FOOTNOTES:

His experiments were begun towards the end of May, 1882, and by September 28 he had obtained a fair earnest of success. The exclusion of all other qualities of light save that with which he desired to operate, was accomplished by using chloride of silver as his sensitive material, that substance being chemically inert to all other but those precise rays in which the corona has the advantage.[544]Plates thus sensitised received impressions which it was hardly possible to regard as spurious. "Not only the general features," Captain Abney affirmed,[545]"are the same, but details, such as rifts and streamers, have the same position and form." It was found, moreover, that the corona photographed during the total eclipse of May 6, 1883, was intermediate in shape between the coronas photographed by Sir William Huggins before and after that event, each picture taking its proper place in a series of progressive modifications highly interesting in themselves, and full of promise for the value of the method employed to record them.[546]But experiments on the subject were singularly interrupted. The volcanic explosion in the Straits of Sunda in August, 1883, brought to astronomers a peculiarly unwelcome addition to their difficulties. The magnificent sunglows due to the diffractive effects on light of the vapours and fine dust flung in vast volumes into the air, and rapidly diffused all round the globe, betokened an atmospheric condition of all others the most prejudicial to delicate researches in the solar vicinity. The filmy coronal forms, accordingly, which had been hopefully traced on the Tulse Hill plates ceased to appear there; nor were any substantially better results obtained by Mr. C. Ray Woods, in the purer air either of the Riffel or the Cape of Good Hope, during the three ensuing years. Moreover, attempts to obtain coronal photographs during the partial phases of the eclipse of August 29, 1886, completely failed. No part of the lunar globe became visible in relief against circumfluous solar radiance on any of the plates exposed at Grenada; and what vestiges of "structure" there were, came out almost betteruponthe moon thanbesideher, thus stamping themselves at once as of atmospheric origin.

That the effect sought is a perfectly possible one is proved by the distinct appearance of the moon projected on the corona, in photographs of the partially eclipsed sun in 1858, 1889, and 1890, and very notably in 1898 and 1900.[547]

In the spring of 1893, Professor Hale[548]attacked the problem ofcoronal daylight photography, employing the "double-slit" method so eminently serviceable for the delineation of prominences.[549]But neither at Kenwood nor at the summit of Pike's Peak, whither, in the course of the summer, he removed his apparatus, was any action of the desired kind secured. Similar ill success attended his and Professor Riccò's employment, on Mount Etna in July, 1894, of a specially designed coronagraph. Yet discouragement did not induce despair. The end in view is indeed too important to be readily abandoned; but it can be reached only when a more particular acquaintance with the nature of coronal light than we now possess indicates the appropriate device for giving it a preferential advantage in self-portraiture. Moreover, the effectiveness of this device may not improbably be enhanced, through changes in the coronal spectrum at epochs of sun-spot maximum.

The prosperous result of the Sohag observations stimulated the desire to repeat them on the first favourable opportunity. This offered itself one year later, May 6, 1883, yet not without the drawbacks incident to terrestrial conditions. The eclipse promised was of rare length, giving no less than five minutes and twenty-three seconds of total obscurity, but its path was almost exclusively a "water-track." It touched land only on the outskirts of the Marquesas group in the Southern Pacific, and presented, as the one available foothold for observers, a coral reef named Caroline Island, seven and a half miles long by one and a half wide, unknown previously to 1874, and visited only for the sake of its stores of guano. Seldom has a more striking proof been given of the vividness of human curiosity as to the condition of the worlds outside our own, than in the assemblage of a group of distinguished men from the chief centres of civilisation, on a barren ridge, isolated in a vast and tempestuous ocean, at a distance, in many cases, of 11,000 miles and upwards from the ordinary scene of their labours. And all these sacrifices—the cost and care of preparation, the transport and readjustment of delicate instruments, the contrivance of new and more subtle means of investigating phenomena—on the precarious chance of a clear sky during one particular five minutes! The event, though fortunate, emphasised the hazard of the venture. The observation of the eclipse was made possible only by the happy accident of a serene interval between two storms.

The American expedition was led by Professor Edward S. Holden, and to it were courteously permitted to be attached Messrs. Lawrance and Woods, photographers, sent out by the Royal Society of London. M. Janssen was chief of the French Academy mission;he was accompanied from Meudon by Trouvelot, and joined from Vienna by Palisa, and from Rome by Tacchini. A large share of the work done was directed to assuring or negativing previous results. The circumstances of an eclipse favour illusion. A single observation by a single observer, made under unfamiliar conditions, and at a moment of peculiar excitement, can scarcely be regarded as offering more than a suggestion for future inquiry. But incredulity may be carried too far. Janssen, for instance, felt compelled, by the survival of unwise doubts, to devote some of the precious minutes of obscurity at Caroline Island to confirming what, in his own persuasion, needed no confirmation—that is, the presence of reflected Fraunhofer lines in the spectrum of the corona. Trouvelot and Palisa, on the other hand, instituted an exhaustive, but fruitless search for the spurious "intramercurian" planets announced by Swift and Watson in 1878.

New information, however, was not deficient. The corona proved identical in type with that of 1882,[550]agreeably to what was expected at an epoch of protracted solar activity. The characteristic aigrettes were of even greater brilliancy than in the preceding year, and the chemical effects of the coronal light proved unusually intense. Janssen's photographs, owing to the considerable apertures (six and eight inches) of his object-glasses, and the long exposures permitted by the duration of totality, were singularly perfect; they gave a greater extension to the coronal than could be traced with the telescope,[551]and showed its forms as absolutely fixed and of remarkable complexity.

The English pictures, taken with exposures up to sixty seconds, were likewise of great value. They exhibited details of structure from the limb to the tips of the streamers, which terminated definitely, and as it seemed actually, where the impressions on the plates ceased. The coronal spectrum was also successfully photographed, and although the reversing layer in its entirety evaded record, a print was caught of some of its more prominent rays just before and after totality. The use of the prismatic camera was baffled by the anomalous scarcity of prominences.

Using an ingenious apparatus for viewing simultaneously the spectrum from both sides of the sun, Professor Hastings noticed at Caroline Island alternations, with the advance of the moon, in the respective heights above the right and left solar limbs of the coronal green line, which were thought to imply that the corona, with its rifts and sheaves and "tangled hanks" of rays, is, after all, merely an illusive appearance produced by the diffraction of sunlight at themoon's edge.[552]But the observation was assuredly misleading or misinterpreted. Atmosphericdiffusionmay indeed, under favouring circumstances, be effective in deceptively enlarging solar appendages; but always to a very limited extent.

The controversy is an old one as to the part played by our air in producing the radiance visible round the eclipsed sun. In its original form, it is true, it came to an end when Professor Harkness, in 1869,[553]pointed out that the shadow of the moon falls equally over the air and on the earth, and that if the sun had no luminous appendages, a circular space of almost absolute darkness would consequently surround the apparent places of the superposed sun and moon. Mr. Proctor,[554]with his usual ability, impressed this mathematically certain truth upon public attention; and Sir John Herschel calculated that the diameter of the "negative halo" thus produced would be, in general, no less than 23°.

But about the same time a noteworthy circumstance relating to the state of things in the solar vicinity was brought into view. On February 11, 1869, Messrs. Frankland and Lockyer communicated to the Royal Society a series of experiments on gaseous spectra under varying conditions of heat and density, leading them to the conclusion that the higher solar prominences exist in a medium of excessive tenuity, and that even at the base of the chromosphere the pressure is far below that at the earth's surface.[555]This inference was fully borne out by the researches of Wüllner; and Janssen expressed the opinion that the chromospheric gases are rarefied almost to the degree of an air-pump vacuum.[556]Hence was derived a general and fully justified conviction that there could be outside, and incumbent upon the chromosphere, no such vast atmosphere as the corona appeared to represent. Upon the strength of which conviction the "glare" theory entered, chiefly under the auspices of Sir Norman Lockyer, upon the second stage of its existence.

The genuineness of the "inner corona" to the height of 5′ or 6′ from the limb was admitted; but it was supposed that by the detailed reflection of its light in our air the far more extensive "outer corona" was optically created, the irregularities of the moon's edge being called in to account for the rays and rifts by whichitsstructure was varied. This view received some countenance from Admiral Maclear's observation, during the eclipse of 1870, of bright lines "everywhere"—even at the centre of the lunar disc. Here,indeed, was an undoubted case of atmospheric diffusion; but here, also, was a safe index to the extent of its occurrence. Light scatters equally in all directions; so that when the moon's face at the time of an eclipse shows (as is the common case) a blank in the spectroscope, it is quite certain that the corona is not noticeably enlarged by atmospheric causes. A sky drifted over with thin cirrus clouds and air changed with aqueous vapour amply accounted for the abnormal amount of scattering in 1870.

But even in 1870 positive evidence was obtained of the substantial reality of the radiated outer corona, in the appearance on the photographic plates exposed by Willard in Spain and by Brothers in Sicily of identical dark rifts. The truth is, that far from being developed by misty air, it is peculiarly liable to be effaced by it. The purer the sky, the more extensive, brilliant, and intricate in the details of its structure the corona appears. Take as an example General Myer's description of the eclipse of 1869, as seen from the summit of White Top Mountain, Virginia, at an elevation above the sea of 5,523 feet, in an atmosphere of peculiar clearness.

"To the unaided eye," he wrote,[557]"the eclipse presented, during the total obscuration, a vision magnificent beyond description. As a centre stood the full and intensely black disc of the moon, surrounded by the aureola of a soft bright light, through which shot out, as if from the circumference of the moon, straight, massive, silvery rays, seeming distinct and separate from each other, to a distance of two or three diameters of the solar disc; the whole spectacle showing as on a background of diffused rose-coloured light."

On the same day, at Des Moines, Newcomb could perceive, through somewhat hazy air, no long rays, and the four-pointed outline of the corona reached at its farthest only asingle semidiameterof the moon from the limb. The plain fact, that our atmosphere acts rather as a veil to hide the coronal radiance than as the medium through which it is visually formed, emerges from further innumerable records.

No observations of importance were made during the eclipse of September 9, 1885. The path of total obscurity touched land only on the shores of New Zealand, and two minutes was the outside limit of available time. Hence local observers had the phenomenon to themselves; nor were they even favoured by the weather in their efforts to make the most of it. One striking appearance was, however, disclosed. It was that of two "white" prominences of unusual brilliancy, shining like a pair of electriclamps hung one at each end of a solar diameter, right above the places of two large spots.[558]This coincidence of diametrically opposite disturbances is of too frequent occurrence to be accidental. M. Trouvelot observed at Meudon, June 26, 1885, two active and evanescent prominences thus situated, each rising to the enormous height of 300,000 miles; and on August 16, one scarcely less remarkable, balanced by an antipodal spot-group.[559]It towered upward, as if by a process ofunrolling, to a quarter of a million of miles; after which, in two minutes, the light died out of it; it had become completely extinct. The development, again from the ends of a diameter, of a pair of similar objects was watched, September 19 and 20, 1893, by Father Fényi, Director of the Kalocsa Observatory; and the phenomenon has been too often repeated to be accidental.

The eclipse of August 29, 1886, was total during about four minutes over tropical Atlantic regions; and an English expedition, led by Sir Norman Lockyer, was accordingly despatched to Grenada in the West Indies, for the purpose of using the opportunity it offered. But the rainy season was just then at its height: clouds and squalls were the order of the day; and the elaborately planned programme of observation could only in part be carried through. Some good work, none the less, was done. Professor Tacchini, who had been invited to accompany the party, ascertained besides some significant facts about prominences. From a comparison of their forms and sizes during and after the eclipse, it appeared that only the growing vaporous cores of these objects are shown by the spectroscope under ordinary circumstances; their upper sections, giving a faint continuous spectrum, and composed of presumably cooler materials, can only be seen when the veil of scattered light usually drawn over them is removed by an eclipse. Thus all modestly tall prominences have silvery summits; but all do not appear to possess the "red heart of flame," by which alone they can be rendered perceptible to daylight observation. Some prove to be ordinarily invisible, because silvery throughout—"sheeted ghosts," as it were, met only in the dark.

Specimens of the class had been noted as far back as 1842, but Tacchini first drew particular attention to them. The one observed by him in 1886 rose in a branching form to a height of 150,000 miles, and gave a brilliantly continuous spectrum, with bright lines at H and K, but no hydrogen-lines.[560]Hence the total invisibility of the object before and after the eclipse. During the eclipse, it was seen framed, as it were, in a pointed arch of coronal light, thesymmetrical arrangement of which with regard to it was obviously significant. Both its unspringing shape, and the violet rays of calcium strongly emitted by it, contradicted the supposition that "white prominences" represent a downrush of refrigerated materials.

The corona of 1886, as photographed by Dr. Schuster and Mr. Maunder, showed neither the petals and plumes of 1871, nor the streamers of 1878. It might be called of a transition type.[561]Wide polar rifts were filled in with tufted radiations, and bounded on either side by irregularly disposed, compound luminous masses. In the south-western quadrant, a triangular ray, conspicuous to the naked eye, represented, Mr. W. H. Pickering thought, the projection of a huge, hollow cone.[562]Branched and recurving jets were curiously associated with it. The intrinsic photographic brightness of the corona proved, from Pickering's measures, to be about 1/54 that of the average surface of the full moon.

The Russian eclipse of August 19, 1887, can only be remembered as a disastrous failure. Much was expected of it. The shadow-path ran overland from Leipsic to the Japanese sea, so that the solar appurtenances would, it was hoped, be disclosed to observers echeloned along a line of 6,000 miles. But the incalculable element of weather rendered all forecasts nugatory. The clouds never parted, during the critical three minutes, over Central Russia, where many parties were stationed, and Professor D. P. Todd was equally unfortunate in Japan. Some good photographs were, nevertheless, secured by Professor Arai, Director of the Tokio Observatory, as well as by MM. Bélopolsky and Glasenapp at Petrovsk and Jurjevitch respectively. They showed a corona of simpler form than that of the year before, but not yet of the pronounced type first associated by Mr. Ranyard with the lowest stage of solar activity.

The genuineness of the association was ratified by the duplicate spectacle of the next-ensuing minimum year. Two total eclipses of the sun distinguished 1889. The first took place on New Year's Day, when a narrow shadow-path crossed California, allowing less than two minutes for the numerous experiments prompted by the varied nature of modern methods of research. American astronomers availed themselves of the occasion to the full. The heavens were propitious. Photographic records were obtained in unprecedented abundance, and of unusual excellence. Their comparison and study placed it beyond reasonable doubt that the radiated corona belonging to periods of maximum sun-spotsgives place, at periods of minimum, to the "winged" type of 1878. Professor Holden perceived further that the equatorial extensions characterising the latter tend to assume a "trumpet-shape."[563]Their extremities diverge, as if mutually repellent, instead of flowing together along a medial plane. The maximum actinic brilliancy of the corona of January 1, 1889, was determined at Lick to be twenty-one times less than that of the full moon.[564]Its colour was described as "of an intense luminous silver, with a bluish tinge, similar to the light of an electric arc."[565]Its spectrum was comparatively simple. Very few bright lines besides those of hydrogen and coronium, and apparently no dark ones, stood out from the prismatic background.

"The marked structural features of the corona, as presented by the negatives" taken by Professors Nipher and Charroppin, were the filaments and the streamers. The filaments issued from polar calottes of 20° radius.

"The impression conveyed to the eye," Professor Pritchett wrote,[566]"is that the equatorial stream of denser coronal matter extends across and through the filaments, simply obscuring them by its greater brightness. The effect is just as if the equatorial belt were superposed upon, or passed through, the filamentary structure. There is nothing in the photographs to prove that the filaments do not exist all round the sun.[567]The testimony from negatives of different lengths of exposure goes to show that the equatorial streamers are made up of numerous interlacing parts inclined at varying angles to the sun's equator."

The coronal extensions, perceptible with the naked eye to a distance of more than 3° from the sun, appeared barely one-third of that length on the best negatives. Little more could be seen of them either in Barnard's exquisite miniature pictures, or in the photographs obtained by W. H. Pickering with a thirteen-inch refractor—the largest instrument so far used in eclipse-photography.

The total eclipse of December 22, 1889, held out a prospect, unfortunately not realized, of removing some of the doubts and difficulties that impeded the progress of coronal photography.[568]Messrs. Burnham and Schaeberle secured at Cayenne some excellent impressions, showing enough of the corona to prove its identicalcharacter with that depicted in the beginning of the year, but not enough to convey additional information about its terminal forms or innermost structure. Any better result was indeed impossible, the moisture-laden air having cut down the actinic power of the coronal light to one-fourth its previous value.

Two English expeditions organized by the Royal Astronomical Society fared still worse. Mr. Taylor was stationed on the West Coast of Africa, one hundred miles south of Loanda; Father Perry chose as the scene of his operations the Salut Islands, off French Guiana. Each was supplied with a reflector constructed by Dr. Common, endowed, by its extremely short focal length of forty-five, combined with an aperture of twenty inches, with a light-concentrating force capable, it was hoped, of compelling the very filmiest coronal branches to self-registration. Had things gone well two sets of coronal pictures, absolutely comparable in every respect, and taken at an interval of two hours and a half, would have been at the disposal of astronomers. But things went very far from well. Clouds altogether obscured the sun in Africa; they only separated to allow of his shining through a saturated atmosphere in South America. Father Perry's observations were the last heroic effort of a dying man. Stricken with malaria, he crawled to the hospital as soon as the eclipse was over, and expired five days later, at sea, on board theComus. He was buried at Barbados. And the sacrifice of his life had, after all, purchased no decisive success. Most of the plates exposed by him suffered deterioration from the climate, or from an inevitably delayed development. A drawing from the best of them by Miss Violet Common[569]represented a corona differing from its predecessor of January 1, chiefly through the oppositely unsymmetrical relations of its parts. Then the western wing had been broader at its base than the eastern; now the inequality was conspicuously the other way.[570]

The next opportunity for retrieving the mischances of the past was offered April 16, 1893. The line of totality charted for that day ran from Chili to Senegambia. American parties appropriated the Andes; both shores of the Atlantic were in English occupation; French expeditions, led by Deslandres and Bigourdan, took up posts south of Cape Verde. A long totality of more than four minutes was favoured by serene skies; hence an ample store of photographic data was obtained. Professor Schaeberle, of the Lick Observatory, took, almost without assistance, at Mina Bronces, a mining station 6,600 feet above the Pacific, fifty-two negatives, eight of them with a forty-foot telescope, on a scale of four and a half inches to thesolar diameter. Not only the inner corona, but the array of prominences then conspicuous, appeared in them to be composed of fibrous jets and arches, held to be sections of elliptic orbits described by luminous particles about the sun's centre.[571]One plate received the impression of a curious object,[572]entangled amidst coronal streamers, and the belief in its cometary nature was ratified by the bestowal of a comet-medal in recognition of the discovery. Similiar paraboloidal forms had, nevertheless, occasionally been seen to make an integral part of earlier coronas; and it remains extremely doubtful whether Schaeberle's "eclipse-comet" was justly entitled to the character claimed for it.

The eclipse of 1893 disclosed a radiated corona such as a year of spot-maximum was sure to bring. An unexpected fact about it was, however, ascertained. The coronal has been believed to have much in common with the chromospheric spectrum; it proved, on investigation with a large prismatic camera, employed under Sir Norman Lockyer's directions by Mr. Fowler at Fundium, to be absolutely distinct from it. The fundamental green ray had, on the West African plates, seven more refrangible associates;[573]but all alike are of unknown origin. They may be due to many substances, or to one; future research will perhaps decide; we can at present only say that the gaseous emission of the corona include none from hydrogen, helium, calcium, or any other recognisable terrestrial element. Deslandres' attempt to determine the rotation of the corona through opposite displacements, east and west of the interposed moon, of the violet calcium-lines supposed to make part of the coronal spectrum, was thus rendered nugatory. Yet it gave an earnest of success, by definitely introducing the subject into the constantly lengthened programme of eclipse-work. There is, however, little prospect of its being treated effectively until the green line is vivified by a fresh access of solar activity.

The flight of the moon's shadow was, on August 9, 1896, dogged by atrocious weather. It traversed, besides, some of the most inhospitable regions on the earth's surface, and afforded, at the best, but a brief interval of obscurity. At Novaya Zemlya, however, of all places, the conditions were tolerably favourable, and, as we have seen, the trophy of a "flash-spectrograph" was carried off. Some coronal photographs, moreover, taken by the late Sir George Baden-Powell[574]and by M. Hansky, a member of a Russian party, were marked by features of considerable interest. They madeapparent a close connection between coronal outflows and chromospheric jets, cone-shaped beams serving as the sheaths, or envelopes, of prominences. M. Hansky,[575]indeed, thought that every streamer had a chromospheric eruption at its base. Further, dark veinings of singular shapes unmistakably interrupted the coronal light, and bordered brilliant prominences,[576]reminding us of certain "black lines" traced by Swift across the "anvil protuberance" August 7, 1869.[577]In type the corona of 1896 reproduced that of 1886, as befitted its intermediate position in the solar cycle.

The eclipse-track on January 22, 1898, crossed the Indian peninsula from Viziadrug, on the Malabar coast, to Mount Everest in the Himalayas. Not a cloud obstructed the view anywhere, and an unprecedented harvest of photographic records was garnered. The flash-spectrum, in its successive phases, appeared on plates taken by Sir Norman Lockyer, Mr. Evershed, Professor Campbell,[578]and others; Professor Turner[579]set on foot a novel mode of research by picturing the corona in the polarised ingredient of its light; Mrs. Maunder[580]practically solved the problem of photographing the faint coronal extensions, one ray on her plates running out to nearly six diameters from the moon's limb. Yet she used a Dallmeyer lens of only one and a half inches aperture. Her success accorded perfectly with Professor Wadsworth's conclusion that effectiveness in delineation by slight contrasts of luminosity varies inversely with aperture. Triple-coated plates, and a comparatively long exposure of twenty seconds, contributed to a result unlikely, for some time, to be surpassed. The corona of 1898 presented a mixed aspect. The polar plumes due at minimum were combined in it with the quadrilateral ogives belonging to spot-maxima. A slow course of transformation, in fact, seemed in progress; and it was found to be completed in 1900, when the eclipse of May 28 revealed the typical halo of a quiescent sun.

The obscurity on this occasion was short—less than 100 seconds—but was well observed east and west of the Atlantic. No striking gain in knowledge, however, resulted. Important experiments were indeed made on the heat of the corona with Langley's bolometer, but their upshot can scarcely be admitted as decisive. They indicated a marked deficiency of thermal radiations, implying for coronal light, in Professor Langley's opinion,[581]an origin analogous to that of the electric glow-discharge, which, at low pressures, wasfound by K. Ångström in 1893 to have no invisible heat-spectrum.[582]The corona was photographed by Professor Barnard, at Wadesborough, North Carolina, with a 61-1/2-foot horizontal "coelostat." In this instrument, of a type now much employed in eclipse operations and first recommended by Professor Turner, a six-inch photographic objective preserved an invariable position, while a silvered plane mirror, revolving by clockwork once in forty-eight hours (since the angle of movement is doubled by reflection), supplied the light it brought to a focus. A temporary wooden tube connected the lens with the photographic house where the plates were exposed. Pictures thus obtained with exposures of from one to fourteen seconds, were described as "remarkably sharp and perfectly defined, showing the prominences and inner corona very beautifully. The polar fans came out magnificently."[583]

The great Sumatra eclipse left behind it manifold memories of foiled expectations. A totality of above six minutes drew observers to the Far East from several continents, each cherishing a plan of inquiry which few were destined to execute. All along the line of shadow, which, on May 18, 1901, crossed Réunion and Mauritius, and again met land at Sumatra and Borneo, the meteorological forecast was dubious, and the meteorological actuality in the main deplorable. Nevertheless, the corona was seen, and fairly well photographed through drifting clouds, and proved to resemble in essentials the appendage viewed a year previously. Negatives taken by members of the Lick Observatory expedition led by Mr. Perrine[584]disclosed the unique phenomenon of a violent coronal disturbance, with a small compact prominence as its apparent focus. Tumbling masses and irregular streamers radiating from a point subsequently shown by the Greenwich photographs to be the seat of a conspicuous spot, suggested the recent occurrence of an explosion, the far-reaching effects of which might be traced in the confused floccular luminosity of a vast surrounding region. Again, photographs in polarised light attested the radiance of the outer corona to be in large measure reflected, while that of the inner ring was original; and the inference was confirmed by spectrographs, recording many Fraunhofer lines when the slit lay far from the sun's limb, but none in its immediate vicinity. On plates exposed by Mr. Dyson and Dr. Humphrys with special apparatus, the coronal spectrum, continuous and linear, impressed itself more extensively in the ultra-violet than on any previous occasion; and Dr. Mitchell succeeded in photographing the reversing layer by means of a grating spectroscope. Finally, Mrs. Maunder, at Mauritius, despite mischievousatmospheric tremors, obtained with the Newbegin telescope an excellent series of coronal pictures.[585]

The principles of explanation applied to the corona may be briefly described as eruptive and electrical. The first was adopted by Professor Schaeberle in his "Mechanical Theory," advanced in 1890.[586]According to this view, the eclipse-halo consists of streams of matter shot out with great velocity from the spot-zones by forces acting perpendicularly to the sun's surface. The component particles return to the sun after describing sections of extremely elongated ellipses, unless their initial speed happen to equal or exceed the critical rate of 383 miles a second, in which case they are finally driven off into space. The perspective overlapping and interlacing of these incandescent outflows was supposed to occasion the intricacies of texture visible in the corona; and it should be recorded that a virtually identical conclusion was reached by Mr. Perrine in 1901,[587]by a different train of reasoning, based upon a distinct set of facts. A theory on very much the same lines was, moreover, worked out by M. Bélopolsky in 1897.[588]Schaeberle, however, had the merit of making the first adequate effort to deduce the real shape of the corona, as it exists in three dimensions, from its projection upon the surface of the sphere. He failed, indeed, to account for the variation in coronal types by the changes in our situation with regard to the sun's equator. It is only necessary to remark that, if this were so, they should be subject to an annual periodicity, of which no trace can be discerned.

Electro-magnetic theories have the charm, and the drawback, of dealing largely with the unknown. But they are gradually losing the vague and intangible character which long clung to them; and the improved definition of their outlines has not, so far, brought them into disaccord with truth. The most promising hypothesis of the kind is due to Professor Bigelow of Washington. His able discussion of the eclipse photographs of January 1, 1889,[589]showed a striking agreement between the observed coronal forms and the calculated effects of a repulsive influence obeying the laws of electric potential, also postulated by Huggins in 1885.[590]Finely subdivided matter, expelled from the sun along lines of force emanating from the neighbourhood of his poles, thus tends to accumulate at "equipotentialsurfaces." In deference, however, to a doubt more strongly felt then than now, whether the presence of free electricity is compatible with the solar temperature, he avoided any express assertion that the coronal structure is an electrical phenomenon, merely pointing out that, if it were, its details would be just what they are.

Later, in 1892, Pupin in America,[591]and Ebert in Germany,[592]imitated the coronal streamers by means of electrical discharges in low vacua between small conducting bodies and strips of tinfoil placed on the outside of the containing glass receptacles. Finally, a critical experiment made by Ebert in 1895 served, as Bigelow justly said, "to clear up the entire subject, and put the theory on a working basis." Having obtained coronoidal effects in the manner described, he proceeded to subject them to the action of a strong magnetic field, with the result of marshalling the scattered rays into a methodical and highly suggestive array. They followed the direction of the magnetic lines of force, and, forsaking the polar collar of the magnetised sphere, surrounded it like a ruffle. The obvious analogy with the aurora polaris and the solar corona was insisted upon by Ebert himself, and has been further developed by Bigelow.[593]According to a recent modification of his hypothesis, the latter appendage is controlled by two opposing systems of forces; the magnetic causing the rays to diverge from the poles towards the equator, and the electrostatic urging their spread, through the mutual repulsion of the particles accumulated in the "wings," from the equator towards either pole. The cyclical change in the corona, he adds, is probably due to a variation in the balance of power thus established, the magnetic polar influence dominating at minima, the electrostatic at maxima. And he may well feel encouraged by the fortunate combination of many experimental details into one explanatory whole, no less than by the hopeful prospect of further developments, both practical and theoretical, along the same lines.

What we really know about the corona can be summed up in a few words. It is certainlynota solar atmosphere. It does not gravitate upon the sun's surface and share his rotation, as our air gravitates upon and shares the rotation of the earth; and this for the simple reason that there is no visible growth of pressure downwards (of which the spectroscope would infallibly give notice) in its gaseous constituents; whereas under the sole influence of the sun's attractive power, their density should be multiplied many million times in the descent through a mere fraction of their actual depth.[594]

They are apparently in a perpetual state of efflux from, and influx to our great luminary, under the stress of opposing forces. It is not unlikely that some part, at least, of the coronal materials are provided by eruptions from the body of the sun;[595]it is almost certain that they are organized and arranged round it through electro-magnetic action. This, however, would seem to be influential only upon their white-hot or reflective ingredients, out of which the streamers and aigrettes are composed; since the coronal gases appear, from observations during eclipses, to form a shapeless envelope, with condensations above the spot-zones, or at the bases of equatorial extensions. The corona is undoubtedly affected both in shape and constitution by the periodic ebb and flow of solar activity, its low-tide form being winged, its high-tide form stellate; while the rays emitted by the gases contained in it fade, and the continuous spectrum brightens, at times of minimum sun-spots. The appendage, as a whole, must be of inconceivable tenuity, since comets cut their way through it without experiencing sensible retardation. Not even Sir William Crookes's vacua can give an idea of the rarefaction which this fact implies. Yet the observed luminous effects may not in reality bear witness contradictory of it. One solitary molecule in each cubic inch of space might, in Professor Young's opinion, produce them; while in the same volume of ordinary air at the sea-level, the molecules number (according to Dr. Johnstone Stoney) 20,000 trillions!

The most important lesson, however, derived from eclipses is that of partial independence of them. Some of its fruits in the daily study of prominences the next chapter will collect; and the harvest has been rendered more abundant, as well as more valuable, since it has been found possible to enlist, in this department too, the versatile aid of the camera.

FOOTNOTES:[512]Vierteljahrsschrift Astr. Ges., Jahrg. xxvi., p. 274.[513]Astr. Jour., vol. iv., p. 33.[514]Proc. Roy. Soc., vol. xvii., p. 116.[515]Comptes Rendus, t. lxvii., p. 757.[516]Comptes Rendus, t. lxvii., p. 839.[517]Month. Not., vol. xxvii., p. 88.[518]Proc. Roy. Soc., vol. xvii., p. 123.[519]Washington Observations, 1867, App. ii., Harkness's Report, p. 60.[520]Am. Jour., vol. xlviii. (2nd series), p. 377.[521]Am. Jour., vol. xi. (3rd series), p. 429.[522]Campbell,Astroph. Jour., vol. x., p. 186.[523]Keeler,Reports on Eclipse of January 1, 1889, p. 47.[524]Everything in such observations depends upon the proper manipulation of the slit of the spectroscope.[525]Mem. R. A. S., vol. xli., p. 435.[526]Comptes Rendus, t. lxvii., p. 1019.[527]Mem. R. A. S., vol. xli., p. 43.[528]Comptes Rendus, t. xciv., p. 1640.[529]Young,Pop. Astr., Oct., 1897, p. 333.[530]J. Evershed,Indian Eclipse, 1898, p. 65;Month. Not., vol. lviii., p. 298;Proc. Roy. Soc., Jan. 17, 1901.[531]Frost,Astroph. Jour., vol. xii., p. 85; Lord,Ibid., vol. xiii., p. 149.[532]Comptes Rendus, t. cxvii., No. 1;Jour. Brit. Astr. Ass., vol. iii., p. 532.[533]Lockyer,Phil. Trans., vol. clvii., p. 551.[534]The rosy envelope of prominence-matter was so named by Lockyer in 1868 (Phil. Trans., vol. clix., p. 430).[535]According to Trouvelot (Wash. Obs., 1876, App. iii., p. 80), the subtracted matter was, at least to some extent, accumulated in the polar regions.[536]Bull. Phil. Soc. Washington, vol. iii., p. 118.[537]Mem. R. A. S., vol. xli., 1879.[538]Astr. Nach., No. 1,737.[539]Correspondence with Newton, pp. 181-184; Ranyard,Mem. Astr. Soc., vol. xli., p. 501.[540]S. P. Langley,Wash. Obs., 1876, App. iii., p. 209;Nature, vol. lxi., p. 443.[541]Schuster (Proc. Roy. Soc., vol. xxxv., p. 154) measured and photographed about thirty.[542]Abney,Phil. Trans., vol. clxxv., p. 267.[543]Proc. Roy. Soc., vol. xxxiv., p. 409. Experiments directed to the same end had been made by Dr. O. Lohse at Potsdam, 1878-80.Astr. Nach., No. 2,486.[544]The sensitiveness of chloride of silver extends fromhto H; that is, over the upper or more refrangible half of the space in which the main part of the coronal light is concentrated.[545]Proc. Roy. Soc., vol. xxxiv., p. 414.[546]Report Brit. Assoc., 1883, p. 351.[547]Maunder,Indian Eclipse, p. 125;Eclipse of 1900, p. 143.[548]Astr. and Astrophysics, vol. xiii., p. 662.[549]Seeinfra, p. 197.[550]Abney,Phil. Trans., vol. clxxx., p. 119.[551]Comptes Rendus, t. xcvii., p. 592.[552]Memoirs National Ac. of Sciences, vol. ii., p. 102.[553]Wash. Obs., 1867, App. ii., p. 64.[554]The Sun, p. 357.[555]Proc. Roy. Soc., vol. xvii., p. 289.[556]Comptes Rendus, t. lxxiii., p. 434.[557]Wash. Obs., 1867, App. ii., p. 195.[558]Stokes, Anniversary Address,Nature, vol. xxxv., p. 114.[559]Comptes Rendus, t. ci., p. 50.[560]Harvard Annals, vol. xviii., p. 99.[561]Wesley,Phil. Trans., vol. clxxx., p. 350.[562]Harvard Annals, vol. xviii, p. 108.[563]Lick Report, p. 20.[564]Ibid., p. 14.[565]Ibid., p. 155.[566]Pub. Astr. Soc. of the Pacific, vol. iii., p. 158.[567]Professor Holden concluded, with less qualification, "that so-called 'polar' rays exist at all latitudes on the sun's surface."Lick Report, p. 19.[568]Holden,Report on Eclipse of December, 1889, p. 18; Charroppin,Pub. Astr. Soc. of the Pacific, vol. iii., p. 26.[569]Published as the Frontispiece to theObservatory, No. 160.[570]Wesley,Ibid., p. 107.[571]Lick Observatory Contributions, No. 4, p. 108.[572]Astr. and Astrophysics, vol. xiii. p. 307.[573]Lockyer,Phil. Trans., vol. clxxxvii., p. 592.[574]He died in London, November 20, 1898.[575]Bull. Acad. St. Pétersbourg, t. vi., p. 253.[576]W. H. Wesley,Phil. Trans., vol. cxc, p. 204.[577]Lick Reports on Eclipse of January 1, 1889, p. 204.[578]Astroph. Jour., vol. xi., p. 226.[579]Observatory, vol. xxi., p. 157.[580]The Indian Eclipse, 1898, p. 114.[581]Science, June 22, 1900;Astroph. Jour., vol. xii., p. 370.[582]Ann. der Physik, Bd. xlviii., p. 528. See also Wood,Physical Review, vol. iv., p. 191, 1896.[583]Science, August 3, 1900.[584]Lick Observatory Bulletin, No. 9.[585]Observatory, vol. xxiv., pp. 321, 375.[586]Lick Report on Eclipse of December 22, 1889, p. 47;Month. Not., vol. l., p. 372.[587]Lick Obs. Bull., No. 9.[588]Bull. de l'Acad. St. Pétersbourg, t. iv., p. 289.[589]The Solar Corona discussed by Spherical Harmonics, Smithsonian Institution, 1889.[590]Bakerian Lecture,Proc. Roy. Soc., vol. xxxix.[591]Astr. and Astrophysics, vol. xi., p. 483.[592]Ibid., vol. xii., p. 804.[593]Am. Journ. of Science, vol. xi., p. 253, 1901.[594]See Huggins,Proc. Roy. Soc., vol. xxxix., p. 108; Young,North Am. Review, February, 1885, p. 179.[595]Professor W. A. Norton, of Yale College, appears to have been the earliest formal advocate of the Expulsion Theory of the solar surroundings, in the second (1845) and later editions of hisTreatise on Astronomy.

[512]Vierteljahrsschrift Astr. Ges., Jahrg. xxvi., p. 274.

[513]Astr. Jour., vol. iv., p. 33.

[514]Proc. Roy. Soc., vol. xvii., p. 116.

[515]Comptes Rendus, t. lxvii., p. 757.

[516]Comptes Rendus, t. lxvii., p. 839.

[517]Month. Not., vol. xxvii., p. 88.

[518]Proc. Roy. Soc., vol. xvii., p. 123.

[519]Washington Observations, 1867, App. ii., Harkness's Report, p. 60.

[520]Am. Jour., vol. xlviii. (2nd series), p. 377.

[521]Am. Jour., vol. xi. (3rd series), p. 429.

[522]Campbell,Astroph. Jour., vol. x., p. 186.

[523]Keeler,Reports on Eclipse of January 1, 1889, p. 47.

[524]Everything in such observations depends upon the proper manipulation of the slit of the spectroscope.

[525]Mem. R. A. S., vol. xli., p. 435.

[526]Comptes Rendus, t. lxvii., p. 1019.

[527]Mem. R. A. S., vol. xli., p. 43.

[528]Comptes Rendus, t. xciv., p. 1640.

[529]Young,Pop. Astr., Oct., 1897, p. 333.

[530]J. Evershed,Indian Eclipse, 1898, p. 65;Month. Not., vol. lviii., p. 298;Proc. Roy. Soc., Jan. 17, 1901.

[531]Frost,Astroph. Jour., vol. xii., p. 85; Lord,Ibid., vol. xiii., p. 149.

[532]Comptes Rendus, t. cxvii., No. 1;Jour. Brit. Astr. Ass., vol. iii., p. 532.

[533]Lockyer,Phil. Trans., vol. clvii., p. 551.

[534]The rosy envelope of prominence-matter was so named by Lockyer in 1868 (Phil. Trans., vol. clix., p. 430).

[535]According to Trouvelot (Wash. Obs., 1876, App. iii., p. 80), the subtracted matter was, at least to some extent, accumulated in the polar regions.

[536]Bull. Phil. Soc. Washington, vol. iii., p. 118.

[537]Mem. R. A. S., vol. xli., 1879.

[538]Astr. Nach., No. 1,737.

[539]Correspondence with Newton, pp. 181-184; Ranyard,Mem. Astr. Soc., vol. xli., p. 501.

[540]S. P. Langley,Wash. Obs., 1876, App. iii., p. 209;Nature, vol. lxi., p. 443.

[541]Schuster (Proc. Roy. Soc., vol. xxxv., p. 154) measured and photographed about thirty.

[542]Abney,Phil. Trans., vol. clxxv., p. 267.

[543]Proc. Roy. Soc., vol. xxxiv., p. 409. Experiments directed to the same end had been made by Dr. O. Lohse at Potsdam, 1878-80.Astr. Nach., No. 2,486.

[544]The sensitiveness of chloride of silver extends fromhto H; that is, over the upper or more refrangible half of the space in which the main part of the coronal light is concentrated.

[545]Proc. Roy. Soc., vol. xxxiv., p. 414.

[546]Report Brit. Assoc., 1883, p. 351.

[547]Maunder,Indian Eclipse, p. 125;Eclipse of 1900, p. 143.

[548]Astr. and Astrophysics, vol. xiii., p. 662.

[549]Seeinfra, p. 197.

[550]Abney,Phil. Trans., vol. clxxx., p. 119.

[551]Comptes Rendus, t. xcvii., p. 592.

[552]Memoirs National Ac. of Sciences, vol. ii., p. 102.

[553]Wash. Obs., 1867, App. ii., p. 64.

[554]The Sun, p. 357.

[555]Proc. Roy. Soc., vol. xvii., p. 289.

[556]Comptes Rendus, t. lxxiii., p. 434.

[557]Wash. Obs., 1867, App. ii., p. 195.

[558]Stokes, Anniversary Address,Nature, vol. xxxv., p. 114.

[559]Comptes Rendus, t. ci., p. 50.

[560]Harvard Annals, vol. xviii., p. 99.

[561]Wesley,Phil. Trans., vol. clxxx., p. 350.

[562]Harvard Annals, vol. xviii, p. 108.

[563]Lick Report, p. 20.

[564]Ibid., p. 14.

[565]Ibid., p. 155.

[566]Pub. Astr. Soc. of the Pacific, vol. iii., p. 158.

[567]Professor Holden concluded, with less qualification, "that so-called 'polar' rays exist at all latitudes on the sun's surface."Lick Report, p. 19.

[568]Holden,Report on Eclipse of December, 1889, p. 18; Charroppin,Pub. Astr. Soc. of the Pacific, vol. iii., p. 26.

[569]Published as the Frontispiece to theObservatory, No. 160.

[570]Wesley,Ibid., p. 107.

[571]Lick Observatory Contributions, No. 4, p. 108.

[572]Astr. and Astrophysics, vol. xiii. p. 307.

[573]Lockyer,Phil. Trans., vol. clxxxvii., p. 592.

[574]He died in London, November 20, 1898.

[575]Bull. Acad. St. Pétersbourg, t. vi., p. 253.

[576]W. H. Wesley,Phil. Trans., vol. cxc, p. 204.

[577]Lick Reports on Eclipse of January 1, 1889, p. 204.

[578]Astroph. Jour., vol. xi., p. 226.

[579]Observatory, vol. xxi., p. 157.

[580]The Indian Eclipse, 1898, p. 114.

[581]Science, June 22, 1900;Astroph. Jour., vol. xii., p. 370.

[582]Ann. der Physik, Bd. xlviii., p. 528. See also Wood,Physical Review, vol. iv., p. 191, 1896.

[583]Science, August 3, 1900.

[584]Lick Observatory Bulletin, No. 9.

[585]Observatory, vol. xxiv., pp. 321, 375.

[586]Lick Report on Eclipse of December 22, 1889, p. 47;Month. Not., vol. l., p. 372.

[587]Lick Obs. Bull., No. 9.

[588]Bull. de l'Acad. St. Pétersbourg, t. iv., p. 289.

[589]The Solar Corona discussed by Spherical Harmonics, Smithsonian Institution, 1889.

[590]Bakerian Lecture,Proc. Roy. Soc., vol. xxxix.

[591]Astr. and Astrophysics, vol. xi., p. 483.

[592]Ibid., vol. xii., p. 804.

[593]Am. Journ. of Science, vol. xi., p. 253, 1901.

[594]See Huggins,Proc. Roy. Soc., vol. xxxix., p. 108; Young,North Am. Review, February, 1885, p. 179.

[595]Professor W. A. Norton, of Yale College, appears to have been the earliest formal advocate of the Expulsion Theory of the solar surroundings, in the second (1845) and later editions of hisTreatise on Astronomy.

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