Part 20

Fourthly, M. Leverrier's reasons for believing that the planet exists are based on the supposition that astronomical observations are more precise than they really are.

Probably, Liais's objections would have had more weight with Leverrier had the fourth point been omitted. It was rash in a former subordinate to impugn the verdict of the chief of the Paris Observatory on a matter belonging to that special department of astronomy which an observatory chief might be expected to understand thoroughly. It is thought daring in the extreme for one outside the circles of official astronomy (as Newton in Flamstead's time, Sir W. Herschel in Maskelyne's, and Sir J. Herschel in the present century), to advance or maintain an opinion adverse to that of some official chief, but for a subordinate (even though no longer so), to be guilty of such rash procedure 'is most tolerable and not to be endured,' as a typical official has said. Accordingly, very little attention was paid by Leverrier to Liais's objections.

Yet, in some respects, what M. Liais had to say was very much to the point.

At the very time when Lescarbault was watching the black spot on the sun's face, Liais was examining the sun with a telescope of much greater magnifying power, and saw no such spot. His attention was specially directed to the edge of the sun (where Lescarbault saw the spot) because he was engaged in determining the decrease of the sun's brightness near the edge. Moreover, he was examining the very part of the sun's edge where Lescarbault saw the planet enter, at a time when it must have been twelve minutes in time upon the face of the sun, and well within the margin of the solar disc. The negative evidence here is strong; though it must always be remembered that negative evidence requires to be overwhelmingly strong before it can be admitted as effective against positive evidence. It seems at a first view utterly impossible that Liais, examining with a more powerful telescope the region where Lescarbault saw the spot, could have failed to see it had it been there; but experience shows that it is not impossible for an observer engaged in examining phenomena of one class to overlook a phenomenon of another class, even when glaringly obvious. All we can say is that Liais was not likely to have overlooked Lescarbault's planet had it been there; and we must combine this probability against Vulcan's existence with arguments derived from other considerations. There is also the possibility of an error in time. As the writer in the 'North British Review' remarks, 'twelve minutes is so short a time that it is just possible that the planet may not have entered upon the sun during the time that Liais observed it.'

The second and third arguments are stronger. In fact, I do not see how they can be resisted.

It is, in the first place, clear from Lescarbault's account that Vulcan must have a considerable diameter--certainly if Vulcan's diameter in miles were only half the diameter of Mercury, it would have been all but impossible for Lescarbault with his small telescope to see Vulcan at all, whereas he saw the black spot very distinctly. Say Vulcan has half the diameter of Mercury, and let us compare the brightness of these two planets when at their greatest apparent distances from the sun, that is, when each looks like a half-moon. The distance of Mercury exceeds the estimated distance of Vulcan from the sun as 27 exceeds 10, so that Vulcan is more strongly illuminated in the proportion of 27 times 27 to 10 times 10, or 729 to 100--say at least 7 to 1. But having a diameter but half as large the disc of Vulcan could be but about a fourth of Mercury's at the same distance from us (and they would be at about the same distance from us when seen as half-moons). Hence Vulcan would be brighter than Mercury in the proportion of 7 to 4. Of course being so near the sun he would not be so easily seen; and we could never expect to see him at all, perhaps, with the naked eye--though even this is not certain. But Mercury, when at the same apparent distance from the sun, and giving less light than at his greatest seeming distance, is quite easily seen in the telescope. Much more easily, then, should Vulcan be seen, if a telescope were rightly directed at such a time, or when Vulcan was anywhere near his greatest seeming distance from the sun. Now it is true astronomers do not know precisely when or where to look for him. But he passes from his greatest distance on one side of the sun to his greatest distance on the other in less than ten days, according to the computed period, and certainly (that is, if the planet exists) in a very short time. The astronomer has then only to examine day after day a region of small extent on either side of the sun, for ten or twelve days in succession (an hour's observation each day would suffice), to be sure of seeing Vulcan. Yet many astronomers have made such search many times over, without seeing any trace of the planet. During total solar eclipses, again, the planet has been repeatedly looked for unsuccessfully--though it should at such a time be a very conspicuous object, when favourably placed, and could scarcely fail of being very distinctly seen wherever placed.

The fourth argument of Lescarbault's is not so effective, and in fact he gets beyond his depth in dealing with it. But it is to be noticed that a considerable portion of the discrepancy between Mercury's observed and calculated motions has long since been accounted for by the changed estimate of the earth's mass as compared with the sun's, resulting from the new determination of the sun's distance. However, the arguments depending on this consideration would not be suited to these pages.

There was one feature in Liais's paper which was a little unfortunate. He questioned Lescarbault's honesty. He said 'Lescarbault contradicts himself in having first asserted that he saw the planet enter upon the sun's disc, and having afterwards admitted to Leverrier that it had been on the disc some seconds before he saw it, and that he had merely inferred the time of its entry from the rate of its motion afterwards. If this one assertion be fabricated, the whole may be so.' 'He considers these arguments to be strengthened,' says the 'North British Review,' 'by the assertion which, as we have seen, perplexed Leverrier himself, that if M. Lescarbault had actually seen a planet on the sun, he could not have kept it secret for nine months.'

This charge of dishonesty, unfortunate in itself, had the unfortunate effect of preventing Lescarbault or the Abbé Moigno from replying. The latter simply remarked that the accusation was of such a nature as to dispense him from any obligation to refute it. This was an error of judgment, I cannot but think, if an effective reply was really available.

The Remarks with which the North British Reviewer closes his account may be repeated now, so far as they relate to the force of the negative evidence, with tenfold effect. 'Since the first notice of the discovery in the beginning of January 1860 the sun has been anxiously observed by astronomers; and the limited area around him in which the planet must be, if he is not upon the sun, has doubtless been explored with equal care by telescopes of high power, and processes by which the sun's direct light has been excluded from the tube of the telescope as well as the eye of the observer, and yet no planet has been found. This fact would entitle us to conclude that no such planet exists if its existence had been merely conjectured, or if it had been deduced from any of the laws of planetary distance, or even if Leverrier or Adams had announced it as the probable result of planetary perturbations. If the finest telescopes cannot rediscover a planet which with the small power used by Lescarbault has a visible disc, within so limited an area of which the sun is the centre, or rather within a narrow belt of that circle, we should unhesitatingly declare that no such planet exists. But the question assumes a very different aspect when it involves moral considerations. If,' proceeds the Reviewer, writing in August 1860, 'after the severe scrutiny which the sun and its vicinity will undergo before and after and during his total eclipse in July, no planet shall be seen; and if no round black spot distinctly separable from the usual solar spots shall be seen on the solar spots' (sic, presumably solar disc was intended), 'we will not dare to say that it does not exist. We cannot doubt the honesty of M. Lescarbault, and we can hardly believe that he was mistaken. No solar spot, no floating scoria, could maintain in its passage over the sun a circular and uniform shape, and we are confident that no other hypothesis but that of an intra-mercurial planet can explain the phenomena seen and measured by M. Lescarbault, a man of high character, possessing excellent instruments, and in every way competent to use them well, and to describe clearly and correctly the results of his observations. Time, however, tries facts as well as speculations. The phenomena observed by the French astronomer may never be again seen, and the disturbance of Mercury which rendered it probable may be otherwise explained. Should this be the case, we must refer the round spot on the sun to some of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.'

The evidence which has accumulated against Vulcan in the interval since this was written is not negative only, but partly positive, as the following instance, which I take from my own narrative at the time in a weekly journal, serves to show:--After more than sixteen years of fruitless watching, astronomers learned last August (1876) that in the month of April Vulcan had been seen on the sun's disc in China. On April 4, it appeared, Herr Weber, an observer of considerable skill, stationed at Pecheli, had seen a small round spot on the sun, looking very much as a small planet might be expected to look. A few hours later he turned his telescope upon the sun, and lo! the spot had vanished, precisely as though the planet had passed away after the manner of planets in transit. He forwarded the news of his observation to Europe. The astronomer Wolff, well known for his sun-spot studies, carefully calculated the interval which had passed since Lescarbault saw Vulcan on March 26, 1859, and to his intense satisfaction was enabled to announce that this interval contained the calculated period of the planet an exact number of times. Leverrier at Paris received the announcement still more joyfully; while the Abbé Moigno, who gave Vulcan its name, and has always staunchly believed in the planet's existence, congratulated Lescarbault warmly upon this new view of the shamefaced Vulcan. Not one of those who already believed in the planet had the least doubt as to the reality of Weber's observations, and of these only Lescarbault himself received the news without pleasure. He, it seems, has never forgiven the Germans for destroying his observatory and library during the invasion of France in 1870, and apparently would prefer that his planet should never be seen again rather than that a German astronomer should have seen it. But the joy of the rest and Lescarbault's sorrow were alike premature. It was found that the spot seen by Weber had not only been observed at the Madrid observatory, where careful watch is kept upon the sun, but had been photographed at Greenwich; and when the description of its appearance, as seen in a powerful telescope at one station, and its picture as photographed by a fine telescope at the other, came to be examined, it was proved unmistakably that the spot was an ordinary sun-spot (not even quite round), which had after a few hours disappeared, as even larger sun-spots have been known to do in even a shorter time.

It is clear that had not Weber's spot been fortunately seen at Madrid and photographed at Greenwich, his observation would have been added to the list of recorded apparitions of Vulcan in transit, for it fitted in perfectly with the theory of Vulcan's real existence. I think, indeed, for my own part, that the good fortune was Weber's. Had it so chanced that thick weather in Madrid and at Greenwich had destroyed the evidence actually obtained to show that what Weber described he really saw, although it was not what he thought, some of the more suspicious would have questioned whether, in the euphonious language of the North British Reviewer, 'the round spot on the sun' was not due 'to one of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.' Of course no one acquainted with M. Weber's antecedents would imagine for a moment that he had invented the observation, even though the objective reality of his spot had not been established. But if a person who is entirely unknown, states that he has seen Vulcan, there is antecedently some degree of probability in favour of the belief that the observation is as much a myth as the planet itself. Some observations of Vulcan have certainly been invented. I have received several letters purporting to describe observations of bodies in transit over the sun's face, either the rate of transit, the size of the body, or the path along which it was said to move, being utterly inconsistent with the theory that it was an intra-mercurial planet, while yet (herein is the suspicious circumstance of such narratives) the epoch of transit accorded in the most remarkable manner with the period assigned to Vulcan. A paradoxist in America (of Louisville, Kentucky) who had invented a theory of the weather, in which the planets, by their influence on the sun, were supposed to produce all weather-changes, the nearer planets being the most effective, found his theory wanted Vulcan very much. Accordingly, he saw Vulcan crossing the sun's face in September, which, being half a year from March, is a month wherein, according to Lescarbault's observation, Vulcan may be seen in transit, and by a strange coincidence the interval between our paradoxist's observation and Lescarbault's exactly contained a certain number of times the period calculated by Leverrier for Vulcan. This was a noble achievement on the part of our paradoxist. At one stroke it established his theory of the weather, and promised to ensure him text-book immortality as one of the observers of Vulcan. But, unfortunately, a student of science residing in St. Louis, after leaving the Louisville paradoxist full time to parade his discovery, heartlessly pointed out that an exact number of revolutions of Vulcan after Lescarbault's March observation, must of necessity have brought the planet on that side of the sun on which the earth lies in March, so that to see Vulcan so placed on the sun's face in September was to see Vulcan through the sun, a very remarkable achievement indeed. The paradoxist was abashed, the reader perhaps imagines. Not in the least. The planet's period must have been wrongly calculated by Leverrier--that was all: the real period was less than half as long as Leverrier had supposed; and instead of having gone a certain number of times round since Lescarbault had seen it, Vulcan had gone twice as many times round and half once round again. The circumstance that if Vulcan's period had been thus short, the time of crossing the sun's face would have been much less than, according to Lescarbault's account, it actually was, had not occurred to the Louisville weather-prophet.

Leverrier's faith in Vulcan, however, has remained unshaken. He has used all the observations of spots which, like Weber's, have been seen only for a short time. At least he has used all which have not, like Weber's, been proved to be only transient sun-spots. Selecting those which fit in well with Lescarbault's observation, he has pointed out how remarkable it is that they show this accord. The possibility that some of them might be explicable as Weber's proved to be, and that some even may have been explicable as completely, but less satisfactorily, in another way, seems to have been thought scarcely worth considering. Using the imperfect materials available, but with exquisite skill--as a Phidias might model an exquisite figure of materials that would presently crumble into dust--Leverrier came to the conclusion that Vulcan would cross the sun's disc on or about March 22, 1876. 'He, therefore,' said Sir G. Airy, addressing the Astronomical Society, 'circulated a despatch among his friends, asking them carefully to observe the sun on March 22.' Sir G. Airy, humouring his honoured friend, sent telegrams to India, Australia, and New Zealand, requesting that observations might be made every two hours or oftener. Leverrier himself wrote to Santiago de Chili and other places, so that, including American and European observations, the sun could be watched all through the twenty-four hours on March 21, 22, and 23. 'Without saying positively that he believed or disbelieved in the existence of the planet,' proceeds the report, 'Sir G. Airy thought, since M. Leverrier was so confident, that the opportunity ought not to be neglected by anybody who professed to take an interest in the progress of planetary astronomy.'

It is perhaps unnecessary to add that observations were made as requested. Many photographs of the sun also were taken during the hours when Vulcan, if he exists at all, might be expected to cross the sun's face. But the 'planet of romance,' as Abbé Moigno has called Vulcan, failed to appear, and the opinion I had expressed last October ('English Mechanic and World of Science,' for October 27, 1876, p. 160), that Vulcan might perhaps better be called the 'planet of fiction' was pro tanto confirmed. Nevertheless, I would not be understood to mean by the word 'fiction' aught savouring of fraud so far as Lescarbault is concerned--I prefer the North Briton's view of Lescarbault's spot, that so to speak, it was

... the blot upon his brain, That would show itself without.

I have left small space to treat of other fancied discoveries among the orbs of heaven. Yet there are some which are not only interesting but instructive, as showing how even the most careful observers may be led astray. In this respect the mistakes into which observers of great and well deserved eminence have fallen are specially worthy of attention. With the description of three such mistakes, made by no less an astronomer than Sir W. Herschel, I shall bring this paper to a close.

When Sir W. Herschel examined the planet Uranus with his most powerful telescope he saw the planet to all appearance girt about by two rings at right angles to one another. The illusion was so complete that Herschel for several years remained in the belief that the rings were real. They were, however, mere optical illusions, due to the imperfect defining qualities of the telescope with which he observed the planet. Later he wrote that 'the observations which tend to ascertain' (indicate?) 'the existence of rings not being satisfactorily supported, it will be proper that surmises of them should either be given up, as ill-founded, or at least reserved till superior instruments can be provided.'

Sir W. Herschel was more completely misled by the false Uranian satellites. He had seen, as he supposed, no less than six of these bodies. As only two of these had been seen again, while two more were discovered by Lassell, the inference was that Uranus has eight satellites in all. These for a long time flourished in our text-books of astronomy; and many writers, confident in the care and skill of Sir W. Herschel, were unable for a long time to believe that he had been deceived. Thus Admiral Smyth, in his 'Celestial Cycle,' wrote of those who doubted the extra satellites:--'They must have but a meagre notion of Sir W. Herschel's powerful means, his skill in their application, and his method of deliberate procedure. So far from doubting there being six satellites' (this was before Lassell had discovered the other two) 'it is highly probable that there are still more.' Whewell, also, in his 'Bridgewater Treatise,' says, 'that though it no longer appears probable that Uranus has a ring like Saturn, he has at least five satellites which are visible to us, and we believe that the astronomer will hardly deny that he' (Uranus, not the astronomer), 'may possibly have thousands of smaller ones circulating about him.' But in this case Sir W. Herschel, anxiously though he endeavoured to guard against the possibility of error, was certainly mistaken. Uranus may, for anything that is known to the contrary, have many small satellites circulating about him, but he certainly has not four satellites (besides those known) which could have been seen by Sir W. Herschel with the telescope he employed. For the neighbourhood of the planet has been carefully examined with telescopes of much greater power by observers who with those telescopes have seen objects far fainter than the satellites supposed to have been seen by the elder Herschel.

The third of the Herschelian myths was the lunar volcano in eruption, which he supposed he had seen in progress in that part of the moon which was not at the time illuminated by the sun's rays. He saw a bright star-like point of light, which corresponded in position with the crater of the lunar mountain Aristarchus. He inferred that a volcano was in active eruption because the brightness of the point of light varied from time to time, and also because he did not remember to have seen it before under the same conditions. There is no doubt something very remarkable in the way in which this part of the moon's surface shines when not illumined by the sun. If it were always bright we should conclude at once that the earth-light shining upon it rendered it visible. For it must be remembered that the part of the moon which looks dark (or seems wanting to the full disc) is illuminated by our earth, shining in the sky of the moon as a disc thirteen times as large as that of the moon we see, and with the same proportion of its disc sunlit as is dark in the moon's disc. Thus when the moon is nearly new our earth is shining in the lunar skies as a nearly full moon thirteen times as large as ours. The light of this noble moon must illumine the moon's surface much more brightly than a terrestrial landscape is illumined by the full moon, and if any parts of her surface are very white they will shine out from the surface around, just as the snow-covered peak of a mountain shines out upon a moonlit night from among the darker hills and dales and rocks and forests of the landscape. But Herschel considered that the occasional brightness of the crater Aristarchus could not be thus explained. The spot had been seen before the time of Herschel's observations by Cassini and others. It has been seen since by Captain Kater, Francis Baily, and many others. Dr. Maskelyne tells us that in March 1794 it was seen by the naked eye by two persons.

Baily thus describes the appearance presented by this lunar crater on December 22, 1835: 'Directed telescope to the moon, and pointing it to the dark part in the vicinity of Aristarchus, soon saw the outline of that mountain very distinctly, formed like an irregular nebula. Nearly in the centre was a light resembling that of a star of the ninth or tenth magnitude. It appeared by glimpses, but at times was brilliant, and visible for several seconds together.'

There can be little doubt, however, that the apparent brightness of this lunar crater, or rather of its summit, is due to some peculiar quality in the surface, which may perhaps be covered by some crystalline or vitreous matter poured out in the far distant time when the crater was an active one. Prof. Shaler, who examined the crater when it was illuminated only by earthshine, with the fine 15-inch telescope of the Harvard Observatory (Cambridge U.S.), says that he has been able to recognise nearly all the craters over 15 miles in diameter in the dark part. 'There are several degrees of brightness,' he says, 'observable in the different objects which shine out by the earth-light. This fact probably explains the greater part of the perplexing statements concerning the illumination of certain craters. It certainly accounts for the volcanic activity which has so often been supposed to be manifested by Aristarchus. Under the illumination by the earth-light this is by far the brightest object on the dark part of the moon's face, and is visible much longer and with poorer glasses than any other object there.'

Here my record of astronomical myths must be brought to a close. It will be noticed that in every instance either the illusion has affected the actual observations of eminent and skilful astronomers, or has caused such astronomers to put faith for a while in illusory observations. Had I cared to include the mistakes which have been made by or have misled observers of less experience, I could have filled many sheets for each page of the present article. But it has seemed to me more instructive to show how error may affect the observations even of the most careful and deservedly eminent astronomers, how even the most cautious may be for a time misled by the mistakes of inferior observers, especially when the fact supposed to have been observed accords with preconceived opinions.

XII.

THE ORIGIN OF THE CONSTELLATION-FIGURES.

Although the strange figures which astronomers still allow to straggle over their star maps no longer have any real scientific interest, they still possess a certain charm, not only for the student of astronomy, but for many who care little or nothing about astronomy as a science. When I was giving a course of twelve lectures in Boston, America, a person of considerable culture said to me, 'I wish you would lecture about the constellations; I care little about the sun and moon and the planets, and not much more about comets; but I have always felt great interest in the Bears and Lions, the Chained and Chaired Ladies, King Cepheus and the Rescuer Perseus, Orion, Ophiuchus, Hercules, and the rest of the mythical and fanciful beings with which the old astronomers peopled the heavens. I say with Carlyle, "Why does not some one teach me the constellations, and make me at home in the starry heavens, which are always overhead, and which I don't half know to this day."' We may notice, too, that the poets by almost unanimous consent have recognised the poetical aspect of the constellations, while they have found little to say about subjects which belong especially to astronomy as a science. Milton has indeed made an Archangel reason (not unskilfully for Milton's day) about the Ptolemaic and Copernican systems, while Tennyson makes frequent reference to astronomical theories. 'There sinks the nebulous star we call the Sun, if that hypothesis of theirs be sound,' said Ida; but she said no more, save 'let us down and rest,' as though the subject were wearisome to her. Again, in the Palace of Art the soul of the poet having built herself that 'great house so royal, rich, and wide,' thither--

... when all the deep unsounded skies Shuddered with silent stars, she clomb, And as with optic glasses her keen eyes Pierced through the mystic dome, Regions of lucid matter taking forms, Brushes of fire, hazy gleams, Clusters and beds of worlds and beelike swarms Of suns, and starry streams: She saw the snowy poles of moonless Mars, That marvellous round of milky light Below Orion, and those double stars Whereof the one more bright Is circled by the other.

But the poet's soul so wearied of these astronomical researches that the beautiful lines I have quoted disappeared (more's the pity) from the second and all later editions. Such exceptions, indeed, prove the rule. Poets have been chary in referring to astronomical researches and results, full though these have been of unspeakable poetry; while from the days of Homer to those of Tennyson, the constellations which 'garland the heavens' have always been favourite subjects of poetic imagery.

It is not my present purpose, however, to discuss the poetic aspect of the constellations. I propose to inquire how these singular figures first found their way to the heavens, and, so far as facts are available for the purpose, to determine the history and antiquity of some of the more celebrated constellations.