Rough Ways Made Smooth. Richard Anthony Proctor
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would correspond to the colour it would have if it consisted in the main of glowing hydrogen. If that really was its constitution, then the theory advanced by one observer of the last eclipse, that at the time of minimum solar disturbance the glowing hydrogen is withdrawn from the corona, would be shown to be incorrect. For 1733 was the actual year of minimum solar disturbance. The pastor of Smoland states that 'during the total obscuration the edge of the moon's disc resembled gilded brass, and the faint ring round it emitted rays in an upward as well as in a downward direction, similar to those seen beneath the sun when a shower of rain is impending.' The mathematical lecturer of the Academy of Charles-stadt, M. Edstrom, observed these rays with special attention: he says that 'they plainly maintained the same position, until they vanished along with the ring upon the re-appearance of the sun.' On the other hand, at Lincopia no rays were seen. On the whole it seems clear from the accounts of this eclipse that the inner corona was bright and narrow; rays issued from the outer faint ring; but they were very delicate phenomena, easily concealed by atmospheric haze, and thus were not everywhere observed. As rays were seen in July 1878, there is nothing in the evidence afforded by the eclipse of 1733, occurring at a time of few spots, which opposes itself definitely to the theory we are considering. But the reddish colour of the corona as already noticed is a doubtful feature: in July, 1878, the bright inner corona was of a pearl colour and lustre.
During the eclipse of February, 1766, the corona presented four luminous expansions, and seems to have presented a greater expansion than we should expect in a year of minimum solar disturbance. Such, however, the year 1766 certainly was. The evidence in this case is unfavourable to our theory—not quite decisively so, but strongly. For we should expect that in the year of actual minimum disturbance the corona would be even narrower than in the year 1878, which was the year following that of least disturbance. And again, a strongly distinctive feature in the corona of July, 1878, was the absence of wide expansions, such as were seen in 1870 and 1871. Now if this peculiarity should really be attributed to the relation existing between the corona and the sun-spots, we should infer that in 1766 the corona would have been still more markedly uniform in shape. The existence of four well marked expansions on that occasion forces us to assume that either the relation referred to has no real existence, or else that the corona may change from week to week as the condition of the sun's surface changes, and that in February, 1766, the sun was temporarily disturbed, though the year, as a whole, was one of minimum disturbance. But as the epoch of actual minimum was the middle of 1766, February 1766 should have been a time of very slight disturbance. I do not know of any observations of the sun recorded for the month of February, 1766. On the whole, the eclipse of 1766 must be regarded as throwing grave doubt on the relation assumed by our theory as existing between the corona and the sun-spots; and as tending to suggest that some wider law must be in question than the one we have been considering—if any association really exists.
The account given by Don Antonio d'Ulloa of the appearance presented by the corona during the total eclipse of 1778, is rendered doubtful by his reference to an apparent rotatory motion of the normal rays. He says that about five or six seconds after totality had begun, a brilliant luminous ring was seen around the dark body of the moon. The ring became brighter as the middle of totality approached. 'About the middle of the eclipse, the breadth of the ring was equal to about a sixth of the moon's diameter. There seemed to issue from it a great number of rays of unequal length, which could be discerned to a distance equal to the moon's diameter.' Then comes the part of d'Ulloa's description which seems difficult to accept. He says that the corona 'seemed to be endued with a rapid rotatory motion, which caused it to resemble a firework turning round its centre.' The colour of the light, he proceeds, 'was not uniform throughout the whole breadth of the ring. Towards the margin of the moon's disc it appeared of a reddish hue; then it changed to a pale yellow, and from the middle to the outer border the yellow gradually became fainter, until at length it seemed almost quite white.' Setting aside the rays and their rotation, d'Ulloa's account of the inner corona may be accepted as satisfactory. The height of this ring was, it seems, about 140,000 miles, or twice that of the ring seen in July 1878. As the year 1779 was one of maximum solar disturbance, there were doubtless many spots in 1778; and the aspect of the corona accorded well with the theory that the corona expands as the number of sun-spots increases.
We come now to three eclipses which are especially interesting as having been all carefully observed, some observers having seen all three—the eclipses, namely, of 1842, 1851, and 1860. Unfortunately the eclipses of 1842 and 1851 occurred when the sun-spots were neither at their greatest nor at their least degree of frequency. For a maximum of sun-spots occurred in 1837, and a minimum in 1844, so that 1842 was on what may be called the descending slope of a sun-spot wave, nearer the hollow than the crest, but not very near either: again, a maximum occurred in 1848, and a minimum in 1856, so that 1851 was also on the descending slope of a sun-spot wave, rather nearer the crest than the hollow, but one may fairly say about midway between them. Still it is essential in an inquiry of this sort to consider intermediate cases. We must not only apply the comparentia ad intellectum instantiarum convenientium, but also the comparentia instantiarum secundum magis ac minus. If the existence of great solar disturbances causes the corona to be greatly enlarged, as compared with the corona seen when the sun shows no spots, we should expect to find the corona moderately enlarged only when the sun shows a considerable but not the maximum number of spots. And again, it is conceivable that we may find some noteworthy difference between the aspect of the corona when sun-spots are diminishing in number, and its aspect when they are increasing. This point seems the more to need investigation when we note that the evidence derived from eclipses occurring near the time either of maximum or of minimum solar disturbance has not been altogether satisfactory. It may be that we may find an explanation of the discrepancies we have recognised, in some distinction between the state of the corona when spots are increasing and when they are diminishing in number.
It is noteworthy that several careful observers of the corona in 1842 believed that they could recognise motion in the coronal rays. Francis Baily compared the appearance of the corona to the flickering light of a gas illumination. O. Struve also was much struck by the appearance of violent agitation in the light of the ring. It seems probable that the appearance was due to movements in that part of our atmosphere through which the corona was observed. The extent of the corona was variously estimated by different observers. Petit, at Montpelier, assigned to it a breadth corresponding to a height of about 200,000 miles; Baily a height of about 500,000 miles; and O. Struve a height of more than 800,000 miles. The last-named observer also recognised luminous expansions extending fully four degrees (corresponding to nearly seven million miles) from the sun. Picozzi, at Milan, noticed two jets of light, which were seen also by observers in France. Rays also were seen by Mauvais at Perpignan, and by Baily at Paria. But Airy, observing the corona from the Superga, could see no radiation; he says 'although a slight radiation might have been perceptible, it was not sufficiently intense to affect in a sensible degree the annular structure by which the luminous appearance was plainly distinguished.' These varieties in the aspect of the corona were doubtless due to varieties in the condition of the atmosphere through which the corona was seen. Now it cannot be questioned that, so far as extension is concerned, the corona seen in 1842 was one which, if the theory we are considering were sound, we should expect to see near the time of maximum rather than of minimum solar disturbance. On the other hand, in brightness the corona of 1842 resembled, if it did not surpass, that of July 1878.
'I had imagined,' says Baily, 'that the corona, as to its brilliant or luminous appearance, would not be greater than that faint crepuscular light which sometimes takes place (sic) in a summer evening, and that it would encircle the moon like a ring. I was therefore somewhat surprised and astonished at the splendid scene which now so suddenly burst upon my view.'
The light of the corona was so bright, O. Struve states, that the naked eye could scarcely endure it; 'many could not believe, indeed, that the eclipse was total, so strongly did the corona's light resemble direct sunlight.' Thus while as to extent the corona in 1842 presented the appearance to be expected at the time of maximum solar disturbance, if our theory is sound, its brightness was that corresponding to a time of minimum disturbance. Its structure corresponded with the former condition. The light of the corona was not uniform, nor merely marked by radiations, but in several places interlacing lines of light could be seen. Arago, at Perpignan, observed with the unaided eye a region