Imagens das páginas
PDF
ePub

267

WHAT, THEN, IS THE CORONA ?

It is not easy for a thoughtful mind to study the evidence bearing on any scientific subject without being led to theorise. Even though the evidence be imperfect, even though—however carefully sifted and analysed—it still leave the problem indeterminate, the mind will yet weigh fact against fact, and probability against probability, adopting then, though but provisionally, the theory which seems best to accord with such facts as have been revealed. As fresh facts are ascertained, the theory may have to be modified or even abandoned ; and often one theory after another may thus be adopted for a while and presently rejected : yet it is only by thus theorising-boldly, but with due deference to facts—that the truth can finally be established. There is no recorded instance—so far as I know-of any difficult problem in science which has been mastered otherwise than by resolute and industrious theorising based on the careful study of all the observed facts bearing upon the subject matter. So Copernicus was enabled to place the sun at the centre of the planetary scheme; so Kepler assigned to the planets the laws according to which they move; so Newton was able to discover the mainspring of the universe. No otherwise, again, did Römer learn how to measure the velocity of light, or Bradley find a

meaning in the aberration-ovals traced out by all the stars upon the heavens. These men, and a hundred others whose names stand highest in the records of science, were theorists; some of them mere theorists; and Newton, the greatest of them all, was (so far as astronomy is concerned) so completely the theorist that not a single astronomical observation of his was worth the paper on which it was recorded.

Therefore I do not think that the fear of being called theorisers by the unthinking should deter us from an attempt to found upon the evidence already obtained respecting the corona such conclusions as that evidence may seem fairly to support. So far as I am myself concerned I am the readier to do this, because I think I shall have to modify very importantly certain opinions in which I had but lately some confidence. I do not indeed find that any theories I had urged as in effect demonstrated are otherwise than strengthened by the evidence lately obtained; but some opinions which had appeared probable to me some time since seem open now to grave objection.

1 There is a sentence in the introductory pages of the Astronomer Royal's admirable ‘Lectures on Astronomy' which reads strangely in connection with the known facts of Newton's life: I mean that sentence in which he divides those who merely take interest in the science of astronomy from persons who are officially attached to observatories, or in other ways professionally cognisant of the technicalities of practical astronomy. How shall Newton, thus judged, retain his place as an astronomer, or rather the greatest of astronomers ? Where are the transits he took ? the star-catalogues he formed ? the physical features he detected in sun, or moon, or planet ? the double stars he divided or measured ? In all that some in our day call astronomical work he did absolutely nothing. Where others worked he only thought; and thus all that he could do was to create modern astronomy.

Much of the evidence on the corona is presented in the preceding paper on that subject; but some facts which only reached me after that paper was written, require to be briefly noticed. For a full account of the scientific details, together with pictures of the corona as photographed, &c. I would refer the reader to the second edition of my • Treatise on the Sun.'

The reader of the preceding paper on the corona will gather that I look on the evidence recently obtained, which proves the coronal radiations to belong to a real solar appendage, as in effect but a demonstration of the demonstrated. No one who had studied the immense mass of evidence acquired during the last two centuries on this point could feel any doubt as to the real existence of these radiations in some amazing solar appendage.

I was prepared therefore to learn that the corona as seen and photographed in Sicily corresponded in all essential respects with the corona as seen and photographed in Spain. This correspondence exists beyond all possibility of question; but when the best records are studied, and when the photographs are carefully examined, something more is revealed which, whatever its interpretation, is undoubtedly full of meaning. Where any great gap or rift appears in the outer or radiated part of the corona, there a depression is seen in the inner and much brighter portion ; and yet, again, where this inner portion is thus depressed, there the coloured prominences are wanting, and the sierra is shallow. As to the former point I shall merely remark

that the peculiarity is very markedly shown in Lieut. Brown's drawing of the corona as seen in Spain; that he referred to it as a fact he had specially noticed; and that both in the Spanish and Sicilian photographs it is most strikingly manifested. As to the latter, I shall quote Professor Roscoe's words respecting Mr. Seabroke's maps of the prominences and Professor Watson's drawing of the corona: On comparing the two drawings thus independently made, a most interesting series of coincidences presented themselves. Wherever on the solar disc a large group of prominences was seen in Mr. Seabroke's map, there a corresponding bulging out of the corona was chronicled on Professor Watson's drawing; and at the positions where no prominences presented themselves, there the bright portions of the corona extended to the smallest distances from the sun's limb.'1 But I must add one piece of evidence directly associating the most distant portion of the corona with the region richest in solar prominences. Mr. Brothers's photographs all show the corona extending much farther towards the west than towards the east. There can be no question,' he writes,' that there was more coronal light on the west side of the moon than at the other points;' and then he calls attention to the fact that the prominences are more numerous on the side where the corona is brightest.

Now here is a fact of the utmost significance—so

It is perhaps necessary to point out that Mr. Seabroke's drawing was not made in the hurry of the eclipse, but (by Dr. Huggins's method) before the eclipse began.

significant, indeed, that it will be well to inquire whether it is in any way supported by the evidence obtained during former total eclipses of the sun.

Fortunately, it is not difficult to find corroborative evidence of the most satisfactory kind. We have only to turn to the account of the corona as seen during the American eclipse of 1869, and to compare the drawings with the photographs, to see that then also this feature was presented. The peculiar trapezoidal figure of the corona as seen on that occasion is most clearly indicated in the much smaller corona shown in the best photographs then taken. And, indeed, one piece of evidence then obtained goes somewhat beyond any that can be deduced from the observations made last December. For at a station where the observers were raised more than 5,500 feet above the sea-level, the quadrangular figure of the corona was seen to be extended into four radial streamers, reaching to a distance equal to three times the moon's apparent diameter. It may be added that this four

II feel compelled to set aside the evidence of Dr. Gould. He saw moving streamers not agreeing with the inner quadrangular radiance : but he alone gives such an account; and surely it would be absurd to reject the numerous accounts pointing to identity and fixedness on the score of one easily explained account of a different kind. Nothing is more natural than that at some station or other atmospheric effects should be mistaken for the real coronal radiations. But to reject on this account the narratives of witnesses describing close resemblance is surely unwarranted. It is as though, after twenty witnesses had stated that a person dressed in a particular way had passed along a certain road, their evidence should be regarded as not relating to one and the same person because one witness had seen a differently-dressed person traverse the same road.

« AnteriorContinuar »