculators, and desired Dr. Simmons to tell the court what he thought of the theories of Burnet and Buffon.[147]

Eight years later, this same "extravagant hypothesis," backed by the powerful recommendation of Sir William Herschel, obtained admittance to the venerable halls of science, there to abide undisturbed for nearly seven decades. Individual objectors, it is true, made themselves heard, but their arguments had little effect on the general body of opinion. Ruder blows were required to shatter an hypothesis flattering to human pride of invention in its completeness, in the plausible detail of observations by which it seemed to be supported, and in its condescension to the natural pleasure in discovering resemblance under all but total dissimilarity.

Sir John Herschel included among the results of his multifarious labours at the Cape of Good Hope a careful study of the sun-spots conspicuously visible towards the end of the year 1836 and in the early part of 1837. They were remarkable, he tells us, for their forms and arrangement, as well as for their number and size; one group, measured on the 29th of March in the latter year, covering (apart from what may be called its outlying dependencies) the vast area of five square minutes or 3,780 million square miles.[148] We have at present to consider, however, not so much these observations in themselves, as the chain of theoretical suggestions by which they were connected. The distribution of spots, it was pointed out, on two zones parallel to the equator, showed plainly their intimate connection with the solar rotation, and indicated as their cause fluid circulations analogous to those producing the terrestrial trade and anti-trade winds.

"The spots, in this view of the subject," he went on to say,[149] "would come to be assimilated to those regions on the earth's surface where, for the moment, hurricanes and tornadoes prevail; the upper stratum being temporarily carried downwards, displacing by its impetus the two strata of luminous matter beneath, the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the sun below. Such processes cannot be unaccompanied by vorticose motions, which, left to themselves, die away by degrees and dissipate, with the peculiarity that their lower portions come to rest more speedily than their upper, by reason of the greater resistance below, as well as the remoteness from the point of action, which lies in a higher region, so that their centres (as seen in our waterspouts, which are nothing but small tornadoes) appear to retreat upwards. Now this agrees perfectly with what is observed during the obliteration of the solar spots, which appear as if filled in by the collapse of their sides, the penumbra closing in upon the spot and disappearing after it."

But when it comes to be asked whether a cause can be found by which a diversity of solar temperature might be produced corresponding with that which sets the currents of the terrestrial atmosphere in motion, we are forced to reply that we know of no such cause. For Sir John Herschel's hypothesis of an increased retention of heat at the sun's equator, due to the slightly spheroidal or bulging form of its outer atmospheric envelope, assuredly gives no sufficient account of such circulatory movements as he supposed to exist. Nevertheless, the view that the sun's rotation is intimately connected with the formation of spots is so obviously correct, that we can only wonder it was not thought of sooner, while we are even now unable to explain with any certainty how it is so connected.

Mere scrutiny of the solar surface, however, is not the only means of solar observation. We have a satellite, and that satellite from time to time acts most opportunely as a screen, cutting off a part or the whole of those dazzling rays in which the master-orb of our system veils himself from over-curious regards. The importance of eclipses to the study of the solar surroundings is of comparatively recent recognition; nevertheless, much of what we know concerning them has been snatched, as it were, by surprise under favour of the moon. In former times, the sole astronomical use of such incidents was the correction of the received theories of the solar and lunar movements; the precise time of their occurrence was the main fact to be noted, and subsidiary phenomena received but casual attention. Now, their significance as a geometrical test of tabular accuracy is altogether overshadowed by the interest attaching to the physical observations for which they afford propitious occasions. This change may be said to date, in its pronounced form, from the great eclipse of 1842. Although a necessary consequence of the general direction taken by scientific progress, it remains associated in a special manner with the name of Francis Baily.

The "philosopher of Newbury" was by profession a London stockbroker, and a highly successful one. Nevertheless, his services to science were numerous and invaluable, though not of the brilliant kind which attract popular notice. Born at Newbury in Berkshire, April 28, 1774, and placed in the City at the age of fourteen, he derived from the acquaintance of Dr. Priestley a love of science which never afterwards left him. It was, however, no passion such as flames up in the brain of the destined discoverer, but a regulated inclination, kept well within the bounds of an actively pursued commercial career. After travelling for a year or two in what were then the wilds of North America, he went on the Stock Exchange in 1799, and earned during twenty-four years of assiduous application to affairs a high reputation for integrity and ability, to which corresponded an ample fortune. In the meantime the Astronomical Society (largely through his co-operation) had been founded; he had for three years acted as its secretary, and he now felt entitled to devote himself exclusively to a subject which had long occupied his leisure hours. He accordingly in 1825 retired from business, purchased a house in Tavistock Place, and fitted up there a small observatory. He was, however, by preference a computator rather than an observer. What Sir John Herschel calls the "archÆology of practical astronomy" found in him an especially zealous student. He re-edited the star-catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Hevelius, Halley, Flamsteed, Lacaille, and Mayer; calculated the eclipse of Thales and the eclipse of Agathocles, and vindicated the memory of the first Astronomer Royal. But he was no less active in meeting present needs than in revising past performances. The subject of the reduction of observations, then, as we have already explained,[150] in a state of deplorable confusion, attracted his most earnest attention, and he was close on the track of Bessel when made acquainted with the method of simplification devised at KÖnigsberg. Anticipated as an inventor, he could still be of eminent use as a promoter of these valuable improvements; and, carrying them out on a large scale in the star-catalogue of the Astronomical Society (published in 1827), "he put" (in the words of Herschel) "the astronomical world in possession of a power which may be said, without exaggeration, to have changed the face of sidereal astronomy."[151]

His reputation was still further enhanced by his renewal, with vastly improved apparatus, of the method, first used by Henry Cavendish in 1797-98, for determining the density of the earth. From a series of no less than 2,153 delicate and difficult experiments, conducted at Tavistock Place during the years 1838-42, he concluded our planet to weigh 5·66 as much as a globe of water of the same bulk; and this result slightly corrected is still accepted as a very close approximation of the truth.

What we have thus glanced at is but a fragment of the truly surprising mass of work accomplished by Baily in the course of a variously occupied life. A rare combination of qualities fitted him for his task. Unvarying health, undisturbed equanimity, methodical habits, the power of directed and sustained thought, combined to form in him an intellectual toiler of the surest, though not perhaps of the highest quality. He was in harness almost to the end. He was destined scarcely to know the miseries of enforced idleness or of consciously failing powers. In 1842 he completed the laborious reduction of Lalande's great catalogue, undertaken at the request of the British Association, and was still engaged in seeing it through the press when he was attacked with what proved his last, as it was probably his first serious illness. He, however, recovered sufficiently to attend the Oxford Commemoration of July 2, 1844, where an honorary degree of D.C.L. was conferred upon him in company with Airy and Struve; but sank rapidly after the effort, and died on the 30th of August following, at the age of seventy, lamented and esteemed by all who knew him.

It is now time to consider his share in the promotion of solar research. Eclipses of the sun, both ancient and modern, were a speciality with him, and he was fortunate in those which came under his observation. Such phenomena are of three kinds—partial, annular, and total. In a partial eclipse, the moon, instead of passing directly between us and the sun, slips by, as it were, a little on one side, thus cutting off from our sight only a portion of his surface. An annular eclipse, on the other hand, takes place when the moon is indeed centrally interposed, but falls short of the apparent size required for the entire concealment of the solar disc, which consequently remains visible as a bright ring or annulus, even when the obscuration is at its height. In a total eclipse, on the contrary, the sun completely disappears behind the dark body of the moon. The difference of the two latter varieties is due to the fact that the apparent diameter of the sun and moon are so nearly equal as to gain alternate preponderance one over the other through the slight periodical changes in their respective distances from the earth.

Now, on the 15th of May, 1836, an annular eclipse was visible in the northern parts of Great Britain, and was observed by Baily at Inch Bonney, near Jedburgh. It was here that he saw the phenomenon which obtained the name of "Baily's Beads," from the notoriety conferred upon it by his vivid description.

"When the cusps of the sun," he writes, "were about 40° asunder, a row of lucid points, like a string of bright beads, irregular in size and distance from each other, suddenly formed round that part of the circumference of the moon that was about to enter on the sun's disc. Its formation, indeed, was so rapid that it presented the appearance of having been caused by the ignition of a fine train of gunpowder. Finally, as the moon pursued her course, the dark intervening spaces (which, at their origin, had the appearance of lunar mountains in high relief, and which still continued attached to the sun's border) were stretched out into long, black, thick, parallel lines, joining the limbs of the sun and moon; when all at once they suddenly gave way, and left the circumference of the sun and moon in those points, as in the rest, comparatively smooth and circular, and the moon perceptibly advanced on the face of the sun."[152]

These curious appearances were not an absolute novelty. Weber in 1791, and Von Zach in 1820, had seen the "beads"; Van Swinden had described the "belts" or "threads."[153] These last were, moreover (as Baily clearly perceived), completely analogous to the "black ligament" which formed so troublesome a feature in the transits of Venus in 1764 and 1769, and which, to the regret and confusion, though no longer to the surprise of observers, was renewed in that of 1874. The phenomenon is largely an effect of what is called irradiation, by which a bright object seems to encroach upon a dark one; but under good atmospheric and instrumental conditions it becomes inconspicuous. The "Beads" must always appear when the projected lunar edge is serrated with mountains. In Baily's observation, they were exaggerated and distorted by an irradiative clinging together of the limbs of sun and moon.

The immediate result, however, was powerfully to stimulate attention to solar eclipses in their physical aspect. Never before had an occurrence of the kind been expected so eagerly or prepared for so actively as that which was total over Central and Southern Europe on the 8th of July, 1842. Astronomers hastened from all quarters to the favoured region. The Astronomer Royal (Airy) repaired to Turin; Baily to Pavia; Otto Struve threw aside his work amidst the stars at Pulkowa, and went south as far as Lipeszk; Schumacher travelled from Altona to Vienna; Arago from Paris to Perpignan. Nor did their trouble go unrewarded. The expectations of the most sanguine were outdone by the wonders disclosed.

Baily (to whose narrative we again have recourse) had set up his Dollond's achromatic in an upper room of the University of Pavia, and was eagerly engaged in noting a partial repetition of the singular appearances seen by him in 1836, when he was "astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified at the sight of one of the most brilliant and splendid phenomena that can well be imagined. For at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory similar in shape and relative magnitude to that which painters draw round the heads of saints, and which by the French is designated an aurÉole. Pavia contains many thousand inhabitants, the major part of whom were, at this early hour, walking about the streets and squares or looking out of windows, in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer, which 'made the welkin ring,' and, for the moment, withdrew my attention from the object with which I was immediately occupied. I had indeed anticipated the appearance of a luminous circle round the moon during the time of total obscurity; but I did not expect, from any of the accounts of preceding eclipses that I had read, to witness so magnificent an exhibition as that which took place.... The breadth of the corona, measured from the circumference of the moon, appeared to me to be nearly equal to half the moon's diameter. It had the appearance of brilliant rays. The light was most dense close to the border of the moon, and became gradually and uniformly more attenuate as its distance therefrom increased, assuming the form of diverging rays in a rectilinear line, which at the extremity were more divided, and of an unequal length; so that in no part of the corona could I discover the regular and well-defined shape of a ring at its outer margin. It appeared to me to have the sun for its centre, but I had no means of taking any accurate measures for determining this point. Its colour was quite white, not pearl-colour, nor yellow, nor red, and the rays had a vivid and flickering appearance, somewhat like that which a gaslight illumination might be supposed to assume if formed into a similar shape.... Splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet I must confess that there was at the same time something in its singular and wonderful appearance that was appalling; and I can readily imagine that uncivilised nations may occasionally have become alarmed and terrified at such an object, more especially at times when the true cause of the occurrence may have been but faintly understood, and the phenomenon itself wholly unexpected.

"But the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. They had the appearance of mountains of a prodigious elevation; their colour was red, tinged with lilac or purple; perhaps the colour of the peach-blossom would more nearly represent it. They somewhat resembled the snowy tops of the Alpine mountains when coloured by the rising or setting sun. They resembled the Alpine mountains also in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona. All the three projections were of the same roseate cast of colour, and very different from the brilliant vivid white light that formed the corona; but they differed from each other in magnitude.... The whole of these three protuberances were visible even to the last moment of total obscuration; at least, I never lost sight of them when looking in that direction; and when the first ray of light was admitted from the sun, they vanished, with the corona, altogether, and daylight was instantaneously restored."[154]

Notwithstanding unfavourable weather, the "red flames" were perceived with little less clearness and no less amazement from the Superga than at Pavia, and were even discerned by Mr. Airy with the naked eye. "Their form" (the Astronomer Royal wrote) "was nearly that of saw-teeth in the position proper for a circular saw turned round in the same direction in which the hands of a watch turn.... Their colour was a full lake-red, and their brilliancy greater than that of any other part of the ring."[155]

The height of these extraordinary objects was estimated by Arago at two minutes of arc, representing, at the sun's distance, an actual elevation of 54,000 miles. When carefully watched, the rose-flush of their illumination was perceived to fade through violet to white as the light returned, the same changes in a reversed order having accompanied their first appearance. Their forms, however, during about three minutes of visibility, showed no change, although of so apparently unstable a character as to suggest to Arago "mountains on the point of crumbling into ruins" through topheaviness.[156]

The corona, both as to figure and extent, presented very different appearances at different stations. This was no doubt due to varieties in atmospheric conditions. At the Superga, for instance, all details of structure seem to have been effaced by the murky air, only a comparatively feeble ring of light being seen to encircle the moon. Elsewhere, a brilliant radiated formation was conspicuous, spreading at four opposite points into four vast luminous expansions, compared to feather-plumes or aigrettes.[157] Arago at Perpignan noticed considerable irregularities in the divergent rays. Some appeared curved and twisted, a few lay across the others, in a direction almost tangential to the moon's limb, the general effect being described as that of a "hank of thread in disorder."[158] At Lipeszk, where the sun stood much higher above the horizon than in Italy or France, the corona showed with surprising splendour. Its apparent extent was judged by Struve to be no less than twenty-five minutes (more than six times Airy's estimate), while the great plumes spread their radiance to three or four degrees from the dark lunar edge. So dazzling was the light that many well-instructed persons denied the totality of the eclipse. Nor was the error without precedent, although the appearances attending respectively a total and an annular eclipse are in reality wholly dissimilar. In the latter case, the surviving ring of sunlight becomes so much enlarged by irradiation, that the interposed dark lunar body is reduced to comparative insignificance, or even invisibility. Maclaurin tells us[159] that during an eclipse of this character which he observed at Edinburgh in 1737, "gentlemen by no means shortsighted declared themselves unable to discern the moon upon the sun without the aid of a smoked glass;" and Baily (who, however, was shortsighted) could distinguish, in 1836, with the naked eye, no trace of "the globe of purple velvet" which the telescope revealed as projected upon the face of the sun.[160] Moreover, the diminution of light is described by him as "little more than might be caused by a temporary cloud passing over the sun"; the birds continued in full song, and "one cock in particular was crowing with all his might while the annulus was forming."

Very different were the effects of the eclipse of 1842, as to which some interesting particulars were collected by Arago.[161] Beasts of burthen, he tells us, paused in their labour, and could by no amount of punishment be induced to move until the sun reappeared. Birds and beasts abandoned their food; linnets were found dead in their cages; even ants suspended their toil. Diligence-horses, on the other hand, seemed as insensible to the phenomenon as locomotives. The convolvulus and some other plants closed their leaves, but those of the mimosa remained open. The little light that remained was of a livid hue. One observer described the general coloration as resembling the lees of wine, but human faces showed pale olive or greenish. We may, then, rest assured that none of the remarkable obscurations recorded in history were due to eclipses of the annular kind.

The existence of the corona is no modern discovery. Indeed, it is too conspicuous an apparition to escape notice from the least attentive or least practised observer of a total eclipse. Nevertheless, explicit references to it are rare in early times. Plutarch, however, speaks of a "certain splendour" compassing round the hidden edge of the sun, as a regular feature of total eclipses;[162] and the corona is expressly mentioned in a description of an eclipse visible at Corfu in 968 A.D.[163] The first to take the phenomenon into scientific consideration was Kepler. He showed, from the orbital positions at the time of the sun and moon, that an eclipse observed by Clavius at Rome in 1567 could not have been annular,[164] as the dazzling coronal radiance visible during the obscuration had caused it to be believed. Although he himself never witnessed a total eclipse of the sun, he carefully collected and compared the remarks of those more fortunate, and concluded that the ring of "flame-like splendour" seen on such occasions was caused by the reflection of the solar rays from matter condensed in the neighbourhood either of the sun or moon.[165] To the solar explanation he gave his own decided preference; but, with one of those curious flashes of half-prophetic insight characteristic of his genius, declared that "it should be laid by ready for use, not brought into immediate requisition."[166] So literally was his advice acted upon, that the theory, which we now know to be (broadly speaking) the correct one, only emerged from the repository of anticipated truths after 236 years of almost complete retirement, and even then timorously and with hesitation.

The first eclipse of which the attendant phenomena were observed with tolerable exactness was that which was central in the South of France, May 12, 1706. Cassini then put forward the view that the "crown of pale light" seen round the lunar disc was caused by the illumination of the zodiacal light;[167] but it failed to receive the attention which, as a step in the right direction, it undoubtedly merited. Nine years later we meet with Halley's comments on a similar event, the first which had occurred in London since March 20, 1140. By nine in the morning of May 3, 1715, the obscuration, he tells us, "was about ten digits,[168] when the face and colour of the sky began to change from perfect serene azure blue to a more dusky livid colour, having an eye of purple intermixt.... A few seconds before the sun was all hid, there discovered itself round the moon a luminous ring, about a digit or perhaps a tenth part of the moon's diameter in breadth. It was of a pale whiteness, or rather pearl colour, seeming to be a little tinged with the colours of the iris, and to be concentric with the moon, whence I concluded it the moon's atmosphere. But the great height thereof, far exceeding our earth's atmosphere, and the observation of some, who found the breadth of the ring to increase on the west side of the moon as emersion approached, together with the contrary sentiments of those whose judgment I shall always revere" (Newton is most probably referred to), "makes me less confident, especially in a matter whereto I confess I gave not all the attention requisite." He concludes by declining to decide whether the "enlightened atmosphere," which the appearance "in all respects resembled," "belonged to sun or moon."[169]

A French Academician, who happened to be in London at the time, was less guarded in expressing an opinion. The Chevalier de Louville declared emphatically for the lunar atmospheric theory of the corona,[170] and his authority carried great weight. It was, however, much discredited by an observation made by Maraldi in 1724, to the effect that the luminous ring, instead of travelling with the moon, was traversed by it.[171] This was in reality decisive, though, as usual, belief lagged far behind demonstration. In 1715 a novel explanation had been offered by Delisle and Lahire,[172] supported by experiments regarded at the time as perfectly satisfactory. The aureola round the eclipsed sun, they argued, is simply a result of the diffraction, or apparent bending of the sunbeams that graze the surface of the lunar globe—an effect of the same kind as the coloured fringes of shadows. And this view prevailed amongst men of science until (and even after) Brewster showed, with clear and simple decisiveness, that such an effect could by no possibility be appreciable at our distance from the moon.[173] Don JosÉ Joaquim de Ferrer, however, who observed a total eclipse of the sun at Kinderhook, in the State of New York, on June 16, 1806, ignoring this refined optical rationale, considered two alternative explanations of the phenomenon as alone possible. The bright ring round the moon must be due to the illumination either of a lunar or of a solar atmosphere. If the former, he calculated that it should have a height fifty times that of the earth's gaseous envelope. "Such an atmosphere," he rightly concluded, "cannot belong to the moon, but must without any doubt belong to the sun."[174] But he stood alone in this unhesitating assertion.

The importance of the problem was first brought fully home to astronomers by the eclipse of 1842. The brilliant and complex appearance which on that occasion challenged the attention of so many observers, demanded and received, no longer the casual attention hitherto bestowed upon it, but the most earnest study of those interested in the progress of science. Nevertheless, it was only by degrees, and through a process of "exclusions" (to use a Baconian phrase) that the corona was put in its right place as a solar appendage. As every other available explanation proved inadmissible and dropped out of sight, the broad presentation of fact remained, which, though of sufficiently obvious interpretation, was long and persistently misconstrued. Nor was it until 1869 that absolutely decisive evidence on the subject was forthcoming, as we shall see further on.

Sir John Herschel, writing to his venerable aunt, relates that when the brilliant red flames burst into view behind the dark moon on the morning of the 8th of July, 1842, the populace of Milan, with the usual inconsequence of a crowd, raised the shout, "Es leben die Astronomen!"[175] In reality, none were less prepared for their apparition than the class to whom the applause due to the magnificent spectacle was thus adjudged. And in some measure through their own fault, for many partial hints and some distinct statements from earlier observers had given unheeded notice that some such phenomenon might be expected to attend a solar eclipse.

What we now call the "chromosphere" is an envelope of glowing gases, by which the sun is completely covered, and from which the "prominences" are emanations, eruptive or flame-like. Now, continual indications of the presence of this fire-ocean had been detected during eclipses in the eighteenth and nineteenth centuries. Captain Stannyan, describing in a letter to Flamsteed an occurrence of the kind witnessed by him at Berne on May 1 (o.s.), 1706, says that the sun's "getting out of the eclipse was preceded by a blood-red streak of light from its left limb."[176] A precisely similar appearance was noted by both Halley and De Louville in 1715; during annular eclipses by Lord Aberdour in 1737,[177] and by Short in 1748,[178] the tint of the ruby border being, however, subdued to "brown" or "dusky red" by the surviving sunlight; while observations identical in character were made at Amsterdam in 1820,[179] at Edinburgh by Henderson in 1836, and at New York in 1838.[180]

"Flames" or "prominences," if more conspicuous, are less constant in their presence than the glowing stratum from which they spring. The first to describe them was a Swedish professor named Vassenius, who observed a total eclipse at Gothenburg, May 2 (o.s.), 1733.[181] His astonishment equalled his admiration when he perceived, just outside the edge of the lunar disc, and suspended, as it seemed, in the coronal atmosphere, three or four reddish spots or clouds, one of which was so large as to be detected with the naked eye. As to their nature, he did not even offer a speculation, further than by tacitly referring them to the moon. The observation was repeated in 1778 by a Spanish Admiral, but with no better success in directing efficacious attention to the phenomenon. Don Antonio Ulloa was on board his ship the Espagne in passage from the Azores to Cape St. Vincent on the 24th of June in that year, when a total eclipse of the sun occurred, of which he has left a valuable description. His notices of the corona are full of interest; but what just now concerns us is the appearance of "a red luminous point" "near the edge of the moon," which gradually increased in size as the moon moved away from it, and was visible during about a minute and a quarter.[182] He was satisfied that it belonged to the sun because of its fiery colour and growth in magnitude, and supposed that it was occasioned by some crevice or inequality in the moon's limb, through which the solar light penetrated.

Allusions less precise, both prior and subsequent, which it is now easy to refer to similar objects (such as the "slender columns of smoke" seen by Ferrer)[183] might be detailed; but the evidence already adduced suffices to show that the prominences viewed with such amazement in 1842 were no unprecedented or even unusual phenomenon.

It was more important, however, to decide what was their nature than whether their appearance might have been anticipated. They were generally, and not very incorrectly, set down as solar clouds. Arago believed them to shine by reflected light,[184] but the AbbÉ Peytal rightly considered them to be self-luminous. Writing in a Montpellier paper of July 16, 1842, he declared that we had now become assured of the existence of a third or outer solar envelope, composed of a glowing substance of a bright rose tint, forming mountains of prodigious elevation, analogous in character to the clouds piled above our horizons.[185] This first distinct recognition of a very important feature of our great luminary was probably founded on an observation made by BÉrard at Toulon during the then recent eclipse, "of a very fine red band, irregularly dentelated, or, as it were, crevassed here and there,"[186] encircling a large arc of the moon's circumference. It can hardly, however, be said to have attracted general notice until July 28, 1851. On that day a total eclipse took place, which was observed with considerable success in various parts of Sweden and Norway by a number of English astronomers. Mr. Hind saw, on the south limb of the moon, "a long range of rose-coloured flames,"[187] described by Dawes as "a low ridge of red prominences, resembling in outline the tops of a very irregular range of hills."[188] Airy termed the portion of this "rugged lines of projections" visible to him the sierra, and was struck with its brilliant light and "nearly scarlet" colour.[189] Its true character of a continuous solar envelope was inferred from these data by Grant, Swan, and Littrow, and was by Father Secchi, after the great eclipse of 1860,[190] formally accepted as established.

Several prominences of remarkable forms, especially one variously compared to a Turkish scimitar, a sickle, and a boomerang, were seen in 1851. In connection with them two highly significant circumstances were pointed out. First, that of the approximate coincidence between their positions and those of sun-spots previously observed.[191] Next, that "the moon passed over them, leaving them behind, and revealing successive portions as she advanced."[192] This latter perfectly well-attested fact was justly considered by the Astronomer Royal and others as affording absolute certainty of the solar dependence of these singular objects. Nevertheless sceptics were still found. M. Faye, of the French Academy, inclined to a lunar origin for them;[193] Feilitsch of Greifswald published in 1852 a treatise for the express purpose of proving all the luminous phenomena attendant on solar eclipses—corona, prominences and "sierra"—to be purely optical appearances.[194] Happily, however, the unanswerable arguments of the photographic camera were soon to be made available against such hardy incredulity.

Thus, the virtual discovery of the solar appendages, both coronal and chromospheric, may be said to have been begun in 1842, and completed in 1851. The current Herschelian theory of the solar constitution remained, however, for the time, intact. Difficulties, indeed, were thickening around it; but their discussion was perhaps felt to be premature, and they were permitted to accumulate without debate, until fortified by fresh testimony into unexpected and overwhelming preponderance.