Chapter XI: The Corona

The corona is exclusively an eclipse phenomenon. No sooner is totality established than it is there. It seems to have emerged from nothingness, to have arrived from nowhere. It starts into view with the abruptness, the inexplicableness, of an apparition. “The sun,” Professor Langley says, recording his impressions of the eclipse of 1869, “went out as suddenly as a blown-out gas-jet, and I became as suddenly aware that all around where it had been, there had been growing into vision a kind of ghostly radiance, composed of separate pearly beams, looking distinct each from each, as though the black circle where the sun once was bristled with pale streamers stretching far away from it in a sort of crown.”[220]

The corona presents various aspects, but it may always be described as composed of extended streamers springing from a much more intensely luminous ring, the so-called “inner corona.” There is no real separation; the entire appendage is evidently framed on the same constructive principle; yet the distinction is obvious visually, and convenient descriptively. “I do not know,” Mr. Francis Galton wrote of the corona visible 18th July 1860, “to what I can justly compare that magnificent meteor. It differed from other objects in the remarkable whiteness and purity of its light, and also in the definition of its shape as combined with a peculiar tenderness of outline.”[221] Both he and Winnecke noticed the curvature of some “long arms of light” protruding from the ring, while other rays took “a more or less tangential direction.” Mädler was struck with the _determinateness_ of the formation. What he saw was no vague light-effusion, but a congeries of sharply terminated beamy sheaves. This is a radical characteristic. The solar corona is a texture of significant pattern. There is indeed much difficulty in laying bare the original design. A spherical agglomeration projected on a plane gives rise to intricate effects of perspective, from which the true relations in solid space of the objects originating them can be deduced only by careful and systematic interpretation on strict geometrical principles. The problem was attacked by Professor Schaeberle[222] with the help of an ingeniously contrived model, photographs of which showed divergent rods inserted over the surface of a globe as apparently intercrossing and interlacing in the flat picture imprinted on the plates. It is, however, well to remember that, while curved rays may be projected so as to seem straight, straight rays can never appear curved. Beams that show flexure are inflected. Nor can a semblance of double curvature be given by perspective to those bent simply in one direction; moreover, rays that are actually normal to the sun’s surface must, from all points of view, appear radial to the limb.[223]

Coronal structure is of immense variety. It is intrinsically of a radiated character, and the fact is of primary importance. There are no signs of concentric arrangement;[224] coronal materials do not form shells or envelopes, such as surround the heads of comets; they are, on the contrary, drawn out into fibres by forces acting upon them in minute detail. Comparisons to spun glass and to silken filaments indicate the delicate nature of the shining tissue spread round the obscured sun. There are indeed differences. The fibres sometimes, as in the corona of 22nd December 1870, look to have been “combed out,” but more or less of derangement is usually prevalent. Tangled hanks of thread are often suggested, or “masses of luminous hair in disorder.”[225] These contorted forms, although their complexity is doubtless augmented by the superposition of sundry groups of twisted rays presented at different angles, afford remarkable evidence of disturbance within the corona itself. Their photographic registration dates from the Sumatra eclipse of 18th May 1901. On plates there taken with the Lick forty-foot telescope, and by Mrs. Maunder at Mauritius, a tumultuous area, agitated as if by the effects of an explosion, was strikingly depicted. “A long thread-like prominence appeared,” it was stated, “to emanate from the same source.”[226]

Fleecy coronal tracts are at times intermixed with regions of striation. The corona of 1868 was perceived at Masulipatam to be “slightly mottled” near the sun, and the “curdled” aspect of the great nebula in Orion has often been recalled to telescopic observers. Sir Norman Lockyer, at Baikul, 12th December 1871, was struck with an “exquisite” and “strongly-developed structure.” “I at once,” he continued, “exclaimed ‘Like Orion!’ Thousands of interlacing filaments varying in intensity were visible; in fact, I saw an extension of the prominence-structure in cooler material.”[227] It may be remarked that nebular tufts, no less than prominence-jets, are resolvable into fibres under the best conditions of seeing. The aureola of 1871 was of such incomparable beauty that M. Janssen could scarcely rouse himself from its delighted contemplation to carry out his programme of work. Numerous coral red prominences were relieved against the “velvet white” of the corona, the exterior shape of which was rudely quadrilateral. Interiorly the streamers leaned together in pairs so as to imitate flower petals, the general effect resembling that of a gigantic lucid dahlia, with the black moon for its heart.

Again and again, in descriptions of successive coronas, the Orion-similitude recurs. In the “density, brightness, and species of its light,” that of 1st January 1889 strongly reminded a Nevada State observer of the nebula, and its slightly greenish tinge of colour completed the likeness.[228] Again, in examining the coronal photographs of 9th August 1896, Mr. W. H. Wesley detected an area “broken up by dark channels into flocculent-looking masses, giving to it somewhat of the _curdled_ appearance of some parts of the nebula in Orion.”[229] The similarity is not merely superficial. Laborious photographic comparisons by Mr. Ranyard (assisted by Mr. Wesley) emphasised the organic analogy between the great nebula and the solar corona.[230] Synclinal forms (as the petal-shaped structures are called) emerge in both, and the branching effusions round the trapezium seem to mimic details legible in many eclipse-pictures.

A chain of “pearly cones” furrowed spirally, 200,000 to 300,000 miles in height, and rising above a long bank of red prominences, were perceived by Professor Cleveland Abbe in the corona of 1869. And in 1893 the sun appears to have been fringed in this manner nearly all round, the individual peaks being projected together into such close array as to be in many places undiscernable apart. “Systems of (approximately) concentric arches” were also distinguished by Professor Schaeberle in his large-scale photographs of the same eclipse.[231]

Inverted shapes are also, though less commonly, met with. Paraboloids, convex towards the limb, now and then replace arcs and cones based upon it. A curious instance was afforded by Schaeberle’s “coronal comet” of 16th April 1893.[232] This object seemed as if _spitted_ upon a slender, solitary, nearly radial streamer, from which it had evidently developed. It was not the only specimen of its class. A well-known drawing by Liais of the corona of 7th September 1858 shows an immense double paraboloid lying behind and partly hidden by a “dahlia petal.” The vertex seemed to be just at the limb. A somewhat similar dusky arc was seen by Winnecke in the corona of 1860.[233] It looked, he said, like a tracing in sepia. Again, during the eclipse of 1868, a bright parabolic outline, “with its vertex towards the sun,” was noted by Weiss. Finally, Homer Lane observed at Des Moines, Iowa, 7th August 1869, two condensations of light which “might well be compared to small telescopic comets, with tails of some length, but without a head, and with no distinct indication of a head at one end rather than the other. They were not far from radial in direction relatively to the sun’s centre, and had their origin above the limb of the moon.”[234] He estimated the length of each at about 130″. These appearances are full of meaning. They plainly assert the subjection of coronal matter to a dual repulsion, such as acts upon the “crystal tresses” of comets. A local centre of condensation throws off a filmy envelope, the constituent particles of which, as they approach the sun, are swept backward into a train by a counter influence proceeding from him. The only genuine “eclipse comet” so far captured was that seen and photographed at Sohag, 17th May 1882. It was sharply characterised as such, the effects of swift motion being unmistakably impressed upon its curved plumage.

An eclipse, visible in the Western States of North America, 29th July 1878, disclosed a surprising spectacle. In lieu of the ordinary radiated corona there were seen “bristles” of light at the sun’s poles, enormous “wings” at each side of the equator. Professor Langley observed the phenomenon from the summit of Pike’s Peak in Colorado, at an elevation of 14,000 feet in a stainless sky. Thus favourably circumstanced, he was able to trace one wide beam to a distance of about five millions of miles from the sun, the other fully twice as far.[235] The direction in which they lay proved, when carefully measured, to agree closely with that of the zodiacal light, and “a faint central rib” emphasised the coincidence. “With the telescope,” he says, “the whole of the bright inner light close to the sun was found to be made up of filaments, more definite even than those seen in sun-spots,” and apparently exempt from the effects of spherical projection; they “fringed the sun’s edge in definite outline, as though it were really but a disc.”

At the time of this eclipse, the sun was in a state of exceptional tranquillity, and a search through the solar archives brought out the notable fact that a similar apparition had, eleven years previously, spots then too being nearly extinct, been described and depicted by Grosch of Santiago. He inferred from it the possession by the sun of “strong magnetic polarity.” And indeed the divergent light-fibres at the poles, in 1867 no less than in 1878, seemed to trace precisely the lines of force in a magnetic field. The concurrence of these phenomena with critical epochs in the sun’s activity started the idea, due, in the first instance, to Mr. Ranyard,[236] of varying coronal types. It was amply borne out by subsequent experience. From eclipse to eclipse, throughout the eleven-year cycle, the corona exhibits changes of form in marked conformity to spot-vicissitudes. In the accompanying plate, the original of which is by M. Hansky, the coronas of 1860, 1870, 1883, and 1893, all of pronounced maximum type, are represented in the first column; those of 1867, 1878, and 1889 in the third. The second and fourth show coronas of intermediate forms. The last figure in the third column has a prophetic character. It shows M. Hansky’s anticipation of the kind of halo due, on the theory of recurring types, in 1900. What was actually photographed is given for comparison in Plate VI., Fig. 1. The correspondence leaps to the eye. A definite law of variation indeed quite obviously regulates the shape of the effluence about the sun. At spot-maxima its component streamers issue from all latitudes indiscriminately; they pay little or no regard to heliographic co-ordinates. Then, as disturbance relaxes, they gradually draw away from the poles, and tend to form “synclinals” above the spot-zones, giving to the whole appendage the “form of a four-rayed star, the points of which are inclined 45° to the sun’s axis.”[237] In the polar regions abandoned by them, “magnetic” filaments meanwhile become noticeable, and what may be called the intermediate type is constituted. It is subject, however, to indefinite variations of detail. A good example is shown in Plate VII., Fig. 1, from photographs taken on the Amur, 9th August 1896, by a Russian party under the leadership of M. Bélopolsky.

One ray, it will be noticed, is greatly longer than the others, and the same peculiarity distinguished the corona of 1898. Only when the tide of solar agitation is dead out, is the finished type of minimum aureola realised. We have then a symmetrical arrangement of crested poles and equatorial extensions, with this one singular qualification to its symmetry, that the wings are not a pair. One is formed of convergent, the other of parallel, or even divergent rays. And they seem to be reversed east and west at alternate epochs. Both are radically double; they are formed by the closing down upon the equator, as spot-activity becomes exhausted, of the synclinal groups previously visible in middle latitudes. It is difficult to realise that these “wings” are merely the profile-shapes of a vast luminous disc completely encompassing the sun. Hence an extreme intricacy of structural details most baffling to efforts towards interpretation.

PLATE V.

Types of the Corona 1860–96, with an Anticipatory Sketch of the Corona
of 1900. Drawn by M. Hansky.

(From the _Observatory_, February 1898.)
]

Coronal modifications are not so entirely isolated as might at first sight appear. Looked at more closely, they are perceived to correspond unmistakably with the cyclical changes in distribution of surface disturbances. This was insisted upon by M. Bélopolsky in 1897.[238] Spots descend into lower latitudes with the approach of each minimum. One after the other, the eleven-year waves of commotion attain their acme in medium zones, and die out near the equator. Coronal development pursues the same course. Its most intimate relations, however, are with chromospheric eruptions. Tacchini[239] was the first to notice that coronal outflows emanate from regions frequented by prominences, which at times of maximum spread all over the sun, but near minimum withdraw from the extensive polar tracts simultaneously denuded of far-spreading streamers. Particular agreement frequently accentuates this general correspondence. Thus the springing of a coronal arch has usually a prominence for its motive. Each pearly pavilion is erected over a red flame. Coincidences of the kind are of perpetual occurrence. Chromospheric jets seemed (and doubtless were) appropriated individually to the “striated cones” observed by Cleveland Abbe in 1869. In Schaeberle’s fine photographs of the eclipse visible in South America 16th April 1893, one-sixth of the sun’s circumference came out clear of prominences; and just over the same segment there is a gap in the elsewhere unbroken range of coronal arches.[240] In some cases arches are buttressed upon prominences; in others they are symmetrical as regards them; coronal streamers appear to be vaulted into domes, or bent together into ogives, through effects of eruptive action in the chromosphere. These relations were especially marked in the corona of 1896. The wide polar rifts were devoid of prominences, but a prominence lay at the root of each great streamer, and a prominence was enclosed by each synclinal structure. These M. Hansky inferred, from their interior darkness, to be hollow,[241] like the tails of certain comets; and he noticed curious effects of coronal transparency, a few beams showing traceably through the substance of those in front of them. The correlation of prominences with coronal forms was somewhat less conspicuous in 1898 than in 1896, and was barely perceptible in 1900. The Lick photographs of 1901, however, showed the envelopment of a prominence by a “series of coronal hoods,” besides other symptoms of community in disturbance between the chromosphere and the silvery aureola.

The more closely the spectrum of the corona is studied, the more interesting and enigmatical it becomes. It has a triple origin. Continuous reflected light is mixed up in it with continuous original light, and these again with bright-line emissions. The three elements are not easily separated and the proportions of them present vary from time to time. The gaseous spectrum is feeble, especially near spot minima. Its leading constituent is a green line at λ 5303, long confounded with the chromospheric λ 5317. Their disconnection first became apparent in Mr. Fowler’s eclipse photographs of 22nd January 1898, and was announced by Sir Norman Lockyer[242] as one result of their examination. The green line characterises the unknown substance designated “coronium,” the distribution of which round the sun can be investigated by its means. Photographs taken during totalities with the “prismatic camera” give separate images of the solar appendages in each quality of discontinuous light emitted by them, and the “green” coronal image proves to be approximately ring-shaped. The gas it is derived from seems to spread through the “inner corona” to an average height of from 160,000 to 200,000 miles, but not to extend into the sheaves and streamers beyond. There are indeed irregularities. The annulus of coronium is far from being uniform or homogeneous. It is wider, perhaps also more condensed, in some places than in others, and spectrograms taken with a slit by Professor Campbell at Jeur suggested, by the distortions of the characteristic ray impressed upon them, the progress of radial movements, such as might well be deemed inevitable in an aerial envelope obviously not in a state of equilibrium.[243]

Ten or a dozen coronal lines besides the green ray have been photographed. The wave-lengths of the most authentically recorded are as follows: 4232, 4086, 3987, 3801, 3643, 3456, 3388, and 3381. No success has hitherto attended efforts to arrange them in a series; nor is it by any means certain that all claim the same chemical origin. On the contrary, the fundamental green line, together with the most refrangible of those above enumerated, appears capable of segregation from the violet ray λ 3987 and the first three of its ultra-visible associates.[244] Two strange gases then, at any rate, are indicated as co-existing in the corona. And they are unmixed with any familiar substance. Evidence collected during recent eclipses testifies strongly to the absence of all the chromospheric materials. Not even the pervasive trio, hydrogen, helium, and calcium, extend into the vast solar halo. Some of their radiations, notably H and K, have _seemed_ to be derived from it, but only through the effects of atmospheric scattering. They come as well from the black disc of the moon. Probably only some peculiar forms of matter, or forms of matter in a peculiar state, constitute the gaseous corona. So far coronium has not been recognised elsewhere.

The continuous light from the interior halo is mainly original. It emanates from incandescent solid or liquid particles. But their incandescence appears to be of an unusual kind. Bolometric experiments, carried out by Messrs. Abbot and Mendenhall of the Smithsonian Observatory during the eclipse of 1900,[245] showed the coronal beams to be almost wholly wanting in thermal power. Compared with them moonlight is a potent source of heat. They include, according to the results in question, next to no infra-red waves, and are surmised to be comparable in quality to the glow of phosphorescent or luminescent substances. Novel inquiries in the laboratory will be needed to ratify these significant conjectures; while it is to be hoped that the eclipse of 1904 will afford some positive data as to the distribution of energy in the coronal spectrum, which may serve as a basis both for photographic investigations and for theoretical conclusions.

Photospheric light _must_ be reflected from the pulverulent materials of the corona, and a reflected ingredient is, accordingly, contained in its radiance. It is, however, small in quantity. Thus the dark Fraunhofer lines are barely distinguishable in it. They were detected by Janssen in 1871; fifteen were photographed by Pluvinel at Senegal in 1893.[246] On the same occasion, nevertheless, Deslandres could obtain no trace of them, and Campbell was equally unsuccessful in 1898.[247] The truth is that where the original emissions are strong they get _drowned out_. They show, and that with difficulty, only in the less luminous sections of the appendage. This was made perfectly obvious by Mr. Perrine’s discussion of the plates taken in Sumatra, upon which thirty-five Fraunhofer-lines impressed themselves, but only in regions remote from the limb. Polarisation-effects give accordant testimony.[248] They are slight but unmistakable, and plainly indicate the action of scattering particles right up to the limb. Some hints as to the manner of their distribution were obtained by Professor Turner in 1898, and again in 1900, through the ingenious device of photographing the corona across a plate of Iceland spar, and so obtaining two pictures in oppositely polarised light.[249]

Much remains to be learned about the nature of coronal radiance, and the opportunities for its investigation are restricted and unsatisfactory. Yet upon their outcome success in coronal portraiture largely depends. Eclipse-photography is an art in itself, and one beset by subtle difficulties. To ensure the best results, the plates employed should have a curve of sensitiveness as nearly as possible coincident with the energy-curve of the coronal radiations, and the form of the latter is entirely unknown. Many questions too arise regarding the quality of the plates, and the development proper to be given them, regarding the best kind of instrument for exposing them with, and, above all, the length of time that should be allotted to the process. And here the obstacle is encountered that no single exposure is suitable to the entire aureola. One long enough to bring out the streamers is too long for the delicate details of the brilliant interior. The choice has to be made between solarisation and incompleteness of representation. Two remedies have been tried. The first is by piecemeal delineation. From photographs of varied exposures, a picture showing the special features rendered by each is compounded by a skilled draughtsman. But it has no longer an autographic value; the forms embodied in it have been deliberately selected and unconsciously emphasised. “To a certain extent,” it has been well said,[250] “the same personality enters into the examination of a photograph that is known to exist in naked-eye observations of the corona.” M. Morin’s drawing of the corona of 1896 (see Plate VII. Fig. 1) is a fine example of what can be done by combining multiplied photographic impressions. The two best of those availed of were taken with an ordinary camera in one and two seconds respectively; the rest with a photoheliograph, getting exposures up to thirty seconds.[251]

The alternative method is purely automatic. It was invented and successfully applied by Professor C. Burckhalter of the Chabot Observatory, California, at the eclipse of 28th May 1900.[252] In his apparatus a system of revolving diaphragms is arranged so as to give exposures graduated to correspond with distance from the sun. The image being progressively covered at successive short intervals, time is allowed for the imprinting of faint coronal extensions, while the bright parts already effectively portrayed are shielded against further prejudicial action. The photograph “controlled” in this way was a striking record, and conveyed some curious intimations of perspective effects in the mutual overlaying of tufts and beams.

The outlying branches of the corona usually baffle the perception of the sensitive plate; for they merge into a dimly illuminated sky-ground, from which they are, unless by special precautions, chemically indistinguishable. The most conspicuous performance in this direction was by Mrs. Maunder in 1898.[253] With a lens of only 1½ inches in aperture, giving small but intensely bright images, exposures were experimentally made of inordinate length, proportionately to the shortness of the focus. The unprecedented result was achieved of photographing rays to a distance of nearly seven solar diameters from the limb. Mr. Wesley’s drawing from the original negatives is reproduced in Plate VII. Fig. 2. Four long narrow divergent rays are visible in it, each starting outward from a synclinal structure. Mr. and Mrs. Maunder hold it probable that such structures have always rod-like extensions, needing only protracted photographic exposures to bring them into view;[254] but this is more than doubtful.

FIG. 15.—Diagram of Markings in the Corona of 1871. Drawn by W. H.
Wesley (_Knowledge_, vol. xxiii. p. 225).
]

FIG. 16.—Diagram of Markings in the Corona of 1896. Drawn by W. H.
Wesley (_Knowledge_, vol. xxiii. p. 226).
]

A very perplexing appearance is that of dark markings in the corona. They are not mere interspaces between brilliant rays. Mr. Wesley, who is an expert in the scrutiny and interpretation of celestial photographs, vouches for their reality. Figs. 15 and 16 copy his diagrams of obscure streaks and veinings in the coronas of 1871 and 1896. In the first case, they cut right across the lustrous branches of the halo; in the second, they are in obvious connection with prominences. Indeed, black coronal and black chromospheric forms belong undeniably to the same order of effect, and cannot be separated causally. They took another shape in the corona of 1900. Mr. Wesley’s beautiful drawing from Mr. Maunder’s photographs (see Plate VIII.) shows rifts apparently darker than the general background of the sky, and hence of a _positive_ character. Their substantial presence was confirmed[255] by negatives taken at Wadesborough, U.S.A., by Miss Gertrude Bacon; but the difficulty of accounting for them is at present insurmountable. That they are due to the interposition of opaque bodies can scarcely be admitted. The objections are prohibitive. Yet the phenomenon is none the less genuine for being incomprehensible. We must wait and compare.

PLATE VII.

1. The Corona of 1896. Drawn by M. Morin from Photographs taken on the
Amur.

2. The Corona of 1898. Drawn by W. H. Wesley from Photographs taken by
Mrs. Maunder. (_Knowledge_, vol. xxi. p. 108.)

_N.B._—The sun’s axis meant to be indicated is a _vertical_ line
bisecting the disc, with which the north-and-south line makes a
small angle.
]

The embarrassments attending coronal photography are enormously enhanced by the effulgence of daylight. Success here is more earnestly desired the less it can be hopefully anticipated; for the prospect is dim of realising Sir William Huggins’s scheme of 1882, or any modification of it. It is true that, during the partial phases of the last couple of eclipses, sensitive plates were impressed by the inner corona, but it only showed as a vague glow throwing into relief the small segment of the moon outside the sun some forty seconds before and after totality.[256] Still, even this scanty measure of success was welcome as an earnest of what the incalculable future might bring. Everything depends upon catching differential effects—upon obtaining plates capable of _feeling_ the delicate gradation between daylight pure and simple and daylight plus corona. And this would be greatly facilitated by acquaintance with the law of intensity in the coronal spectrum. It ought to be stronger in the upper reaches than the ordinary solar spectrum, since the corona escapes the heavy toll of blue absorption exacted from the photosphere by the “smoke-veil”; the question is, can this presumable superiority be rendered predominant enough for the ends of portraiture? The use of coloured screens, letting through the more refrangible rays, and barring out those lower ones in which mere glare has the advantage, has proved ineffectual; and the “double-slit method,” so splendidly helpful in other departments of solar physics, has also been tried in vain.[257] In coronal photography, no bright line can serve the end in view because the gaseous spectrum belongs only to the inner corona, and the record aspired after is more especially of the outer corona, with its characteristic plumes, streamers, and aigrettes. Until the changes these undergo can be followed day by day, little will be satisfactorily known of their intimate relations with the different orders of solar phenomena, and still less of the underlying cause of variation.

It has yet to be determined whether the corona rotates with the sun. Opposite motion-displacements above the east and west limbs, of the green line or one of its companions, would settle the point; but they show, if at all, most evasively. M. Deslandres first attempted such measurements at Fundium in West Africa, 16th April 1893.[258] He chose, however, as the object of his attack the K-line of calcium, since ascertained to be non-coronal, so that his results were null and void. Yet they marked a starting-point, for they sufficed to introduce the research definitively into the eclipse-programme; nor will it be dropped out of it, we may hope, until a substantial increase of knowledge has been gained. Subsequent experiments, although legitimately conducted, have been indecisive. Mr. Newall, in those tried by him at Pulgaon, 22nd January 1898, went too far afield for their materials.[259] He directed his spectroscope to points 8′—corresponding to upwards of 200,000 miles—from each limb, whence no bright line could be obtained, since they lay outside the limits of the gaseous corona. Professor Campbell,[260] on the other hand, found in spectrograms taken east and west of the sun during the same eclipse, a difference in position of the green ray giving an ostensible radial velocity of 3·1 kilometers per second, suggesting rotation at half that speed; but he regarded its genuineness as open to grave doubt.

PLATE VIII.

The Corona of 1900. Drawn by W. H. Wesley from Photographs taken by E.
W. Maunder. (_Knowledge_, vol. xxiii. p. 227.)
]

More hopeful than the method of simple displacements is perhaps the method of _inclinations_ recommended by Deslandres.[261] A small, intensely luminous image of the corona being thrown upon the slit of a powerful spectroscope, the varied deviations of the bright lines derived from its different parts should tell something as to the mode of motion prevalent throughout the appendage. If it rotates like a solid body, all in one piece, the velocity increases outward, and the lines would lie aslant on the plate in corresponding directions. If, however, the regimen be analogous to that governing Saturn’s ring-system, in which every component particle revolves as an independent satellite, then the speed of coronal matter slackens with increase of distance from the sun, and the spectral rays emitted by it should be deflected the opposite way. The criterion, if it prove applicable, will be highly discriminative as regards rival hypotheses.

These may be classified as ejective, meteoric, and electro-magnetic. Professor Schaeberle analyses the phenomenon into “streams of matter ejected from the lower latitudes of the sun.”[262] To the materials of the longest rays he ascribes initial velocities up to 400 miles a second, and excursions outward to the remote neighbourhood of Jupiter or even Saturn; ogives and arches being due to eruptions of less violence; while the interplay of innumerable curving jets, foreshortened in all possible ways, explains the complex aspect of the sun’s lucent crown. Mr. and Mrs. Maunder, again, consider the prominences to “represent centres of strong eruptive action, and that in consequence of such action coronal matter is driven upward from the sun over a very wide area in domelike forms.”[263] A succession of arches results, “the outer being less definite and complete than the inner ones. Outside all we find the curves defining the boundaries of the synclinal group.”

There is much plausibility in these inferences. Prominences are visibly spouted or flung upward, and the lustrous filagree vaults often rising above them can scarcely have a totally dissimilar origin. But the whole secret is not thus laid bare. The rationale by eruptive action is no sooner sought to be made exclusive than it meets contradictory facts. It offers no admissible explanation of varying coronal types; it ignores the mysterious coronium envelope; it seems to be negatived by the bolometric observations of Abbot in 1900; for obviously the light of the supposed bombs, projectiles, or pulverulent ejected streams should contain the ordinary solar proportion of heat-rays, which are nevertheless markedly deficient.

The corona, according to Sir William Huggins, must consist of “incandescent fog.” And Professor Newcomb,[264] following a similar train of ideas, avers it to be made up of detached particles, wholly or imperfectly vaporised. They might be most sparsely distributed. Intense radiance would, he informs his readers, result from the occurrence of a single fragment of dust in every cubic mile of space about the sun. But how is the dust (if dust there be) supplied? Does it come from within or from without? Here the upholders of the meteoric theory join issue with the eruptionists. There must be rings and streams of meteors revolving quite close to the sun in orbits of all possible inclinations and considerably varied eccentricities. These furnish, we are told, the materials of the corona, which—as Dr. Scheiner has recently shown[265]—are raised to a temperature of incandescence by direct solar radiation. This view, nevertheless, like the eruption-hypothesis, is scarcely tenable in view of the non-thermal quality of coronal light.

There remains the electrical theory. Formally enounced by Sir William Huggins in 1885,[266] it remains unverified indeed, yet unrefuted. Coronal streamers are regarded by it as analogous to comets’ tails; they issue forth under the influence of a repulsive force emanating from the sun; they are illuminated by electrical discharges due probably to differences of potential at their bases and extremities.[267] The magnetic relations of the phenomenon, vividly indicated by the minimum forms exemplified at the two eclipses of 1889, were ably discussed by Professor Bigelow.[268] He analysed coronal structure by spherical harmonics, on the supposition of its dependence upon some mode of action similar to that of free electricity, “the rays being lines of force, and the coronal matter being discharged from the body of the sun, or arranged and controlled” by a power proceeding from it. He further showed that the power was of a repulsive nature and varied inversely as the square of the distance;[269] but he avoided speaking of it as “electrical” out of “deference to the doubt that free electricity can exist at such high temperatures as prevail on the sun’s surface,” content to have proved “that some force is present acting on the corona according to the laws of electric potential.”

The diagrammatic halo laid down on these principles was indeed the very “twin Dromio” of the corona of 1889; but they stood ill the test of prediction. The corona of 1893 failed to exhibit the special features anticipated for it by Professor Bigelow. Plainly the assertion that an illuminated magnetic field surrounds the sun, although scarcely deniable, does not comprise the whole truth. The same may be said of Hermann Ebert’s “electro-magnetic theory.”[270] He defines the corona as “the visible reaction of the finely-divided matter in the vicinity of the sun upon the dielectric polarisation proceeding from the different parts of the sun.” Luminosity is evoked by Hertzian oscillations propagated outward with the velocity of light, and its filamentous texture corresponds to differences in dielectric stress connected with the distribution of electricity on the sun’s surface.

The auroral aspect of the corona has often been commented upon. M. Ebert remarks[271] that the magnetic lines of force near the earth are not more definitely traced out by the play of polar lights than are those about the sun by the disposition of coronal rays. The truth of this, however, is patent only as regards aureolas of the minimum type. In those visible at maximum it is at any rate disguised. For, as the sun’s internal activity augments, beamy outflows predominate over tufted effluences, although both kinds of radiance may be simultaneously present. Professor Holden considered the sun to be _hairy_ all over, and not merely at the poles. “There is no latitude,” he wrote, “at which we can say that here the polar rays end and a new species—equatorial rays—begins.”[272] This was also recognised by M. Hansky in his study of the corona of 1896;[273] but the superposed streamers and arches can with difficulty be included in any magnetic theory. On the other hand, Mr. Pupin of Columbia College obtained in 1892 striking imitations of the maximum type of corona by means of electrical discharges through partially exhausted bulbs.[274] Effects of polarity being, however, wholly absent, the reproduction failed to convey one fundamental characteristic of the real phenomenon. But the deficiency was supplied, three years later, by Ebert’s experiment of subjecting “coronoidal” tubes to the action of a powerful magnetic field. The organising effect upon the light-effluence was just what was needed, according to Bigelow’s contention, to give nature’s own imprimatur to his “magnetic theory of the solar corona.”[275]

A fact of high import in this connection is that the coronal bright lines are not reversed in the Fraunhofer spectrum. Coronium, and the gases associated with it, exercise no perceptible absorption upon the light transmitted through them. Now this kind of inertness, according to M. Cantor’s experiments,[276] is distinctive of substances glowing by electrical stimulation, so that we have here—as the late Professor Fitzgerald pointed out—a confirmation, absolutely _sui generis_, of the conjecture that coronal emissions are analogous to those of an illuminated vacuum-tube.

Yet none of the views propounded on the subject are completely satisfactory. They have points of contact with truth, but they do not closely embrace it. This, indeed, could hardly be expected at so comparatively early a stage of coronal research. For the questions involved are beyond measure baffling and intricate. We may re-enumerate them.

One that is fundamental relates to coronal heat. Its virtual absence, attested bolometrically at Wadesborough in 1900, must be confirmed during future eclipses before Deslandres’s contrary inference[277] as to the copious presence of long waves in coronal light can be finally dismissed.

The coronium-envelope offers a problem which stands almost apart from that of the stellate appendage surrounding it. And it is one that can scarcely yet be grappled with. No familiar substance enters into its composition. Its ingredients are altogether exotic. They do not diffuse into the chromosphere, while those of the chromosphere are as strictly excluded from the corona. The cause of this extraordinary circumstance will perhaps long remain obscure. Meanwhile, through the attempted apportionment of the coronal rays photographed in totalities between sundry hypothetical substances we are led to regard coronium as only one of a group of gases foreign to terrestrial experience.

The rotation of the corona can be measured only by the most refined methods; but there is little doubt that they will be successfully applied. Movements of other kinds may also be spectroscopically determined, since they are likely, in many cases, to take directions oblique to the limb, and therefore to have large components along the line of sight to the earth. Notwithstanding the lasting, and, in some respects, the growing importance of securing the best possible picture-photographs of the corona during totalities, novel revelations are scarcely to be expected from them. Their leading interest just now centres, first in the structural relationship of coronal arches with prominences, next, in the information they may afford about dark markings in the solar appendages. For the rest, we must look to daylight photography. When the great desideratum is attained of getting behind (as it were) the veil of atmospheric glare, we shall be able to trace the progress of coronal change, to follow the unbuilding and rebuilding of the typical aureolas, to witness, perhaps, sudden coronal developments in sympathy with chromospheric outbursts. From the vantage-ground thus gained, in short, the true function of the corona in the solar economy can be systematically investigated. During the few crowded moments of eclipse this is not possible.