William Herschel and his work/Chapter 8
"THE CONSTRUCTION OF THE HEAVENS"
The writer who described Herschel's papers as "lively and amusing" may have intended a sneer, but he did a great wrong to inquiries and facts as novel as they were inspiring. Whatever helps to lift man's thoughts above the littlenesses of life and time is a distinct gain to the human race, altogether irrespective of the uses to which, in course of time, it may be applied. Herschel's papers on The Construction of the Heavens were of this nature. They were among the first he wrote; they were also among the last. He wrote at least eight papers on the subject, covering three hundred and thirty quarto pages: he began the series in 1784, he finished it in 1818, and he left the work as a legacy to his son, who nobly honoured his father's memory by doing for the southern hemisphere what the father did for the northern. Even though these labours had been nothing more than an attempt on man's part to penetrate the workshop of nature and ascertain the hidden processes of an Almighty Worker, they would have been invaluable as a serviceable hypothesis for future efforts. Boldly and with all reverence, he set himself to open the closed hand of Almighty Wisdom, and find what that Power had kept hid. Others laboured in this cause before him, but "we are indebted solely to the genius and industry of Dr. Herschel for perfecting their sagacious views, and supporting them by a body of evidence amounting nearly to demonstration."
The first point he laid down was that there is ample reason "strongly to suspect that there is not, in strictness of speaking, one fixed star in the heavens." Fixed stars is a name we have been led to use, because, unlike the planets or wanderers, they seem never to change their places in the sky; but absolute rest in any one of these stars is impossible except, it may be, as a result of nicely balanced forces. Herschel was beginning in 1783 A.D. at the same starting-point as the famous Hipparchus nearly two thousand years before, who "observed a new star which appeared in his own day, and which led him to believe that the same thing might happen frequently, and that the stars considered fixed might be in motion." The proper motion, as it is called, of some of the brightest stars was suspected nearly a century before Herschel's time and was afterwards fully proved. What the nature of that motion may be, might be guessed by astronomers, but was really a fruitful field for genius and perseverance to cultivate. "Its causes and laws are hid for the present in almost equal obscurity," was the judgment of Dr. Maskelyne, then Astronomer-Royal; but it pointed to changes among the stars, which a shrewd observer would endeavour to ascertain and account for. Herschel undertook the work. Availing himself of a catalogue of 2884 stars published in 1723 by Flamsteed, the first Astronomer-Royal, he compared the heavens of his own day with the appearance they presented then. He had no star charts such as astronomers have since constructed, and which, when compared with a revised edition a century hence, may reveal much that is at present dark regarding the motions and destiny of the small but beautiful home of our shortlived race. He had no photographic plates to expose or consult. From beginning to end it was eye-labour and hand-labour with this intrepid traveller among these far-away suns. So laborious was the comparison that he had "many a night, in the course of eleven or twelve hours of observation, carefully and singly examined not less than 400 celestial objects, besides taking measures of angles and positions of some of them with proper micrometers, and sometimes viewing a particular star for half an hour together, with all the various powers of his telescope." During that interval of sixty years he found that stars had been lost or had vanished, that they had undergone some capital change of position or magnitude, or had come into sight where they were not previously seen, although "it is not easy to prove a star to be newly come" into any part of the sky. If a star suddenly shone out so as to attract the eyes of the common people, where a practised observer was sure there was no star visible an hour or two before, was he to conclude that it had flared up as if it were on fire, and that it would go out as the fire died down? Or, if he saw a star brightening, paling, going out, and brightening again every three or four days, or weeks or months, or every three or four years, was he to infer that dark bodies of vast size were thrusting themselves between that distant sun and our eyes, eclipsing it, in fact; or that immense reaches of unlighted space, or dark regions on its surface, were turned for a time towards us, as it revolved on its axis? Dark spots on a sunny star's surface and a rotation more or less rapid were the causes accepted by Herschel from previous astronomers for this change of brightness in what are called changing or variable stars. He examined seven that were then known. Their periods were 3, 5, 6, 7, 331, 394, and 497 days. He felt, however, that his views were discredited by the sudden bound from 7 days to 331. Unless a star were found bridging the gulf between these two, he would not have had confidence to give his theory to the world. But the star α Herculis seemed to him to bridge the gap, and satisfy the theory. Its period was found to be about 60 days. These and other changes on the face of the heavens, known for many years and registered in books, formed Herschel's prelude to the work he had set his heart on, The Construction of the Heavens. That they are a building, a wonderful temple consecrated to Almighty Power and Wisdom, he never doubted. To discover the plan on which the All-wise Worker proceeded was his aim and ambition.
Stars had been seen by Flamsteed which Herschel could no longer find. A century had elapsed, and Herschel put these stars down as "lost." He meant that a star thus noted was not to be seen when he looked for it, "but that possibly at some future time, if it be a changeable or periodical star, it may come to be visible again." In other cases he entered in his journal the remark, "Does not exist," when Flamsteed had not himself seen the star. Herschel, however, does not appear to have considered that these "lost stars" may have been comets, or wanderers like his own Uranus, or specks like the numerous body of asteroids and satellites, that were then undiscovered. In a paper written at a later period he found that he had treated as faultless a catalogue of stars which required correction. His conclusions regarding lost or changing stars were thus premature. But neither the poetic beauty nor the possibility of a "lost" star can be denied. Perhaps he was only borrowing a phrase that was used nearly two thousand years earlier by Hipparchus, who, by his catalogue of the fixed stars, gave future generations the means of ascertaining "if stars could be lost and reappear, if they changed their place, their size, their brightness."
Dissatisfied with the principles on which stars visible to the naked eye are classed, according to their brightness, as stars of the first, or second down to the sixth magnitude, he began, about 1782, to adopt a new and more effective but certainly a very laborious method of settling degrees of brightness among the stars, and of determining to what extent the brightness changed from year to year, or from age to age. By this method actual inspection would at once decide whether a star was increasing or diminishing in brightness compared with other stars. It was an attempt to ascertain the advance of life or the vigour of youth, the beginnings of decay or the promise of a long continuance of brightness among the countless suns in creation. Of the importance of these investigations he entertained no doubt, nor should we. "The great number of alterations of stars that we are certain have happened within the last two centuries, and the much greater number that we have reason to suspect to have taken place," are curious features in the history of the heavens, as curious as the slow wearing away of the landmarks of our earth on mountains, on river banks, on ocean shores. "If we consider how little attention has formerly been paid to this subject," he goes on to say, "and that most of the observations we have are of a very late date, it would perhaps not appear extraordinary were we to admit the number of alterations, that have probably happened to different stars, to be a hundred; this compared with the number of stars that have been examined, with a view to ascertain their changes, which we can hardly rate at three thousand, will give us a proportion of 1 to 30 . . . even 1 to 300 is sufficiently striking to draw our attention." These were the words of a wise observer, uttered long before geologists had begun to use similar language in their own researches.
The conclusion which Herschel drew from these alterations, real or imagined, in the light of the stars was that they will "much lessen the confidence we have hitherto placed upon the permanency of the equal emission of light of our sun. Many phenomena in natural history seem to point out some past changes in our climates. Perhaps the easiest way of accounting for them may be to surmise that our sun has been formerly sometimes more and sometimes less bright than it is at present. At all events it will be highly presumptuous to lay any great stress upon the stability of the present order of things; and many hitherto unaccountable varieties that happen in our seasons, such as a general severity or mildness of uncommon winters or burning summers, may possibly meet with an easy solution in the real inequality of the sun's rays." If our sun be a variable star diffusing heat in greater or less degrees at different times, or if it be a star growing old and burning out, the credit of the idea as well as of "lost" stars in the ocean of infinitude may justly be claimed, in our day at least, for this poetic and musical observer of the heavens. To shed a ray of light on this question of sunshine Herschel sought, but sought in vain, for temperatures in ages that were past. He could get none. He was not aware of the thermometers made by the school of Galileo and lost to sight till Libri discovered them, and made them the common property of science. But, resolved not to be baffled, Herschel turned to the rise and fall of the price of wheat at Windsor as an indication of the warmth or coldness of the sun's rays. It was his only resource, and it was an idea worthy of a baffled man of science. But critics in the highest quarters attacked and ridiculed this seeker after truth as if he were guilty of supreme folly. Leaders of thought in every branch of science and in every department of life have to bear the brunt of ridicule from learned ignorance!
These were the first steps taken by Herschel, it may be said, in his quest after the plan on which Almighty Wisdom built the world of suns and systems. A farther step forward was made when he addressed himself to ascertain the motion of the sun and solar system through space. That there was such a motion he did not doubt. Some had held the same faith before him; astronomers as able had refused it a hearing. He converted it from faith to fact. What it means is that our sun with his most distant planet and comet, with every particle of matter that owns his sway, is travelling onward through space, round a centre of force apparently, and constrained by Newton's law of gravitation. Are these facts or fancies, leading features in the plan of creation or dreams of a mere enthusiast? Herschel not only believed they were facts; he set himself to prove it.
When he had proceeded some way in his inquiries, he received from a friend a copy of a catalogue of eighty stars made by Mayer of Göttingen in 1756, "and compared with the same stars as given by Roemer in 1706." Both Roemer and Mayer were men of the highest ability. Previously he knew this catalogue only in an extract which he found in a French book on astronomy. Setting to work on the new material thus furnished, and laying aside thirteen or fourteen of the stars as those he had already examined, he separated the others into two classes, those which went for his view of a motion of the sun through space, and those whose motions "must be ascribed to a real motion in the stars themselves" Mayer, admirable astronomer though Herschel admitted him to be, did not countenance the idea of a motion of the sun with all its planets through space. "Were it so," he wrote in 1760, "were the sun and all the planets and our home, the earth, advancing towards some quarter, all the stars in that part of the heavens would seem to open out, and those in the opposite quarter to come together, just as, when you are walking through a wood, the trees which are in front of you seem to separate from each other, and those which are behind to draw closer." Herschel, seizing on Mayers illustration of trees in a wood, declared that these very changes were taking place among stars in the heavens. At the same time he was encouraged by a short tract sent him by the author. Dr. Alexander Wilson, Professor of Astronomy at Glasgow, and printed in 1777, entitled Thoughts on General Gravitation and Views thence arising as to the State of the Universe. A friendship sprang up between the two men, and Glasgow seems to have become a favourite place of summer pilgrimage to Herschel. It was clear that he was favoured by the flowing tide of scientific thought. He took it at the flood: he even guided it into the channels along which it has since flowed in an ever increasing volume. It "is an arduous task," he said of this quest after our solar system's movement in space, "which we must not hope to see accomplished in a little time; but we are not to be discouraged from the attempt. Let us, at all events, endeavour to lay a good foundation for those who are to come after us." And this good foundation, by precept and example, he did lay.
With the boldness of a man who had confidence in himself and his instruments, he wrote: "I think we are no more authorised to suppose the sun at rest than we should be to deny the diurnal motion of the earth, except in this respect, that the proofs of the latter are very numerous, whereas the former rests only on a few, though capital testimonies." He founded this conclusion on a discussion of the motions observed in seven of the principal fixed stars. But in support of his view he also quoted a table of the proper motions of twelve stars in fifty years given by Lalande, motion in the two directions known to astronomers as right ascension and declination, corresponding to longitude and latitude on the earth. Twenty-seven motions altogether had to be accounted for. On the hypothesis of a general movement of the solar system through space, twenty-two out of these twenty-seven movements were explained. The live exceptions he "resolved into the real proper motion of the stars." He did not then know whether the motion was of one star round a companion star, or round some far greater and immensely more distant sun.
The conclusion which Herschel arrived at was that the whole solar system was at that time moving towards the constellation Hercules in the northern sky, and that the star "λ Herculis is possibly as well chosen as any we can fix upon in that part of the heavens" for the point we are moving towards. He modified this view in 1804 on receiving more correct measurements from the Astronomer-Royal: "It will be necessary to mention that I have no longer supposed the solar motion to be directed towards λ Herculis. A point at no very great distance from this star has been chosen." As the direction of the tangent to the sun's orbit is constantly changing, this change of direction from age to age is unavoidable. He did not attempt to estimate precisely the rate of motion, but, "in a general way," he considered that it "cannot certainly be less than that which the earth has in her annual orbit." At the same time he expected that future astronomers would assist him in determining the direction of the solar motion; and he added that he had "begun a series of observations upon several zones of double stars," with the view of establishing or overturning his hypothesis. His estimate of the rate of the sun's motion may not be correct. Probably it is only from five to nine miles a second, or less than half what he made it: but science has accepted his view of the point, to which the solar system has, for an hundred years, been advancing. Recently α Lyræ (Vega) has been claimed as the point we are now making for.
In the years that followed his first papers on The Construction of the Heavens, Herschel, with wider views, a better instrument, and a clearer insight into what he considered "the Laboratories of the universe, wherein the most salutary remedies for the decay of the whole are prepared," essayed a bolder flight into a world of "things, unattempted yet in prose or rhyme." Stars, clusters of stars, and nebulæ were the building stones, so to speak, out of which Almighty Wisdom constructed the starry sphere around our earth. How many of them exist, what are their relations to each other, and how are they arranged in space? were some of the questions to which he sought an answer. When he began the work of observation, he "surmised that several nebulæ might yet remain undiscovered for want of sufficient light to detect them. . . . The event has plainly proved that my expectations were well founded; for I have already found 466 new nebulæ and clusters of stars, none of which, to my present knowledge, have been seen before by any person." Great though the discovery was, it was only the beginning of others still greater. These nebulæ or little white clouds were similar to the Milky Way in the colour of their light, but apparently of immensely less extent. The first known of them, properly so called, was that of Andromeda, to which the attention of astronomers was directed by Simon Marius in 1612. Others were seen and recorded during the next century and a half, but the Magellanic clouds were visible to the naked eye and formed a striking spectacle in the southern heavens. The Dutch, who saw them in their voyages to India round South Africa, called them the Clouds of the Cape. Astronomers were slowly feeling their way to a fuller knowledge of the "white clouds" they were discovering among the stars. La Caille, when working at a catalogue of about ten thousand stars in South Africa, set down the places of forty-two, which he saw in the telescope. He divided them into three classes; fourteen in which there was no appearance of stars; fourteen which were clearly composed of small stars; and fourteen which combined the characters of both these classes, small stars surrounded or attended by white spots. His labours were published in 1755. Herschel followed at the end of the century, vastly extended our knowledge of these singular objects, and completed the classification which the Frenchman began.
Turning his attention to the broad band of light known as the Milky Way, of which the various nebulæ "seemed to be portions, spread out in different parts of the heavens," Herschel at once solved the puzzle that then divided the astronomical world. Is it the diffused light of innumerable stars, or a shining gas? He describes it as beyond doubt "a most extensive stratum of stars of various sizes"; and "that our sun is actually one of the heavenly bodies belonging to it is as evident." These were two steps forward, but he did not stop with them. He examined that shining zone in all directions with a powerful telescope—a 20-feet reflector—piercing to the borders of its length, breadth, and thickness. He even undertook to count the number of stars he saw. He called this census of stars gauging the heavens. Four years afterwards, he called it analysing them, and spoke of his method as "perhaps the only one by which we can arrive at a knowledge of their construction." He admits, however, that, in course of time, "many things must have been suggested by the great variety in the order, the size, and the compression of the stars as they presented themselves to his view." As the number of stars he counted increased, the brightness of the Milky Way increased; as the number diminished, its apparent brightness to the naked eye diminished also. The law of gravitation he felt certain existed among that vast multitude of suns and systems, just as it exists in pulling a stone to the ground. At first this was mere suspicion. More than twenty years elapsed before he could say it was an established fact.
He continued his review of the heavens, or his gauging of the stars. The results were so marvellous that all the world—men of science, the common people, even children at school—wondered. Sometimes he saw, in a small celestial space, as many as 250, or 340, or 424, or 588 stars; at other times he counted only 3 or 4, 5 or 6. The star-wealth of some of these regions was so vast that in one only 5° in breadth—a very small part of the whole vault of the heavens—there were about 330,000 shining suns or stars! The Chancellor of the University of Halle, who visited Herschel shortly before his death, evidently got from the astronomer himself that he had "often known more than 50,000 pass before his sight within an hour," and he records his own wonder, and the wonder of men generally, while these discoveries were still fresh in their minds, that "after the invention of his instruments, I. H. Schroeter, the celebrated astronomer of Lilienthal, might well compute the fixed stars in the southern and northern hemispheres at more than twelve millions in number."
The average of many hundreds of these gauges gave him what he called "the contents of the heavens." Where the stars were exceedingly crowded, "no more than half a field was counted, and even sometimes only a quadrant"; but the result of these vast labours was that the Milky Way could not be described as other than "a very extensive, branching, compound congeries of many millions of stars; which, most probably, owes its origin to many remarkably large as well as pretty closely scattered small stars, that may have drawn together the rest." Imagination stands appalled at the thought of millions of shining stars, each of the same kindred as our sun, and each, it may be supposed, with a train of habitable worlds like his planets, all circling round their central orb. The littleness of man, the smallness of human life, the meanness of its petty details, that usually fill the whole horizon of human thought, are dwarfed into nothingness in presence of these stupendous realities, till even they become insignificant before the nobler and more inspiring conception of the grandeur of the soul, which measures and weighs these innumerable suns, which takes them up in the hollow of its hand, which deals with them as playthings for its leisure moments, and which says to every one of them, I am greater and of more worth than thou, yes, greater than all your millions put together. "There is no speech nor language where their voice is not heard."
By these star gauges Herschel did a service to the world, for which humanity can never be sufficiently thankful. The plan as well as the labour of thus estimating "the contents of the heavens," and lifting man's mind to a higher level than it ever attained before, were altogether his own, unless we add that his devoted sister Caroline shared the labour and, it must be added, the dangers of the work. What a vista of eternity and infinitude was unfolded by the musician of Bath! It seemed as if he had built a bridge for thought to span the gulf which separates the finite from the infinite, the temporal from the eternal, in this incredible profusion of suns and systems, of inconceivable spaces and times.
Of the length, breadth, and thickness of these strata of millions of stars that form the Milky Way, we have but the faintest conception. Still, Herschel made an estimate, which shows the immensity of space covered by this island of stars in the ocean of infinitude, if we may still presume to speak of it in these terms. "In the sides of the stratum opposite to our situation in it, where the gauges often run below 5, our nebula"—the white cloud called the Milky Way—"cannot extend to 100 times the distance of Sirius." But we know now, what Herschel did not know, that light, which darts from the sun to our earth in eight minutes, takes about ten years at the same rate to travel the distance between Sirius and us. One hundred times that distance would be traversed by light in 1000 years. And, if the farthest-off stars of the Milky Way are nearly five hundred times as far away from our earth as Sirius, the swift messenger who brings us tidings of them would be five thousand years on his journey, and could only tell us what was then taking place, opt what may be happening now. Herschel believed that his telescope sounded space to this and far greater depths without finding traces of nebulosity—gas or star dust—in the regions it reached. He said also that his telescope sounded the depths of past time not less than of space. Be his ideas reality or romance, they give us a sublime conception of the greatness and worth of the human mind buried in its pigmy house of clay, and chafing against the chains that bind it to earth and time.
Sublime though Herschel's conceptions were, he did not conceal from himself or others that "a certain degree of doubt may be left about the arrangement and scattering of the stars" in the Milky Way. They were founded on the supposition of "numberless stars of various sizes, scattered over an indefinite portion of space in such a manner as to be almost equally distributed throughout the whole." This was a large supposition to make; it is not correct, and it was a corner-stone that might be knocked away at any moment. The barriers he required to overleap were the distance and the relative sizes of the stars. These barriers remained insurmountable during his lifetime. It was next assumed, for it could not be said to be proved, that "there is but little room to expect a connection between our nebula"—the Milky Way—"and any of the neighbouring ones; . . . for if our nebula is not absolutely a detached one, I am firmly persuaded that an instrument may be made large enough to discover the places where the stars continue onwards. A very bright, milky nebulosity must there undoubtedly come on." At that time Herschel imagined space to be a vast ocean of light-bearing ether, studded with continents and islands of stars, which he called nebulæ, clusters, or groups. The Milky Way, with its many millions of shining suns, is one of these thickly peopled islands, separated from many others as rich or perhaps richer of worlds, in this infinite ocean. Of these nebulæ or clusters, or star islands, he had, up to that time, counted "more than 900, many of which, in all probability, are equally extensive with that which we inhabit; and yet they are all separated from each other by very considerable intervals. Some there are that seem to be double and treble; and though with most of these it may be that they are at a very great distance from each other, yet we allow that some such conjunctions really are to be found. But then these compound or double nebulæ still make a detached link in the great chain." He fell from some of these views at a later period, wholly or in part.
Herschel delighted in these attractive speculations. In a paper on the power of telescopes to penetrate space, one of the conclusions he came to was that, while his 20-feet reflector "might possibly have reached to some distance beyond the apparent bounds of the Milky Way," his 40-feet would reach stars from which light would take about two millions of years to reach our earth. A ray of light revealing to us the history of stars as it was two millions of years ago! If such things are dreams or miscalculations, they soar into the sublimest regions of mortal thought. More amenable to arithmetic is his calculation, that it will require not less than 598 years, of 100 working hours each, to take a census of the stars by looking with his 40-feet "only one single moment into each part of space, and, even then, so much of the southern hemisphere will remain unexplored as will take up 213 years more to examine." In these numbers Herschel was perhaps mistaken. Struve at Pulkowa found 80 nights suitable out of 120 clear nights; but Sir John Herschel's experience at the Cape of Good Hope gave him the whole or parts of 131 nights in 1836, and at least 100 in the following year. The estimate of 598 years, or rather 811, by Sir William Herschel may be set down as excessive.
Herschel does not appear to have been altogether satisfied with the position he had taken up. It was not warranted by pure and inductive science. The foundation on which alone he could build with confidence had not been laid, the distance of fixed stars and nebulæ. "To these arguments," he says, "which rest on the firm basis of a series of observation, we may add the following considerations drawn from analogy." Science demands something more trustworthy than arguments and analogy. Mathematical science is not content with probability: it demands demonstration, and this he could not give. He had a distinct idea of an ocean, we shall say, of ether, transmitting light. In that ocean are thousands of floating islands, each composed of myriads or millions of shining worlds, all communicating with each other by far-piercing sunbeams. What the telegraphic messages thus sent from sun to sun, from island to island, may be, Herschel had no means at first of knowing. He came to understand and even read some of these messages in later years. We are able to read more of them now, for they tell the sizes of suns, their rates of motion, their direction of motion, and other pieces of star history incredibly interesting to curious man. Herschel did not imagine that this ocean of ether is in any degree impervious to light. His friend Dr. Olbers, of Bremen, suggested that it might be. Precisely as the glass or the horn, through which rays of light pass, keeps part of them back or absorbs them, the infinite ocean of ether may have a similar effect, though in a vastly less degree. This apprehension remains a mere speculation to this day. Sometimes these islands of stars were broken into clusters of stars showing magnificent colours, and forming the most splendid objects that can be seen in the heavens. They seemed to concentrate round a centre. The Milky Way is one of these islands, of which the population consists in suns and worlds. Others, separated from it and from each other, and even apparently changing their shape from age to age, are "generally seen upon a very clear and pure ground without any star near them that might be supposed to belong to them." With all this sublimity of exposition and explanation, Herschel at the same time asks for consideration from critics and readers, "for, this subject being so new, I look upon what is here given partly as only an example to illustrate the spirit of the method."
The idea Herschel formed and then figured of the shape of the Milky Way may be best understood by comparing it to the palm of the hand with only two fingers—the middle and the forefinger—and these stretched fully out. Our sun he supposed to be near the roots of the fingers, looking out into open space through the interval between them. He had the idea also that our star-island "has fewer marks of antiquity upon it than the rest." He believed that its stars "are now drawing towards various secondary centres, and will in time separate into different clusters so as to occasion many subdivisions." In fact, he "ascribes a certain air of youth and vigour to many very regularly scattered regions in our sidereal stratum." He imagined also that "some parts of our system seem to have sustained greater ravages of time than others," so much so that "in the body of the Scorpion is an opening or hole" of at least four degrees broad, through which, as through a window, infinite space can be surveyed outside, till telescopes of greater power pierce the darkness, and, it may be, reveal to our eye Milky Ways in the far Beyond. One of them, near the constellation called the Southern Cross, had long been familiar to sailors in southern seas as the Coal Sack of the Milky Way, a pear-shaped oval almost destitute of stars, with which the regions around are crowded and brilliant. "The purity and clearness of the heavens are remarkable," he says, "when we look out of our stratum at the sides towards Leo and Virgo on the one hand, and Cetus on the other; whereas the ground of the heavens becomes troubled as we approach towards the length or height of it." These troubled appearances seemed to arise "from distant, straggling stars that yield hardly light enough," till, after a long examination of these troubled spots, the eye gets accustomed to the dimness, and the stars that caused the troubling come into view.
When Sir John Herschel went to the Cape of Good Hope in 1833, to survey the southern heavens as his father had surveyed the northern half a century before, his aunt Caroline wrote to him, "It is not clusters of stars I want you to discover in the body of the Scorpion (or thereabout), for that does not answer my expectation, remembering having once heard your father, after a long awful silence, exclaim, 'Hier ist wahrhaftig ein Loch in Himmel!' and, as I said before, stopping afterwards at the same spot, but leaving it unsatisfied." The nephew attended to her wishes, rummaged Scorpio with the telescope, and found many blank spaces "without the smallest star. . . . Then come on the globular clusters, then more blank fields, then suddenly the Milky Way comes on in large milky nebulous irregular patches and banks."
Other Milky Ways than the star-island, to which we belong, "which cannot well be less but are probably much larger," Herschel at one time believed he saw in the white clouds, which float in the depths of space, unseen by the naked eye. Sometimes his telescope resolved them into brilliant star-dust, scattered like shining jewels on the dark background of the heavens: and sometimes not. That they are at immense, at inconceivable distances from the solar system and from each other, is evident. How far, it would be rash to say. But Herschel's enthusiasm overleaped all boundaries of prudent reticence. Some of them may be "600 times the distance of Sirius from us"; other clusters "cannot well be supposed to be at less than six or eight thousand times that distance." Light, the swiftest messenger we know, light, which can journey round the earth eight times in a second, would take six thousand years to bring us a message from the nearest of these clusters, or more than eighty thousand years from the more remote. If his views prove correct, a messenger of wing so swift, and of foot so tireless, may well be regarded as an angel of the Almighty.
Speculations so attractive by a watcher with an eye so keen to detect chinks in the armour, that concealed nature's most secret workings, could not fail to be affected by new facts, as they forced themselves on his observation. He found in course of years that "the hypothesis of an equality and an equal distribution of stars is too far from being strictly true to be laid down as an unerring guide in this research. . . . This consideration is fully sufficient to shew that, how much truth soever there may be in the hypothesis of an equal distribution and equality of stars, when considered in a general view, it can be of no service in a case where great accuracy is required." Fifteen years later he wrote: "When we examine the Milky Way, or the closely compressed clusters of stars, this supposed equality of scattering must be given up." It is clear that, until the distance and mutual relations of the fixed stars were ascertained, mere speculations on their size and brilliance were out of place. He found also that Cassini's classification of nebulæ was at least incomplete or defective. He was leaning to the belief that some of the nebulæ are masses of shining gas, while there may be vast masses or regions of it still dark; but these and other matters must be referred to another chapter. It is enough in the meanwhile to say that twenty-five years of further research wrought a change on the views he once expressed. But they also brought into distincter prominence the changeful character of even the starry heavens. They had wrought no change on the awe with which his contemporaries, however trifling they might be, regarded "the profusion of worlds on worlds" revealed to their view. The immense multiplication of life on our little earth is on the same scale and partakes of the same procedure as this profusion in creating worlds. Unity of design to the remotest bounds of nature is a conclusion that plainly results from Herschel's discoveries.
The worst objection taken to the writings of this midnight watcher was the strange English he sometimes used. "Stupendous as Mr. Herschel's investigations are," Horace Walpole wrote to a friend, "and admirable as are his talents, his expression of 'our retired corner of the universe' seems a little improper. When a little emmet, standing on its anthill, could get a peep into infinity, how could he think he saw a retired corner in it? . . . If there are twenty millions of worlds, why not as many, and as many, and as many more? Oh, one's imagination cracks!" "To the inhabitants of the nebulæ of the present catalogue," Herschel wrote, "our sidereal system must appear either as a small nebulous patch; an extended streak of milky light; a large resolvable nebula; a very compressed cluster of minute stars hardly discernible; or as an immense collection of large scattered stars of various sizes." Well may we repeat in sobriety and humility what the poet, in contempt and fun, uttered about the same time,
"Oh wad some Power the giftie gie us
To see oorsels as ithers see us."
The last two papers which Herschel wrote on The Construction of the Heavens were given to the world about four years before his death. They show the same grasp of details, the same enthusiasm in working, out a lofty theme, the same insight into general principles, as illumined the first paper he wrote on the subject thirty-five years before. Although his sun was nearing its going down, there was no loss of its morning brilliance. "Of all the celestial objects consisting of stars not visible to the eye," he writes, "the Milky Way is the most striking. . . . Its general appearance, without applying a telescope to it, is that of a zone surrounding our situation in the solar system, in the shape of a succession of differently condensed patches of brightness, intermixed with others of a fainter tinge." But his latest observations led him to believe that the Milky Way is a fathomless, and comparatively thin stratum of stars, of which his 40-feet reflector would sound the depths "to the 2300th order of distances and would then fail us." He imagined also he had "shown how, by an equalisation of the light of stars of different brightness, we may ascertain their relative distances from the observer, in the direction of the line in which they are seen." Among these last words was his expressed conviction that the Milky Way is the most brilliant, and beyond all comparison the most extensive sidereal system. He thus held to the end that it was one of many systems, of which it bulked in his eyes as a great continent in an ocean of ether, while the nebulæ are outlying islands. Within the bounds of the Milky Way he believed that all our stars, visible to the naked eye, are contained. If an 18-inch globe represented all these stars, it would require a line 45 feet long to be added to express the distance of the 734th order of stars, and, while he saw many of the 900th or 980th order, he was convinced that his 40-feet telescope would penetrate space to the 2300th order. We can only say with Horace Walpole on looking at these figures, One's imagination cracks! But definite distances had not been determined then, and are not determined yet.
Whether these be the dreams of an enthusiastic romancer, or the sober facts of science, there can be no doubt that the observations on which they rest are a delightful mixture of poetry and scientific truth. Thickly strewn over the pages of a scientific memoir are such entries as these: "The stars are so exceedingly close and small that they cannot be counted"; "a beautiful cluster of stars"; "stars are so small that I can but just perceive some and suspect others"; "light without stars"; "a brilliant cluster"; "a coarse cluster of large stars of different sizes"; "a rich cluster of very compressed stars." The wealth of the heavens passes both the language and the comprehension of man. Star-dust, sparkling with more than diamond lustre on the dark background of the heavens, has become a common figure of speech. Jewels of silver, jewels of gold, rubies, diamonds, and sapphires are seen in admirably distinct disorder in the great mirror of the telescope. The prose of the heavens surpasses the brightest poetry of earth.
Whether William Herschel was justified in holding to the theory of an ocean of ether with thousands of dimly seen Milky Ways floating about in it, or whether he modified his view into a belief that the starry worlds, seen from our earth, are parts of a connected whole, is of little consequence in these days. Perhaps he was himself in doubt which view to take. But he was nearer to realising infinitude of space and eternity of time—if the phrase be allowable—than any man ever was before him. He marks an era in the progress of human thought and experience, for his words leave on the mind of a reader an awful impression of unspeakable vastness in space and time, of multitudinous arrangements for working out with singular ease and success some vast whole, and of undiscovered purposes in the designs of a Being to whose nature ours is of kin, though we feel ourselves to be but nothings, or less than nothings, in His presence. To ignore or deny this impression is to do an injustice to humanity.
- This is Herschel's own phrase, taken probably from the notice of Ptolemy's Almagest (145 A.D.) in Lalande's Astronomy (1771 A.D.), where its title is given in Latin, Magna Constructio (i. 156). The phrase does not deserve the condemnation it received from an Edinburgh Reviewer in January 1803; but a later Reviewer accepts it in July 1848, "to use a phrase which Sir W. Herschel introduced" (p. 105). "Introduced" is scarcely correct.
- Sir David Brewster in his edition of Ferguson's Astronomy (1823), ii. 298. He is referring specially to nebulæ, of which Herschel "observed the position, magnitude, and structure of no fewer than 2500."
- Lalande, i. 152; Pliny, ii. 26.
- Phil. Trans. (1796), pp. 455-56. Professor Holden gives the numbers as 3, 5, 6, 7, 334, 404, and 494 (p. 133).
- The most ancient catalogue of the stars is that of Ptolemy (140 A.D.) of Alexandria, which was probably a revised transcript of that of Hipparchus (160 B.C.). It contains 1022 stars. Tycho's catalogue (1572 A.D.) contains 777 principal stars, to which Kepler afterwards added 280, taken probably from Tycho's own manuscripts. Hevelius (1690 A.D.) published a catalogue containing 950 stars of former lists, 603 observed by himself, and 377 southern stars observed by Halley from Saint Helena. "But the most perfect and the largest catalogue which had been made" was the British catalogue published by Flamsteed in 1712, and afterwards in better condition in 1725. It contains about 3000 stars. See Lalande, i. 284.
- The tables he took advantage of were those given by Adam Smith in The Wealth of Nations. The ridicule that was heaped upon him may be seen in the Edinburgh Review, and in a letter signed J. M., Scots Magazine, 1807, p. 829.
- Phil. Trans., pp. 249, 247 (100 times), 497 times.
- "Here indeed is a hole in the Heavens!"
- Letters, vi. 461, 258.
- Phil. Trans., 1818, pp. 437-50.