The progress of discovery during the past century has been so rapid, and, compared to the previous ages of the world, so epoch-making, that not a few recognized thinkers in different lines of scientific research have expressed the opinion that the age of really great discoveries has passed; that what remains to be done is the perfection of the sciences rather than the laying of foundations for future development along new lines. These critics appear to consider the temple of human knowledge essentially complete in its general aspect, with perhaps a pillar to be placed here and a pediment to be completed there, but with no great wings or new outlines yet to be disclosed to coming ages. Such an opinion is logically a conceivable one, and in some lines of scientific activity is indeed justified by the present state of our knowledge. The sciences of elementary geometry and human anatomy, for example, are well-nigh finished; and the same is true of ancient biography and Homeric criticism, at least until new discoveries shall alter the present aspects of these subjects. The history of Greek philosophy and of Greek art affords little or no field for new explorations, after the century of searching inquiry to which it has been subjected by German scholarship; but obviously the same degree of exhaustion has not been attained in other notable fields of historical criticism, particularly in those pertaining to the Middle Ages. The achievement of ideal perfection, a state of development permitting of imitation, without material improvement or hope of higher ideals—such, for example, as followed the epoch of Phidias in sculpture and that of Raphael in painting, and has so frequently been exhibited in the literature and architecture of different ages and countries—is seldom, if ever attainable in the natural sciences, which admit of greater and greater perfection, wider and wider extension. This essential distinction between art and literature, on the one hand, and the physical sciences, on the other, was clearly pointed out by Laplace about a century ago, and has perhaps been more or less realized by the principal scientific thinkers of every age; yet so great an authority as Professor Haeckel, of Jena, has only recently taken a somewhat different view, and declared that the work of the future will consist mainly in perfecting the structure of the sciences on their present foundations.
It cannot but appear a little remarkable that so lucid a thinker as Professor Haeckel should take so inadequate a view of the future of the natural sciences. Possibly his own experience in stretching the theories of organic evolution somewhat beyond their natural limit, and the reaction which inevitably followed in the minds of conservative biological thinkers, may have contributed to this temper of mind. Whatever be the causes—and the uncertainty of the theories of life, and the resulting unsatisfactory state of the many biological inquiries, is evidently one of them—it seems that Haeckel's criticism is of doubtful validity as regards the exact sciences of mathematics, astronomy, physics, chemistry, mechanics, geology, or indeed any of the natural sciences.
When science has attained a definite state of development, it frequently is not possible to assert in what direction a new advance will take place; even the most penetrating and discerning minds will often view a subject from different standpoints. But as regards general progress in some direction, I am not aware of any philosophic authority who regards the natural sciences as either finished or nearing completion, even in the matter of principles, still less in the matter of applications, and of verifications relative to the infinitely varied phenomena so abundantly diffused throughout nature. Rash as it may appear to some, I, for one, believe that all the physical sciences are still in their infancy, and that a considerable number of the generalizations now provisionally accepted are destined to be cast aside when more light is shed upon the true phenomena of the physical world. Such has been uniformly the result of past experiences, and a similar outcome is strongly indicated by fresh discoveries in many lines. There is indeed nothing in recent progress to indicate that the resources of the human mind have been exhausted. We are, beyond doubt, still profoundly ignorant of most great natural phenomena; and any attempt such as Herbert Spencer has made to write the sum and substance of the final philosophy will necessarily be in a very large degree a failure. It may, however, serve some such purpose for our times as the writings of Aristotle did for those of the Greeks. This ancient Greek philosophy does not look well in the light of modern research; and so it will be with any attempt now made to write a final philosophy, even in the light of the experience of the nineteenth century, which is distinguished above every other age of the world for the output of exact scientific knowledge. It will be the aim of the following pages to point out the tendency of some recent discoveries in astronomy, and to indicate their probable bearing upon our general conceptions of the physical universe.
A word should first be said in relation to the division between the great sciences of astronomy and physics. The consideration of the luminiferous ether, common hypothesis of both, is usually regarded as a branch of physics, but it also has a very important bearing on astronomy, as the properties of the ethereal medium are based largely on phenomena, derived from the observation of the heavenly bodies. The velocity of light was discovered by Roemer in 1675, from the observations of the eclipses of Jupiter's satellites, which recurred in such a way as to show conclusively that about sixteen minutes are required for the propagation of light diametrically across the Earth's orbit. Roemer's discovery is generally conceded to be one of the most remarkable in history. Besides exerting a great influence on the philosophy of the sciences, it has led, during the last half of the nineteenth century, to some of the finest physical experiments of all time. Sixty years before Roemer's memorable achievement the immortal Galileo had discovered the satellites of Jupiter by means of a telescope of his own construction, which consisted of a simple lens, and an eyepiece fitted in a leaden tube about two inches in diameter and some three feet in length. In accordance with the doctrine of the ancients, handed down from time immemorial, the velocity of light was then supposed to be infinite; and Galileo naturally had no idea that the moons of Jupiter might be used for investigating its rate of propagation, which in fact proved to be so rapid as to be practically instantaneous for all terrestrial distances, and could probably never have been discovered but for the fortunate use of the satellites of Jupiter, whose distance from us varies about 186,000,000 miles during the year, owing to the orbital motion of the Earth about the Sun.
In the first half of the eighteenth century another capital discovery was made by the English astronomer Bradley, known as the "aberration of light," which confirmed the discovery of Roemer, in showing that the motion of the Earth had an apparent effect on the places of the fixed stars, each luminous point describing a small ellipse during the year, as a result of the combination of the Earth's orbital motion with that of the light from the celestial objects. The discovery of the aberration of light proved of high importance for exact astronomy, and in turn gave the physicists some ground for hope of devising experimental means for measuring the velocity of light by suitable apparatus upon the surface of the Earth. This was accomplished during the nineteenth century by a number of eminent men: first by Fizeau, then by Foucault and Cornu, in France, and finally, in America, by Michelson and Newcomb, whose classic determinations have come into use among men of science everywhere.
The studies on the velocity of light have led to experimental searches for motion of the ether near the surface of the Earth. Though great pains have been bestowed upon these inquiries by Michelson and Morley in this country, and by Lodge in England, and perhaps by others, it has not yet been possible to prove that the ether near the Earth's surface suffers any change owing to the forward motion of the Earth in its orbit; nor has it been possible to communicate to the ether between two disks, revolving with the utmost rapidity and separated by a sensible space, any motion whatever. These experiments are thought to offer an apparent contradiction to the observed phenomena of aberration, and some further investigations of a thoroughgoing character will be required to throw light upon the cause of the discrepancy. In view of such phenomena, it is perhaps unnecessary to point out how intimate is the relation between the sciences of astronomy and physics, and how little of this common ground has yet been occupied. That the luminiferous ether fills the visible heavens, and is a medium of like nature and qualities throughout, seems established beyond doubt by the appearances of the stellar universe. It seems to transmit the light from the most distant stars, without sensible loss, due to imperfection of the intervening medium; and if any light is absorbed, it must come, according to Professor Brace, from all wave lengths alike, so that the most distant stars exhibit no increase of coloration relative to those comparatively close to our planetary system.
Moreover, Lord Kelvin has brought to light a very singular fact—that the properties of the luminiferous ether correspond closely to those of an elastic solid. Although usually spoken of as a fluid, it acts like a solid, a true jelly, transmitting all vibrations communicated to it almost perfectly. In proportion to its density it is exceedingly rigid, a veritable elastic solid; and so great an authority as Lord Kelvin has even suggested that the medium may be occasionally broken or cracked by the violent shocks to which it is subjected by material bodies. It does not seem to be affected by the attraction of gravity from such bodies as the Sun and planets, but appears to be equally dense in all parts of space, without regard to the presence of ponderable masses, which are scattered very unequally in different parts of the sky. Dr. Thomas Young held the opinion that the ether might be so continuous a medium that it passed through or around ordinary material bodies in motion, like a stream of wind through the tops of the trees. In this way he accounted for the seeming common motion of all portions of the ether near the surface of our terrestrial globe. It would thus be undisturbed by the motion of material bodies through it; the ether would freely press round and allow them to pass. There appears to be still some serious defect in our knowledge of the aberration, since different methods give somewhat different results. One cannot but feel that in these discrepancies lies a beautiful discovery, awaiting the attention of a patient and thoroughgoing investigator.
Notwithstanding the great importance of the general subject of the luminiferous ether for both physics and astronomy, it must yet be conceded that the branch of physics of most direct bearing on astronomy is a particular development of the wave theory, known as "spectrum analysis," or "astrophysics," which is scarcely half a century old. Everybody remembers Newton's decomposition of white light into the primary colors of the spectrum, and how he afterwards verified his experiment by reversing the operation, and again obtained white light by recombining the separate colored rays into one white beam. For a century after this famous achievement the progress of pure and applied optics was very considerable; yet it seems that no one possessed the apparatus for, or had considered the careful study of, the spectra of the actual heavenly bodies. About 1826, Wollaston, of Edinburgh, approached the subject from a new point of view, and obtained some remarkable results. His work was soon to be superseded, however, by the great explorations of Fraunhofer, of Munich, who combined theoretical and practical optical knowledge with a mechanical talent of the highest order, which together created a new epoch in the manufacture of prisms and lenses for achromatic telescopes, and led him to recognize for the first time the great variations and even the distinct classes among the spectra of the heavenly bodies.
The subsequent invention of the spectroscope by Kirchhoff and Bunsen, about 1860, laid a new foundation for the physics of the heavens. These distinguished investigators were the first to study attentively the spectrum of the Sun, and to inquire into the spectra of a multitude of terrestrial substances subjected to experimentation in the laboratory. When the spectra of various bodies, such as sodium, lithium, iron, magnesium, and hydrogen, were studied, it was found that each substance had a characteristic spectrum, and when incandescent consisted of bright lines having definite positions in the spectrum; that is, the light consisted of vibrations from molecules oscillating in particular periods, and thus having particular wavelengths. And it was found that the substances in the sun gave dark lines in the place of the bright ones produced by the flames in the laboratory. The identity of the substances in the Sun and on the Earth was clear enough on grounds of probability; and the darkness of the lines in the solar spectrum was readily explained by the absorption of the solar atmosphere, which cut down the intensity of the vibrations without altering the periods.
No sooner had Kirchhoff and Bunsen laid the foundations of spectrum analysis than it occurred to Sir William Huggins, then a young man, to apply the new method of research to the study of the heavenly bodies generally. As early as 1864 he had examined the spectra of the Sun, Moon, planets, numerous stars, nebulae, and even comets, each of which had its own peculiar type and an interest commensurate with the novelty of the subject. The subsequent history of astrophysics, always under the leadership of Sir William Huggins, and shaped by the special researches of Secchi, Rutherford, Draper, Vogel, Young, Langley, Pickering, Jansen, Thollon, and others, need not be recounted here; perhaps it will suffice to say that the new science experienced such rapid growth that it now occupies the attention of nearly one third of the observatories of the world.
In the year 1840, Christian Doppler, of Prague, announced, as a result of his studies in the wave theory of light, that stars moving toward us would give more light waves per second, and stars moving away fewer waves per second, than an ideal star at rest relative to the Earth. The result would be a slight modification of the spectra of all bodies moving toward or from this planet. This would have the effect of shifting all the lines in the star spectrum by a slight amount; and if the amount of this displacement—which is toward the blue for stars approaching, toward the red for stars receding—could be accurately determined, it would afford a measure of the velocity of approach or recession. This problem of measuring the motion in the line of sight was taken up by Sir William Huggins in 1867. By using as a basis of comparison the spectral lines of hydrogen, iron, and other substances volatilized in the laboratory, it was possible to determine with the utmost nicety the amount of motion in the line of sight. At that time, however, all comparisons were necessarily made by the eye, since photography had not yet been applied to the study of spectra. The extreme difficulty of measuring by eye observation the slight displacement of faint and often hazy lines was such that it is not surprising that the early work of Huggins proved to be qualitative rather than quantitative. Yet there was no difficulty in showing by these early experiments that the method would eventually be capable of great possibilities. And during the past twenty years these anticipations have been more than fulfilled by the large and unprecedented developments of spectrum photography.
With the largest telescopes, it is now possible to photograph and measure for motion in the line of sight spectra of stars as faint as the sixth or seventh magnitude. This would give for both hemispheres some six thousand stars which could be used for determining this important element. Up to the present time probably not more than six hundred stars have been measured in this way; yet these few objects, about one tenth of the number which can be measured with existing instruments, have yielded results of the greatest interest. Most of these results have been achieved at the Lick Observatory, in California, by Professor W. W. Campbell, the distinguished American astronomer, whose discoveries bid fair to constitute a veritable epoch in modern astronomy.
Before taking up the details of this work, however, we must allude briefly to the present state of double-star astronomy, which is intimately connected with Campbell's famous work at the Lick Observatory. It is well known that the science of double stars was founded by the illustrious Sir William Herschel, about one hundred and twenty years ago. While observing closely associated stars for relative parallax, due to the orbital motion of the Earth, he accidentally discovered that certain double and triple stars constitute genuine double and triple systems; and the lapse of twenty years showed that their components move in ellipses, and obey the same laws as Kepler had found to hold true in the solar system. This implied with the highest degree of probability that the law of gravitation is really universal, and not confined in its application to the bodies revolving about our Sun, whose motions had been so fully studied by the immortal geometers Newton and Laplace. The pioneer work of Sir William Herschel on the double stars of the northern hemisphere was extended by his son, Sir John Herschel, to the double stars of the southern hemisphere. This hurried survey was completed at the Cape of Good Hope between 1834 and 1838. This latter year is famous, also, for the publication by the renowned William Struve, of Poulkowa, of his monumental work on 3112 double and multiple stars measured at Dorpat, Russia, between 1824 and 1837. In this splendid work we have the first secure foundation of an exact knowledge of the stellar systems within one hundred and five degrees of the north pole. It has since been supplemented by the explorations and measures of Otto Struve and Glasenapp, in Russia; Dembowski and Schiaparelli, in Italy; Burnham and Hough, Hall and See, Hussey and Aitken, in America; and finally by those of Russell, of Australia, and Innes, of the Cape of Good Hope.
All together, something like eleven thousand double stars have now been catalogued; but of this total number only about five thousand are of real permanent interest. In the explorations which have been made to discover and measure these five thousand important double stars, probably not less than one million of the brighter stars of the heavens have been examined with powerful telescopes. If we could suppose that no double stars were overlooked in this examination, this result would indicate that on the average one star in every two hundred is an important double. The experience of the writer, who examined something like two hundred thousand fixed stars in the southern hemisphere, would indicate that at least one in every hundred, under the average conditions, is double; while under the best conditions to be had in the dry climates of Arizona and at the City of Mexico, the indications were that one in twenty-five might be resolved with the twenty-four-inch refracting telescope of the Lowell Observatory. This would indicate that, under the best conditions afforded by modern instruments, four out of every hundred stars are probably double, and could be so recognized by exhaustive study, in a clear, dry climate, with a good telescope. Our search for double stars was usually confined to the brighter objects for two reasons : (1) they are the most interesting on general grounds, as being on the average the closer members of the sidereal system; (2) the closer members of the sidereal system will be the more easily separated into their constituents, since the remoter the object, the smaller will be its angular separation as seen in the telescope. From these considerations it appears that while our explorations have been confined chiefly to the brighter stars, and have been more thorough in the northern than in the southern hemisphere, yet there is not the slightest doubt that if we had sufficiently powerful telescopes, and could use them efficiently through the disturbing atmosphere which covers the globe, we should find double stars, genuine stellar systems, extending to the utmost bounds of the sidereal universe.
In this connection attention may be called to the great desirability of having a large telescope in the southern hemisphere, for the study of an extensive zone around the south pole which is still very largely unexplored. It is a misfortune, hitherto apparently unavoidable, that nearly all the principal instruments of the world are in the northern hemisphere, which includes the great civilized nations of the Earth, and the only peoples devoted to the cultivation of the sciences. The result is that a large space, beneath our horizon, round the southern celestial pole, including three eighths of the celestial sphere, and incomparably rich in objects of surpassing interest, is almost as little known as the antarctic continent. A few of the more obvious phenomena have been studied, either hurriedly or with inferior instruments, and enough attention has been given to the contents of that part of the universe to assure us of its exceeding richness; but there has been no general and exhaustive survey of that part of the sky, such as is demanded by the present state of our knowledge of the northern heavens. The largest telescope in the southern hemisphere is an eighteen-inch refractor at the Cape of Good Hope, where, unfortunately, the climate is so poor that little can be done in the way of discovery; while the northern hemisphere has at least twenty telescopes of greater power than any one in the southern hemisphere.
The dry climate and elevated plains of Peru offer atmospheric conditions probably unsurpassed on the face of the terrestrial globe; and this location above all others is to be recommended to the builders of our future great telescopes. Explorations in this region will be pioneer work; their value to the future progress of astronomical science will be priceless. The Harvard College Observatory, fully alive to the advantage of this southern location, already has a magnificent station at Arequipa, Peru, devoted to the photographic study of the southern stars and their spectra. Discoveries of the highest interest have recently been made at this site, which, it is interesting to note, was recommended by Alexander von Humboldt nearly a century ago. In his account of the exploration of the countries west of the Andes he points out that this is a dry and elevated plain, where the air is so steady that the stars scarcely twinkle when at any considerable elevation, but rather shine with a steady lustre, like the planets in our own climate. This steadiness of the atmosphere enables the telescope to perform to its full theoretical capacity, and would enable one powerful instrument in Peru to do more important work of discovery than a dozen great telescopes in the northern hemisphere.
So far as can now be estimated, it is safe to say that several thousand new stellar systems of great interest would be disclosed by an adequate exploration of the zone within sixty degrees of the south pole, which includes the constellations Scorpius, Centaurus, Lupus, Crux, Toucana, Grus, Eridanus, Corona Australis, Phoenix, and the great ship Argo, besides many less famous groups. The two wonderful Magellanic Clouds adorn this area, and the equally renowned voids known as the Coal Sacks. These latter are so named because they appeared to the early navigators as black holes in the densest portion of the Milky Way, near the Southern Cross. It is difficult to overestimate the high interest attaching to this part of the sidereal universe, which in point of variety of remarkable objects surpasses in importance every other portion of the celestial sphere. No area of the same extent in either hemisphere has so many promising objects for exploration, and no other portion of the sky is so truly a coelum incognitum. Under the circumstances, it cannot be considered singular that all astronomers hope for the early exploration of this interesting region by a powerful telescope, which will alone enable us to form a correct estimate of the extent and variety of bodies composing the material universe.
The observations since the time of Herschel show that the double stars obey the law of gravitation. This law, being established for many individual cases, is inferred to be true universally; and hence, in the few instances where certain anomalies appear, it is inferred that the regular motion is disturbed by unknown bodies, usually dark and wholly unseen. The discovery of double and multiple stars from the effects of the gravitational attraction on their luminous components is known as the "Astronomy of the Invisible." It was first suggested by the illustrious Bessel about 1840, to account for certain irregularities in the proper motions of the two dog stars, Sirius and Procyon; both of which have since been shown to be real binaries, the bright stars being in both cases attended by faint but massive satellites. More recently, Professor Seeliger, of Munich, Mr. Lewis, of Greenwich, the writer, and others have added to the Astronomy of the Invisible by showing that certain double stars are in reality triple, with one component yet to be disclosed. But the greatest extension of the Astronomy of the Invisible has been made by Professor Campbell, of the Lick Observatory. In the course of the regular work on the motion of stars in the line of sight, carried out with a powerful spectroscopic apparatus presented to the Observatory by Hon. D. O. Mills, of New York, he has investigated during the past five years the motion of several hundred of the brighter stars of the northern heavens. The velocities toward and from the Earth developed in different cases were, of course, very different; and with this splendid spectrograph, which Professor Campbell has used with decisive effect, the accuracy attainable is little short of marvelous. An error in the final result of one mile per second is quite impossible. With such unprecedented telescopic power and a degree of precision in the spectrograph which can be safely depended upon, it is not unnatural that some new and striking phenomena should be disclosed. These consisted of a large number of spectra with double lines, which undergo a periodic displacement, showing that the stars in question were in reality double, made up of two components, moving in opposite directions—one approaching, the other receding from, the Earth. There were thus disclosed spectroscopic binary stars, systems with components so close together that they could not be separated in any existing telescope, yet known to be real binary stars by the periodic behavior of the lines of the spectra so faithfully registered on different days by the powerful Mills spectrograph attached to the thirty-six-inch telescope at the Lick Observatory. Some of the more famous of these new stars are Capella, Polaris, Xi Ursae Majoris, Kappa Pegasi, Castor, Spica, Algol, Beta Lyre, and Eta Aquilae. In all, about fifty such stars are now known.
It appears from the investigations so far made that the brilliant star Capella is made up of two nearly equal components, which revolve in a period of one hundred and four days. The period of Polaris is about four days. In other cases the periods vary according to the objects: some being very short indeed, say only two days; others amounting to a considerable portion of a year, or even as much as three years in the case of Beta Capricorni.
It should be pointed out that these are not indeed the first spectroscopic binaries ever discovered. Professors Pickering and Vogel led in the initial search for these remarkable objects; yet with the means at their disposal they found only a few isolated examples, such as Beta Aurigae, Alpha Virginis, and Zeta Ursae Majoris. Campbell's work at the Lick Observatory derives increased importance from its systematic character, which enables us to draw some general conclusions of the greatest interest. He has thus far made known the results of his study of the spectra of two hundred and eighty of the brighter stars of the northern heavens. Out of this number he finds thirty-one spectroscopic binaries, or one ninth of the whole number of objects studied. Professor Campbell also points out that as some of the stars are multiple in character, composed of three or more components, with periods ranging from a few days to a year, or even several years, it cannot be assumed that all the spectroscopic binaries have been found in the first study of his photographic plates. In fact, it seems certain that a more thorough study will materially increase the number of spectroscopic binaries; and Professor Campbell thinks one sixth, or even one fifth, of all the objects studied may eventually prove to be binary or multiple systems. Such an extraordinary generalization opens up to our contemplation an entirely new view of the sidereal universe. If there be five or six thousand stars in both hemispheres which are sufficiently bright for study with the powerful apparatus now in use at the Lick Observatory, it will indicate that there are at least one thousand spectroscopic binary stars awaiting exploration—a number of stellar systems decidedly inferior, to be sure, to those of the visual class, yet undeniably impressive, and ample for furnishing us the general laws for all such objects, seen and unseen, throughout the immensity of space. If the labors of the next twenty years should give us accurate knowledge of even forty spectroscopic binaries, these would enable us to obtain a good estimate of the probable character of all such systems whatsoever. So far as they have been studied, it appears that the double stars observed visually in our telescopes are remarkable for two chief characteristics : (1) the high eccentricities of their orbits, which average about 0.5, or are twelve times larger than the eccentricities prevailing in the solar system; (2) the masses composing the systems, which are equal or comparable, not enormously disproportionate, like those of the planets relative to the Sun, or those of the satellites relative to the planets about which they revolve. Thus the stellar systems heretofore discovered are of a very different type from what we find in our own solar system, where the satellites are insignificant compared to the planets, and the planets insignificant compared to the Sun, and all the orbits nearly circular. And the number of such stellar systems, both visual and spectroscopic, appears to be truly enormous. Campbell finds that the general characteristics of high eccentricities and comparable masses, first attributed to double stars by the writer of these lines, some years ago, are true also of the spectroscopic binaries, which therefore are likewise of a different type from anything found in the solar system.
Since our telescopes do not enable us to recognize bodies anything like as faint as the planets attending the fixed stars, it is obviously impossible to affirm that no other systems similar to the solar system exist in the immensity of space; yet it is very clear that a vast number of systems of a radically different type are widely diffused. Some of these systems are self-luminous, like ordinary double stars; others probably are burnt out and already comparatively dark, so that they are correctly classed with the Astronomy of the Invisible; while yet others are spectroscopic in character, composed of one, two, or more associated bright and dark bodies revolving under the action of their mutual gravitation.
If we accept the conclusion that with our finest telescopes, in the best climates, on the average one star in twenty-five is visually double, it will follow from Campbell's work on some three hundred stars that five times that number are spectroscopically double. Thus, although over a million stars have been examined visually, and some five thousand interesting systems disclosed by powerful telescopes, the concluded ratio would give us, at last analysis, four million visual systems among the hundred million objects assumed to compose the stellar universe. On the other hand, the large ratio of spectroscopic binaries to the total number of stars examined by Campbell would lead us to conclude that in the celestial spaces there exist in reality no less than twenty million spectroscopic binary stars! Could anything be more impressive than the view thus opened to the human mind? Millions and millions of systems, of all sizes and representing all stages of cosmical evolution; with light, dark, and semi-obscure masses, all moving in orbits of considerable eccentricity, and by gravitational attraction generating in their fluid globes enormous bodily tides, which, working and reacting through the ages, modify the shape and size of the orbits and the stability of the systems! Since there are doubtless many millions of dark bodies, both large and small, as yet wholly unseen and even unsuspected, it seems not unreasonable to suppose that probably the great majority of the stars are in some way attended by satellites. The mass of matter composing the stupendous arch of the Milky Way is thus very much greater than has been supposed by those who have enumerated the stars disclosed by our telescopes, and computed the total amount of it on the assumption that all of the star dust is luminous.
It may indeed well be that the dark and unseen portion of the universe is even greater than that which is indicated by our most powerful telescopes. Half a century ago Bessel remarked: "There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others."
If, therefore, certain stars are called "runaway " stars, because their velocities appear to be too great to be accounted for by the attraction of the luminous bodies composing the sidereal universe, we should perhaps ask whether the unknown mass of matter scattered throughout space as dark stars, comets, meteors, and nebulae might not, after all, account for the discrepancy. For my part, I am satisfied that it probably would, and that the universe is much more massive than has been generally supposed. In this fact will doubtless be found the explanation of the great velocities of the runaway stars.
These discoveries shed an interesting light upon the general theories of the material universe, and show that the ultimate exploration of the heavens has, in fact, only begun. Moreover, it is now recognized that the self-luminous stars are fluid masses, and therefore binaries are of necessity agitated by tidal oscillations. In considering some recent observations bearing on this subject, Campbell has found in certain subsidiary displacements of the spectral lines of a few binary stars evidence of the enormous tidal waves which sweep over their flaming globes.
It is well known that our original conception of tides arose from the oscillations in the waters covering the Earth, first noted by the early navigators of our seas. These periodic motions of the oceans were correctly explained by Newton in 1687. The theory of the tides has since been placed on an adequate mathematical basis by the labors of numerous geometers; and as the law of gravitation is shown to hold among the double stars, we assume that the rotations and orbital motions of such systems are disturbed by the gigantic tidal waves generated in their globes of flaming fluid. Some years ago I explained in this way the high eccentricities of the stellar orbits, and, following the younger Darwin, pointed out tidal friction as a physical cause operating with more or less effect throughout the heavens. Since the generation of bodily tides depends merely on the mutual attractions of two connected fluid globes, the resulting tidal effects are obviously as universal as gravitation itself.
For the natural philosopher to be enabled to ascend from the comparatively minute and unimportant oscillations of our terrestrial seas, generated by the attractions of the Sun and Moon, to the bodily tides in the stars composing the Milky Way, which are great pulsating globes of self-luminous fluid; and to trace in this manner the effects of tidal friction, which with the flight of ages has enlarged and elongated the orbits of double and multiple stars, is a generalization which at least need cause no feeling of humiliation! A chain of reasoning connecting such grand phenomena may justly impress a philosopher of any age or country as alike honorable and gratifying to the human mind. And since this achievement is of comparatively recent origin, it may be cited as a specific proof that all the great generalizations of nature are not yet accomplished. Far from it!
Though three hundred years have elapsed since the death of Tycho Brahe, and the scientific world has only recently joined in celebrating worthily his immortal memory, it appears that we are in many lines almost as far from the ultimate goal as when he began the great work of exploring the skies, before the days of Kepler, when all Europe was slumbering in intellectual darkness. The science of the stars, indeed, has been refined and perfected in an unparalleled degree, and infinitely extended in all directions; but with the bounds of darkness pushed back step by step, the goal is not, and never will be, in sight. An infinity of objects and causes and an endless variety of phenomena are yet to be explored, and the work of the mind is rather a process of development to the perfect understanding of the universe than the solution of a simple mathematical problem. We cannot therefore subscribe to the doctrine announced by Professor Haeckel. If we did so, we should come back to the mental position of the schoolmen of the Middle Ages and of the unproductive Arabians. With them, the most that an acute and daring mind could hope for was to comment on the writings of Plato and Aristotle, or perhaps remeasure the earth and catalogue the stars by the methods of Ptolemy. Such an attitude indicates a mental condition unaccustomed to, and without hope of, solid progress, ill fitted to cope with real philosophic problems, such as have been handled successfully by the great natural philosophers of the past three centuries. And for my part, also, I am unwilling to believe that the universe is so simple or so easily exhausted that even a great number of the acutest minds could unravel its principal mysteries in a few centuries, flattering as such an achievement would be to the age in which we live.
It may be said that in some lines of applied science we have indeed well-nigh reached the appointed goal. Within the memory of this generation the Earth has been girdled with iron and steel, and the electric telegraph and the cable have practically annihilated terrestrial space: these modes of communication have come to stay, and they are ultimate. Whatever be the future progress of the world, it seems certain that nothing more rapid or more general will ever be used by the children of men. The velocity of electricity is the same as that of light, and no swifter messenger is possible or even desirable. The same approach to ultimate standards of speed may be observed in other lines of activity, as railroading and navigation, where the limits are fixed by the nature of organic life and by the physical properties of matter. But such physical limits do not restrict the powers of the mind for researches in pure science, whether in the biological or in the physical world. And if we continue to make discoveries throwing light upon the phenomena and principles underlying the arrangement and growth of the universe, who can doubt that some of them will augment continually the mental and physical comforts of mankind?