CHAPTER V.

THE INFLUENCE OF HERSCHEL’S CAREER ON MODERN ASTRONOMY.

The powers of the telescope were so unexpectedly increased, that they may almost be said to have been discovered by William Herschel. No one before him had considered the advantages of large apertures. No one had seemed to remember that the primary function of an instrument designed to aid vision is to collect light. The elementary principle of space-penetration had not been adverted to. It devolved upon him to point out that the distances of similar objects are exactly proportional to the size of the telescopes barely sufficing to show them. The reason is obvious. Compare, for instance, a one-inch telescope with the naked eye. The telescope brings to a focus twenty-five times as much light as can enter the pupil, taken at one-fifth of an inch in diameter; therefore it will render visible a star twenty-five times fainter than the smallest seen without its help; or--what comes to the same thing--an intrinsically equal star at a five-fold distance. A one-inch glass hence actually quintuples the diameter of the visible universe, and gives access to seventy-five times the volume of space ranged through by the unassisted eye.

This simple law Herschel made the foundation-stone of his sidereal edifice. He was the first to notice it, because he was the first practically to concern himself with the star-depths. The possibility of gauging the heavens rose with him above the horizon of science. Because untiring in exploration, he was insatiable of light; and being insatiable of light, he built great telescopes.

His example was inevitably imitated and surpassed. Not through a vulgar ambition to “beat the record,” but because a realm had been thrown open which astronomers could not but desire to visit and search through for themselves. Lord Rosse’s six-foot reflector was the immediate successor of Herschel’s four-foot; Mr. Lassell’s beautiful specula followed; and the series of large _metallic_ reflectors virtually closed with that of four-feet aperture erected at Melbourne in 1870. The reflecting surface in modern instruments is furnished by a thin film of silver deposited on glass. It has the advantage of returning about half as much again of the incident light as the old specula, so that equal power is obtained with less size. Dr. Common’s five-foot is the grand exemplar in this kind; and it is fully equivalent to the Parsonstown six-foot.

The improvement of refractors proceeded more slowly. Difficulties in the manufacture of glass stood in the way, and difficulties in the correction of colour. The splendid success, however, of the Lick thirty-six inch, and the fine promise of the Yerkes forty-inch, have turned the strongest current of hope for the future in the direction of this class of instrument. But all modern efforts to widen telescopic capacity primarily derive their impulse from Herschel’s passionate desire to see further, and to see better, than his predecessors.

His observations demonstrate the rare excellence of his instruments. Experiments made on the asteroid Juno, in 1805, for the purpose of establishing a valid distinction between real and fictitious star-discs, prove, in Professor Holden’s opinion, the reflector employed to have been of almost ideal perfection; and his following of Saturn’s inner satellites right up to the limb, with the twenty-foot and the forty-foot, was a _tour de force_ in vision scarcely, if ever, surpassed.

In the ordinary telescopes of those days really good definition was unknown; they showed the stars with rays or tails, distorted into triangles, or bulged into “cocked hats;” clean-cut, circular images were out of the question. Sitting next Herschel one day at dinner, Henry Cavendish, the great chemist, a remarkably taciturn man, broke silence with the abrupt question--“Is it true, Dr. Herschel, that you see the stars round?” “Round as a button,” replied the Doctor; and no more was said until Cavendish, near the close of the repast, repeated interrogatively, “Round as a button?” “Round as a button,” Herschel briskly reiterated, and the conversation closed.

It seems probable that Herschel’s _caput artis_ lost some of its fine qualities with time. Great specula are peculiarly liable to deterioration. Their figure tends to become impaired by the stress of their own weight; their lustre is necessarily more or less evanescent. Re-polishing, however, is a sort of re-making; and the last felicitous touches, upon which everything depends, can never be reckoned upon with certainty. Hence, the original faultlessness of the great mirror was, perhaps, never subsequently reproduced.

“Such telescopes as Herschel worked with,” Dr. Kitchiner wrote in 1815, “could only be made by the man who used them, and only be used by the man who made them.” The saying is strictly true. His skill in one branch promoted his success in the other. He was as much at home with his telescopes as the Bedouin are with their horses. Their peculiarities made part of his most intimate experience. From the graduated varieties of his specula he picked out the one best suited to the purpose in hand. It was his principle never to employ a larger instrument than was necessary, agility of movement being taken into account no less than capacity for collecting light. The time-element, indeed, always entered into his calculations; he worked like a man who has few to-morrows.

His sense of sight was exceedingly refined, and he took care to keep it so. In order to secure complete “tranquility of the retina,” he used to remain twenty minutes in the dark before attempting to observe faint objects; and his eye became so sensitive after some hours spent in “sweeping,” that the approach of a third-magnitude star obliged him to withdraw it from the telescope. A black hood thrown over his head while observing served to heighten this delicacy of vision. He despised no precaution. Details are “of consequence,” he wrote to Alexander Aubert, an amateur astronomer, “when we come to refinements, and want to _screw an instrument up to the utmost pitch_.”

This was said in reference to his application of what seemed extravagantly high magnifying powers. He laid great stress upon it in the earlier part of his career. The method, he said, was “an untrodden path,” in which “a variety of new phenomena may be expected.” With his seven-foot Newtonian he used magnifications up to nearly 6,000, proceeding, however, “all along experimentally”--a plan far too much neglected in “the art of seeing.” “We are told,” he proceeded, “that we gain nothing by magnifying _too much_. I grant it, but shall never believe I magnify too much till by experience I find that I can see better with a lower power.” The innovation was received with a mixture of wonder, incredulity, and admiration.

Herschel showed his customary judgment in this branch of astronomical practice. He established the distinctions still maintained, and laid down the lines still followed. It is true he went far beyond the point where modern observers find it advisable to stop. The highest power brought into use with the Lick refractor is 2,600; and Herschel’s instruments bore 5,800 (nominally 6,500) without injury to definition. But only at exceptional moments. His habitual sweeping power was 460; he “screwed-up” higher only for particular purposes, and under favourable conditions. Although his strong eye-pieces seem, for intelligible reasons, to have been laid aside on the adoption of the “front-view” form of construction, they had served him well in the division of close pairs, as well as for bringing faint stars into view--an effect correctly explained by him as due to the augmented darkness, under high powers, of the sky-ground. But the most important result of their employment was the discovery that the stars have no sensible dimensions. This became evident through the failure of attempts to magnify them; the higher the power applied, the smaller and more intense they appeared. Herschel accordingly pronounced stellar telescopic discs “spurious,” but made no attempt to explain their origin through diffraction.

He never possessed an instrument mounted equatoreally--that is, so as automatically to follow the stars. In its absence, his work, had it not been accomplished, would have seemed to modern ideas impossible. No clockwork movement kept the objects he was observing in the field of view. His hands were continually engaged in supplying the deficiency. How, under these circumstances, he contrived to measure hundreds of double stars, and secure the places of thousands of nebulæ, would be incomprehensible but for the quasi-omnipotence of enthusiasm.

The angle made with the meridian by the line joining two stars (their “position angle”) was never thought of as a quantity useful to be ascertained until Herschel, about 1779, invented his “revolving-wire micrometer.” This differed in no important respect from the modern “filar micrometer;” only spider-lines have been substituted for the original silk fibres. For measuring the distances of the wider classes of double stars, he devised in 1782 a “lamp-micrometer;” while those of the closest pairs were estimated in terms of the discs of the components. In compiling his second catalogue, however, he used the thread-micrometer for both purposes. It is true that “even in his matchless hands”--in Dr. Gill’s phrase--the results obtained were “crude;” but the fact remains that the whole system of micrometrical measurement came into existence through Herschel’s double-star determinations.

Their consequences have developed enormously within the last few years. Mr. Burnham’s discoveries of excessively close pairs have been so numerous as to leave no reasonable doubt that their indefinite multiplication is only a question of telescopic possibility. Then in 1889, another power came into play; the spectroscope took up the work of resolving stars. Or rather, the spectroscope in alliance with the photographic camera; for the spectral changes indicating the direction and velocity of motion in the line of sight can be systematically studied, as a rule, only when registered on sensitive plates. The upshot has been to bring within the cognisance of science the marvellous systems known as “spectroscopic binaries.” They are of great variety. Some consist of a bright, others of a bright and dark, pair. Those that revolve in a plane nearly coinciding with our line of vision undergo mutual occultations. A further detachment seem to escape eclipse, yet vary in light for some unexplained reason, while they revolve. Others, like Spica Virginis, revolve without varying. Their orbital periods are counted by hours or days. The study of the disturbances of these remarkable combinations promises to open a new era in astronomical theory. For they are most likely all multiple. Irregularities indicating the presence of attractive, although obscure bodies, have, in several cases, been already noticed.

The revolutions of spectroscopic binary stars can be studied to the greatest advantage when they involve light-change; and photometric methods have accordingly begun to play an important part in the sidereal department of gravitational science. And here again we meet with Herschel’s initiative. His method of sequences has been already explained; and he made the first attempt to lay down a definite scale of star-magnitudes. He failed, and it was hardly desirable that he should succeed. On his scale, the ratio of change from one grade to the next constantly diminished. In the modern system it remains always the same. A star of the second magnitude is by definition two and a-half (2·512) times less bright than one of the first; a star of the third magnitude is two and a-half times less bright than one of the second, the series descending without modification until beyond telescopic reach. This uniformity in the _proportionate_ value of a magnitude is indispensable for securing a practicable standard of measurement. Herschel, however, took the great step of introducing a principle of order.

His estimates of stellar lustre were purely visual. And although various instruments, devised for the purpose, have since proved eminently useful, the ultimate appeal in all is to the eye. But there are many signs that, in the photometry of the future, not the eye but the camera will be consulted. Their appraisements differ markedly. Herschel’s incidental remark on the disturbance of light-valuation by colour touches a point of fundamental importance in photographic photometry. The chemical method gives to white stars a great advantage over yellow and red ones. They come out proportionately much brighter on the sensitive plate than they appear to the eye. And to these varieties of hue correspond spectral class-distinctions, the spectrum of an object being nothing but its colour written at full length. This systematic discrepancy between visual and photographic impressions of brightness, while introducing unwelcome complications in measures of magnitude, may serve to bring out important truths. The inference, for example, has been founded upon it that the Milky Way is composed almost exclusively of white, or “Sirian” stars; and there can be no question but that the arrangement of stars in space has some respect to their spectral types.

Herschel’s plan of inquiry into the laws of stellar distribution by “photometric enumeration,” or gauging by magnitudes, was a bequest to posterity which has been turned to account with very little acknowledgment of its source. Argelander’s review of the northern heavens (lately completed photographically by Dr. Gill to the southern pole) afforded, from 1862, materials for its application on a large scale; but the magnitudes assigned to his 324,000 stars do not possess the regularity needed to make deductions based on them perfectly trustworthy. Otherwise the distance from the earth of the actual aggregations in the Milky Way could have been ascertained in a rough way from the numerical representation of the various photometric classes. As it is, the presumption is strong that the galactic clouds are wholly independent of stars brighter than the ninth magnitude--that they only begin to gather at a depth in space whence light takes _at least_ a thousand years to travel to our eyes. Confirmatory evidence, published in 1894, has been supplied by M. Easton’s research, based on the same principle, into the detailed relations of stars of various magnitudes to Milky Way structure. They are exhibited only by those of the ninth magnitude, or fainter; for with them sets in a significant crowding upon its condensed parts, attended by a scarcity over its comparative vacuities. Counts by magnitudes have, besides, made it clear that the stars, in portions of the sky removed from the Milky Way, thin out notably before the eleventh magnitude is reached; so that, outside the galactic zone, the stellar system is easily fathomed.

Also on the strength of photometric enumerations, Dr. Gould, of Boston, came to the conclusion, in 1879, that there is an extra thronging of stars about our sun, which forms one of a special group consisting of some four or five hundred members. The publication, in 1890, of the “Draper Catalogue,” of 10,530 photographed stellar spectra, has thrown fresh light on this interesting subject. Mr. Monck, of Dublin, gave reasons for holding stars physically like the sun to be generally nearer to us than stars of the Sirian class; and Professor Kapteyn, of Gröningen, as the result of a singularly able investigation, concluded with much probability that the sun belongs to a strongly condensed group of mostly “solar” stars, nearly concentric with the galaxy. It might, in fact, be said that we live in a globular cluster, since our native star-collection should appear from a very great distance under that distinctive form.

This modern quasi-discovery was anticipated by Herschel. He was avowedly indebted, it is true, to Michell’s “admirable idea” of the stars being divided into separate groups; but Michell did not trouble himself about the means of its possible verification, and Herschel did. He always looked round to see if there were not some touchstone of fact within reach.

His discussion of the solar cluster, though brief and incidental, is not without present interest. He found the federative arrangement of the stars to be “every day more confirmed by observation.” The “flying synods of worlds” formed by them must gravitate one towards another as if concentrated at their several centres of gravity. Accordingly, “a star, or sun, such as ours, may have a proper motion within its own system of stars, while the whole may have another proper motion totally different in quantity and direction.” We may thus, he continued, “arrive in process of time, at a knowledge of all the real, complicated motions of the planet we inhabit; of the solar system to which it belongs; and even of the sidereal system of which the sun may possibly be a member.” He proceeded to explain how stars, making part of the solar cluster, might be discriminated from those exterior to it; the former showing the perspective influence only of the sun’s translation among themselves, while the latter would be affected besides by a “still remoter parallax”--a secular drift, compounded of the proper motion of the sun within its cluster, and of its cluster relatively to other clusters.

The possibility of applying Herschel’s test is now fully recognised. Each fresh determination of the solar apex is scrutinised for symptoms of the higher “systematical parallax;” although as yet with dubious or negative results. Associated stellar groups are, nevertheless, met with in various parts of the sky. Herschel not only anticipated their existence, but suggested “a concurrence of proper motions” as the fittest means for identifying them.

His anticipation has been realised by Mr. Proctor’s detection of “star-drift.” Several stars in the Plough thus form a squadron sailing the same course; and similar combinations, on an apparently smaller scale, have been pieced together in various constellations. But the principle of their connection has yet to be discovered. They are evidently not self-centred systems; hence their companionship, however prolonged, must finally terminate. The only pronounced cluster with a common proper motion is the Pleiades; and its drift seems to be merely of a perspective nature--a reflection of the sun’s advance.

Bessel said of Herschel that “he aimed at acquiring knowledge, not of the motions, but of the constitution of the heavenly bodies, and of the structure of the sidereal edifice.” This, however, is a defective appreciation. He made, indeed, no meridian observations, and computed no planetary or cometary perturbations; yet if there ever was an astronomer who instinctively “looked before and after,” it was he. Could he have attained to a complete knowledge of the architecture of the heavens, as they stood at a given moment, it would not have satisfied him. To interpret the past and future by the present was his constant aim; from his “retired situation” on the earth, he watched with awe the grand procession of the sum of things defile through endless ages. He could not observe what was without at the same time seeking to divine what had been, and to forecast what was to come.

His nebular theory is now accepted almost as a matter of course. The spectroscope has lent it powerful support by proving the _de facto_ existence of the “lucid medium,” postulated by him as a logical necessity. This was done August 1st, 1864, when Dr. Huggins derived from a planetary nebula in Draco a spectrum characteristic of a gaseous body, because consisting of bright lines. Their wave-lengths, which turned out to be identical for all objects of the kind, with one or two possible exceptions, indicated a composition out of hydrogen mixed with certain unfamiliar aeriform substances. Herschel’s visual discrimination of gaseous nebulæ was highly felicitous. Modern science agrees with him in pronouncing the Orion nebula, as well as others of the irregular class, planetaries, diffused nebulosities, and the “atmospheres” of “cloudy stars,” to be masses of “shining fluid.” As for his “ambiguous objects,” they remain ambiguous still. “Clusters in disguise” through enormous distance, give apparently the same quality of light with irresolvable nebulæ. His inference that stars and nebulæ form mixed systems has, moreover, been amply confirmed. No one now denies their significant affinity, and very few their genetic relationship.

Herschel gave a list in 1811 of fifty-two dim, indefinite nebulosities, covering in the aggregate 152 square degrees. “But this,” he added, “gives us by no means the real limits” of the luminous appearance; “while the depth corresponding to its superficial extent may be far beyond the reach of our telescopes;” so “that the abundance of nebulous matter diffused through such an expansion of the heavens must exceed all imagination.”

“The prophetic spirit of these remarks,” Professor Barnard comments, “is being every day made more evident through the revelations of photography.” He is himself one of the very few who have telescopically verified any part of these suggestive observations.

“I am familiar,” he wrote in _Knowledge_, January, 1892, “with a number of regions in the heavens where vast diffusions of nebulous matter are situated. One of these, in a singularly blank region, lies some five or six degrees north-west of Antares, and covers many square degrees. Another lies north of the Pleiades, between the cluster and the Milky Way; a portion of this has recently been successfully photographed by Dr. Archenhold. There is a large nebulous spot in that region, easily visible to the naked eye, which I have seen for many years. When sweeping there with a low power, the whole region between the Pleiades and the Milky Way is perceived to be nebulous. These great areas of nebulosity make their presence known by a singular dulling of the ordinarily black sky, as if a thin veil of dust intervened.” They “are specially suitable for the photographic plate, and it is only by such means that they can be at all satisfactorily located.”

Some of Herschel’s milky tracts have been thus pictured; notably one in the Swan, shown on Dr. Max Wolf’s plates to involve the bright star Gamma Cygni; and another immense formation extending over sixty square degrees about the belt and sword of Orion, and joining on, Herschel was “pretty sure,” to the great nebula. This, never unmistakably _seen_ except by him, portrayed itself emphatically in 1886 in Professor E. C. Pickering’s photographs. Herschel’s persuasion of the subordinate character of the original “Fish-mouth nebula” was well-grounded. On plates exposed by Professors W. H. Pickering and Barnard, it is disclosed as the mere nucleus of a tremendous spiral, sweeping round from Bellatrix to Rigel.

Diffused nebulosities appear in photographs as far from homogeneous. They are not simple volumes of gas indefinitely expanding in all directions, after the manner of simple aeriform fluids. They possess, on the contrary, characteristic shapes. Structureless nebulæ, like structureless protoplasm, seem to be non-existent. In all, an organising principle is at work.

Minute telescopic stars showed to Herschel as prevalently red, owing, he conjectured, to the enfeeblement of their blue rays during an uncommonly long journey through space “not quite destitute of some very subtle medium.” The argument is a remarkable one. It would be valid if the ethereal vehicle of light exercised absorption after the manner of ordinary attenuated substances. There is, however, reason to suppose that the symptomatic redness was only a subjective impression, not an objective fact. His colour-sense was not quite normal. The lower, to his perception, somewhat overbalanced the higher end of the spectrum, and his mirrors added to the inequality by reflecting a diminished proportion of blue light. Thus he recorded many stars as tinged with red which are now colourless, yet lie under no suspicion of change.

Herschel was, in the highest and widest sense, the founder of sidereal astronomy. He organised the science and set it going; he laid down the principles of its future action; he accumulated materials for its generalisations, and gave examples of how best to employ them. His work was at once so stimulating and so practical that its abandonment might be called impossible. Others were sure to resume where he had left off. His son was his first and fittest successor; he was the only one who undertook in its entirety the inherited task. Yet there are to be found in every quarter of the world men imbued with William Herschel’s sublime ambitions. Success swells the ranks of an invading army; and the march of astronomy has, within the last decade, assumed a triumphal character. The victory can never be completely won; the march can never reach its final goal; but spoils are meanwhile gathered up by the wayside which eager recruits are crowding in to share. The heavens are, year by year, giving up secrets long and patiently watched for, while holding in reserve many others still more mysterious. There is no fear of interest being exhausted by disclosure.

Herschel’s dim intuition that something might be learned about the physical nature of the stars from the diverse quality of their light, was verified after sixty-five years, by the early researches of Secchi, Huggins, and Miller; but he could not suspect that, through the chemical properties, which he guessed to belong in varying degrees to the different sections of their spectra, pictures of the heavenly bodies would be obtained more perfect than the telescopic views he rapturously gazed at. Still less could he have imagined that, owing to its faculty of accumulating impressions too weak to affect the eye separately, the chemical would, in great measure, supersede the telescopic method in carrying out the designs he had most at heart.

Those designs have now grown to be of international importance. At eighteen northern and southern observatories a photographic review of the heavens is in progress. The combined results will be the registration, in place and magnitude, of fifteen to twenty millions of stars. The gauging of the skies will then be complete down to the fourteenth magnitude; and the “construction of the heavens” can be studied with materials of the best quality, and almost indefinite in quantity. By simply “counting the gauges” on Herschel’s early plan, much may be learnt; the amount of stellar condensation towards the plane of the Milky Way, for instance, and the extent of stellar denudation near its poles. A marked contrast between the measures of distribution in these opposite directions will most likely be brought into view. The application of his later method of enumeration by magnitudes ought to prove even more instructive, but may be very difficult. The obstacles, it is to be hoped, will not be insurmountable; yet they look just now formidable enough.

The grand problem with which Herschel grappled all his life involves more complicated relations than he was aware of. It might be compared to a fortress, the citadel of which can only be approached after innumerable outworks have been stormed. That one man, urged on by the exalted curiosity inspired by the contemplation of the heavens, attempted to carry it by a _coup de main_, and, having made no inconsiderable breach in its fortifications, withdrew from the assault, his “banner torn, but flying,” must always be remembered with amazement.