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and also with its relative brightness or darkness (as indicative of greater or less temperature), we have a means of studying those conditions of motion, pressure, and temperature, respecting which the telescope alone can give us no information whatever.

It is wonderful, indeed, to consider that that analysis of the dark lines of the solar spectrum which seemed half a century ago so unmeaning, those speculations of Doppler which but a quarter of a century ago were rejected by many as wholly fanciful, and those inquiries as to the almost evanescent wave-lengths of light which from the days of Newton downwards had been ridiculed as a complete waste of time and thought, should have resulted, under the labours of Bunsen and Kirchhoff, of Plucker, Huggins, and Frankland, and, finally, of Ängström and Van der Willingen, in a means of dealing with problems so recondite and seemingly so hopeless. By an observation not occupying many seconds any clear-sighted observer, armed by our opticians with adequate spectroscopic power, can measure the swiftness of the solar windstorm, can gauge the pressure of the solar atmosphere, and can estimate the relative temperature of spot and faculæ, of photosphere and chromosphere, and, lastly, of the higher regions to which eruptions cast those masses of glowing vapour which form the solar prominences.

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CHAPTER IV.

STUDY OF THE SUN'S SURFACE.

WE may regard the discovery of the spots on the Sun as the commencement of that long series of telescopic researches to which we owe our present knowledge of the solar orb. It is highly probable, indeed, that spots on the Sun had been seen and even watched for long intervals, when as yet astronomers were not aided by the powers of the telescope. But there is no reason to believe that the nature of the spots so seen, or even the fact that they are true solar phenomena, had ever been suspected by astronomers. Whatever opinion we

*Kepler supposed that in lines 441 and 454 of Virgil's first Georgic, the solar spots were referred to. For if any one,' he reasons, 'should refuse to see anything else than an allusion to our clouds in the words

'Ille ubi nascentem maculis variaverit ortum,'

I shall oppose to the interpretation this other verse:

Sin maculæ incipient rutilo immiscerier igni.'

But the latter verse is quite as applicable to clouds as the former. As regards the occasional recognition of spots by the ancients, however, there seems less room for doubt. We learn from Father Mailla that the Chinese recorded the appearance of spots on the Sun in the year 321 A.D., and Acosta tells us that the natives of Peru told the Spanish invaders that the Sun's face had in former times been marked with spots. In the year 807, a large spot was seen on the Sun for eight

may form of ancient records of solar obscurations, we must turn to the telescopic discovery of the spots for the real commencement of astronomical researches into the Sun's physical condition.

I do not propose to enter here into the discussion which has been raised respecting the astronomer to whom the credit of having first seen the solar spots is to be assigned. This discussion has been pursued by grave authorities with an earnestness which would really seem to imply that they have regarded the matter as of serious importance. Let us simply recognise the fact that the credit of the discovery is not worth contending about, and proceed to consider

successive days, and was supposed by those who were little familiar with the laws of planetary motion to be the planet Mercury. Arago is of opinion, also, that the transits of Mercury said to have been witnessed by Averrhoes, Scaliger, and Kepler himself (May 28, 1607) were only observations of Sun spots. It is worthy of notice that the ancients could very well have observed Sun spots, and have even traced the progress of these spots across the solar disc, had they employed the method which Gassenius adopted in observing the transit of Mercury in 1631. He admitted the Sun's rays into a darkened chamber through a small aperture in a shutter, and thus obtained an inverted image of the Sun, on which, when the transit had begun, he could perceive the disc of Mercury. Although no spots are ever seen which, even in the nucleus, are so dark as Mercury, yet many (or rather all the noteworthy spots) present a much more conspicuous appearance than Mercury in transit. Fabricius, indeed, as we shall presently see, did actually apply this method.

*It has been justly remarked by an eminent astronomer of our own time, that the discovery of the spots was a necessary sequel of the invention of the telescope; and whether Galileo, or Fabricius, or Scheiner, or Harriot, first set eyes on these objects, is a matter which can in no way increase the reputation of any one of these astronomers. To discuss the question of priority seems therefore to be simply a waste of time.

what the first observers actually saw

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From a work by Fabricius,* we learn that in the commencement of the year 1611, while observing the Sun just after sunrise with a telescope of inconsiderable power, he noticed a black spot upon its disc which he supposed to be a terrestrial cloud. He found, however, that the object, whatever it was, belonged to the Sun. As the Sun rose he had to discontinue his observations, for he possessed no means of mitigating the brilliancy of the Sun's light. My father and I,' he says, passed the rest of the day and the whole night in great impatience, trying to think what this spot might be. "If it is in the Sun," I said, "we shall no doubt see it again; if it is not, its motion will have carried it away from the Sun's disc, and so we shall be unable to see it." On the next morning, however, to my delight, I saw the spot again. But it was not in the same place-a peculiarity which increased our perplexity. We determined to obtain an image of the Sun on a sheet of paper by permitting his rays to pass through a small hole in a darkened chamber, and in this way we saw the spot quite clearly in the form of an elongated cloud. For three days we were prevented by bad weather from continuing our observations; but at the end of that period we again saw the spot, which had crossed obliquely towards the western side of the Sun's disc. Another smaller one

* Entitled De Maculis in Sole observatis, &c., and published at Wittemberg in 1611.

had made its appearance near the eastern edge, and in a few days this second spot reached the middle of the disc. Lastly, a third spot appeared. The three spots vanished in the order of their appearance. I was hopeful that they would be seen again, but yet perplexed by doubts and fears; however, ten days afterwards the first re-appeared on the eastern side of the disc. I knew then that it had revolved completely round (the Sun), and since then I have convinced myself that this is really the case.' Fabricius studied the import of his observations, and came to the conclusion that the spots are probably upon the body of the Sun itself. We invite the students of science,' he says, to profit by our description; they will doubtless conclude that the Sun has a motion of rotation, as Jordanus Bruno has asserted, and, more lately, Kepler. Indeed, I do not know what we could make of these spots on any other supposition.'

Galileo, at Florence, and Father Scheiner, a German Jesuit, besides independently discovering the spots, investigated the laws which regulate the motion of these objects. Scheiner was at first disposed to regard the spots as due to the existence of planets travelling round the Sun close to its surface, and indeed for a while these imagined planets were admitted as true members of the solar family under the title of the Borbonian stars.* But Galileo having pointed out

They are referred to under this name in Burton's Anatomy of Melancholy (a very short time after their discovery), in that singularly interesting chapter which he entitles, quaintly enough, a

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