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.