A Popular History of Astronomy During the Nineteenth Century / Fourth Edition

by Agnes M. Clerke

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 the moon's edge.[552] But the observation was assuredly misleading or misinterpreted. Atmospheric diffusion may 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 which its structure 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 a single semidiameter of 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.