Part 6 (1/2)
[Footnote 123: _Phil. Trans._, vol. cxxiii., p. 503.]
[Footnote 124: _Results_, etc., p. 136.]
[Footnote 125: Loomis, _Month. Not._, vol. xxix., p. 298.]
[Footnote 126: See the Author's _System of the Stars_, pp. 116-120.]
[Footnote 127: _Outlines of Astr._, App. I.]
[Footnote 128: _Phil. Trans._, vol. cxix., p. 27.]
[Footnote 129: Dr. Dreyer's New General Catalogue, published in 1888 as vol. xlix. of the Royal Astronomical Society's _Memoirs_, is an enlargement of Herschel's work. It includes 7,840 entries, and was supplemented, in 1895, by an ”Index Catalogue” of 1,529 nebulae discovered 1888 to 1894. _Mem. R. A. S._, vol. li.]
[Footnote 130: A list of 10,320 composite stars was drawn out by him in order of right ascension, and has been published in vol. xl. of _Mem. R.
A. S._; but the data requisite for their formation into a catalogue were not forthcoming. See Main's and Pritchard's _Preface_ to above, and Dunkin's _Obituary Notices_, p. 73.]
CHAPTER III
_PROGRESS OF KNOWLEDGE REGARDING THE SUN_
The discovery of sun-spots in 1610 by Fabricius and Galileo first opened a way for inquiry into the solar const.i.tution; but it was long before that way was followed with system or profit. The seeming irregularity of the phenomena discouraged continuous attention; casual observations were made the basis of arbitrary conjectures, and real knowledge received little or no increase. In 1620 we find Jean Tarde, Canon of Sarlat, arguing that because the sun is ”the eye of the world,” and the eye of the world _cannot suffer from ophthalmia_, therefore the appearances in question must be due, not to actual specks or stains on the bright solar disc, but to the transits of a number of small planets across it! To this new group of heavenly bodies he gave the name of ”Borbonia Sidera,”
and they were claimed in 1633 for the House of Hapsburg, under the t.i.tle of ”Austriaca Sidera” by Father Malapertius, a Belgian Jesuit.[131] A similar view was temporarily maintained against Galileo by the justly celebrated Father Scheiner of Ingolstadt, and later by William Gascoigne, the inventor of the micrometer; but most of those who were capable of thinking at all on such subjects (and they were but few) adhered either to the _cloud theory_ or to the _slag theory_ of sun-spots. The first was championed by Galileo, the second by Simon Marius, ”astronomer and physician” to the brother Margraves of Brandenburg. The latter opinion received a further notable development from the fact that in 1618, a year remarkable for the appearance of three bright comets, the sun was almost free from spots; whence it was inferred that the cindery refuse from the great solar conflagration, which usually appeared as dark blotches on its surface, was occasionally thrown off in the form of comets, leaving the sun, like a snuffed taper, to blaze with renewed brilliancy.[132]
In the following century, Derham gathered from observations carried on during the years 1703-11, ”That the spots on the sun are caused by the eruption of some new volcano therein, which at first pouring out a prodigious quant.i.ty of smoke and other opacous matter, causeth the spots; and as that fuliginous matter decayeth and spendeth itself, and the volcano at last becomes more torrid and flaming, so the spots decay, and grow to umbrae, and at last to faculae.”[133]
The view, confidently upheld by Lalande,[134] that spots were rocky elevations uncovered by the casual ebbing of a luminous ocean, the surrounding penumbrae representing shoals or sandbanks, had even less to recommend it than Derham's volcanic theory. Both were, however, significant of a growing tendency to bring solar phenomena within the compa.s.s of terrestrial a.n.a.logies.
For 164 years, then, after Galileo first levelled his telescope at the setting sun, next to nothing was learned as to its nature; and the facts immediately ascertained, of its rotation on an axis nearly erect to the plane of the ecliptic, in a period of between twenty-five and twenty-six days, and of the virtual limitation of the spots to a so-called ”royal”
zone extending some thirty degrees north and south of the solar equator, gained little either in precision or development from five generations of astronomers.
But in November, 1769, a spot of extraordinary size engaged the attention of Alexander Wilson, professor of astronomy in the University of Glasgow. He watched it day by day, and to good purpose. As the great globe slowly revolved, carrying the spot towards its western edge, he was struck with the gradual contraction and final disappearance of the penumbra _on the side next the centre of the disc_; and when on the 6th of December the same spot re-emerged on the eastern limb, he perceived, as he had antic.i.p.ated, that the shady zone was now deficient _on the opposite side_, and resumed its original completeness as it returned to a central position. In other spots subsequently examined by him, similar perspective effects were visible, and he proved in 1774,[135] by strict geometrical reasoning, that they could only arise in vast photospheric excavations. It was not, indeed, the first time that such a view had been suggested. Father Scheiner's later observations plainly foreshadowed it;[136] a conjecture to the same effect was emitted by Leonard Rost of Nuremburg early in the eighteenth century;[137] both by Lahire in 1703 and by J. Ca.s.sini in 1719 spots had been seen as notches on the solar limb; while in 1770 Pastor Schulen of Essingen, from the careful study of phenomena similar to those noted by Wilson, concluded their depressed nature.[138] Modern observations, nevertheless, prove those phenomena to be by no means universally present.
Wilson's general theory of the sun was avowedly tentative. It took the modest form of an interrogatory. ”Is it not reasonable to think,” he asks, ”that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark, and that this immense and dark globe is encompa.s.sed with a thin covering of that resplendent substance from which the sun would seem to derive the whole of his vivifying heat and energy?”[139] He further suggests that the excavations or spots may be occasioned ”by the working of some sort of elastic vapour which is generated within the dark globe,” and that the luminous matter, being in some degree fluid, and being acted upon by gravity, tends to flow down and cover the nucleus. From these hints, supplemented by his own diligent observations and sagacious reasonings, Herschel elaborated a scheme of solar const.i.tution which held its ground until the physics of the sun were revolutionised by the spectroscope.
A cool, dark, solid globe, its surface diversified with mountains and valleys, clothed in luxuriant vegetation, and ”richly stored with inhabitants,” protected by a heavy cloud-canopy from the intolerable glare of the upper luminous region, where the dazzling coruscations of a solar aurora some thousands of miles in depth evolved the stores of light and heat which vivify our world--such was the central luminary which Herschel constructed with his wonted ingenuity, and described with his wonted eloquence.
”This way of considering the sun and its atmosphere,” he says,[140]
”removes the great dissimilarity we have hitherto been used to find between its condition and that of the rest of the great bodies of the solar system. The sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface, the rotation upon its axis, and the fall of heavy bodies, leads us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circ.u.mstances of that vast globe.”
We smile at conclusions which our present knowledge condemns as extravagant and impossible, but such incidental flights of fancy in no way derogate from the high value of Herschel's contributions to solar science. The cloud-like character which he attributed to the radiant sh.e.l.l of the sun (first named by Schroter the ”photosphere”) is borne out by all recent investigations; he observed its mottled or corrugated aspect, resembling, as he described it, the roughness on the rind of an orange; showed that ”faculae” are elevations or heaped-up ridges of the disturbed photospheric matter; and threw out the idea that spots may ensue from an excess of the ordinary luminous emissions. A certain ”empyreal” gas was, he supposed (very much as Wilson had done), generated in the body of the sun, and rising everywhere by reason of its lightness, made for itself, when in moderate quant.i.ties, small openings or ”pores,”[141] abundantly visible as dark points on the solar disc.
But should an uncommon quant.i.ty be formed, ”it will,” he maintained, ”burst through the planetary[142] regions of clouds, and thus will produce great openings; then, spreading itself above them, it will occasion large shallows (penumbrae), and mixing afterwards gradually with other superior gases, it will promote the increase, and a.s.sist in the maintenance, of the general luminous phenomena.”[143]
This partial antic.i.p.ation of the modern view that the solar radiations are maintained by some process of circulation within the solar ma.s.s, was reached by Herschel through prolonged study of the phenomena in question. The novel and important idea contained in it, however, it was at that time premature to attempt to develop. But though many of the subtler suggestions of Herschel's genius pa.s.sed unnoticed by his contemporaries, the main result of his solar researches was an unmistakable one. It was nothing less than the definitive introduction into astronomy of the paradoxical conception of the central fire and hearth of our system as a cold, dark, terrestrial ma.s.s, wrapt in a mantle of innocuous radiance--an earth, so to speak, within--a sun without.
Let us pause for a moment to consider the value of this remarkable innovation. It certainly was not a step in the direction of truth. On the contrary, the crude notions of Anaxagoras and Xeno approached more nearly to what we now know of the sun, than the complicated structure devised for the happiness of a n.o.bler race of beings than our own by the benevolence of eighteenth-century astronomers. And yet it undoubtedly const.i.tuted a very important advance in science. It was the first earnest attempt to bring solar phenomena within the compa.s.s of a rational system; to put together into a consistent whole the facts ascertained; to fabricate, in short, a solar machine that would in some fas.h.i.+on work. It is true that the materials were inadequate and the design faulty. The resulting construction has not proved strong enough to stand the wear and tear of time and discovery, but has had to be taken to pieces and remodelled on a totally different plan. But the work was not therefore done in vain. None of Bacon's aphorisms show a clearer insight into the relations between the human mind and the external world than that which declares ”Truth to emerge sooner from error than from confusion.”[144] A definite theory (even if a false one) gives holding-ground to thought. Facts acquire a meaning with reference to it.
It affords a motive for acc.u.mulating them and a means of co-ordinating them; it provides a framework for their arrangement, and a receptacle for their preservation, until they become too strong and numerous to be any longer included within arbitrary limits, and shatter the vessel originally framed to contain them.
Such was the purpose subserved by Herschel's theory of the sun. It helped to _clarify_ ideas on the subject. The turbid sense of groping and viewless ignorance gave place to the lucidity of a possible scheme.
The persuasion of knowledge is a keen incentive to its increase. Few men care to investigate what they are obliged to admit themselves entirely ignorant of; but once started on the road of knowledge, real or supposed, they are eager to pursue it. By the promulgation of a confident and consistent view regarding the nature of the sun, accordingly, research was encouraged, because it was rendered hopeful, and inquirers were shown a path leading indefinitely onwards where an impa.s.sable thicket had before seemed to bar the way.