motions are taking place, resulting in motions of recess or approach with reference to the terrestrial observer. In all such cases we may expect to find peculiarities in the affected lines, corresponding to varieties in the motion or rates of motion of the parts examined.

I give a few examples, illustrating the way in which such peculiarities are to be interpreted. I consider, for convenience, motions taking place in that coloured envelope (whence the solar prominences seem to spring) which has been called the chromosphere :

Suppose ss to represent the portion of the Sun and chromosphere under examination, s s' being the Sun's

limb, c c' the (invisible) outline of the chromosphere. Now, if the F line appeared as in I., we should conclude that the hydrogen in the part of the chromosphere under examination was quiescent near the Sun's surface, as far as motions of approach or recess are concerned (though it might be moving very rapidly in a direction square to the line of sight), but that at some distance from the Sun's surface it was moving very rapidly towards the eye, the rate of motion increasing with the vertical height above the Sun. If, on the other hand, the spectrum appeared as at II. (fig. 40), we should come to a similar conclusion, substituting only a motion

of recession for one of approach. If the spectrum appeared as at III. we should conclude that to a certain level above the Sun's limb there was a gradually increasing motion of approach, but that at and above that level there was a motion of recession tolerably uniform in rate to a considerable height. The case would resemble those instances in our own atmosphere where an upper air current blows in a different direction than the air nearer the sea-level. If, lastly, the spectrum appeared as at IV., we should conclude that to a considerable height above the Sun's surface there was no motion of recess or approach; but that in higher regions of the chromosphere there were masses (within the long range of chromospheric matter really included in the direction of the visual line) moving both from and towards the eye at an enormously rapid rate. The greater or less width of different parts of the bright line would indicate the greater or less pressure at which the hydrogen existed at the corresponding levels during the time of observation. Hence, a bulb on any part of the bright line would indicate a corresponding layer of relatively compressed hydrogen, while a marked narrowing would indicate a layer of hydrogen existing for the time at relatively low pressure.

Similar considerations apply to the spectroscopic analysis of solar spots, or of faculous regions of the Sun's surface, or generally of any regions where disturbances may produce solar atmospheric currents of approach or recess. Combining the observed shifting of portions of a spectral line with its observed thickness

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

I shall

'Ille ubi nascentem maculis variaverit ortum,'

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.