Great Astronomers Part 10

Maskelyne declared he could detect no error in their graduation so large as two seconds and a half. But Ramsden maintained that even better results would be obtained by superseding the entire quadrant by the circle. He obtained the means of testing this prediction when he completed a superb circle for Palermo of five feet diameter.

Finding his antic.i.p.ations were realised, he desired to apply the same principles on a still grander scale. Ramsden was in this mood when he met with Dr. Ussher. The enthusiasm of the astronomer and the instrument-maker communicated itself to the Board, and a tremendous circle, to be ten feet in diameter, was forthwith projected.

Projected, but never carried out. After Ramsden had to some extent completed a 10-foot circle, he found such difficulties that he tried a 9-foot, and this again he discarded for an 8-foot, which was ultimately accomplished, though not entirely by himself.

Notwithstanding the contraction from the vast proportions originally designed, the completed instrument must still be regarded as a colossal piece of astronomical workmans.h.i.+p. Even at this day I do not know that any other observatory can show a circle eight feet in diameter graduated all round.

I think it is Professor Piazzi Smith who tells us how grateful he was to find a large telescope he had ordered finished by the opticians on the very day they had promised it. The day was perfectly correct; it was only the year that was wrong. A somewhat remarkable experience in this direction is chronicled by the early reports of the visitors to Dunsink Observatory. I cannot find the date on which the great circle was ordered from Ramsden, but it is fixed with sufficient precision by an allusion in Ussher's paper to the Royal Irish Academy, which shows that by the 13th June, 1785, the order had been given, but that the abandonment of the 10-foot scale had not then been contemplated. It was reasonable that the board should allow Ramsden ample time for the completion of a work at once so elaborate and so novel. It could not have been finished in a year, nor would there have been much reason for complaint if the maker had found he required two or even three years more.

Seven years gone, and still no telescope, was the condition in which the Board found matters at their first visitation in 1792. They had, however, a.s.surances from Ramsden that the instrument would be completed within the year; but, alas for such promises, another seven years rolled on, and in 1799 the place for the great circle was still vacant at Dunsink. Ramsden had fallen into bad health, and the Board considerately directed that "inquiries should be made." Next year there was still no progress, so the Board were roused to threaten Ramsden with a suit at law; but the menace was never executed, for the malady of the great optician grew worse, and he died that year.

Affairs had now a.s.sumed a critical aspect, for the college had advanced much money to Ramsden during these fifteen years, and the instrument was still unfinished. An appeal was made by the Provost to Dr. Maskelyne, the Astronomer Royal of England, for his advice and kindly offices in this emergency. Maskelyne responds--in terms calculated to allay the anxiety of the Bursar--"Mr. Ramsden has left property behind him, and the College can be in no danger of losing both their money and the instrument." The business of Ramsden was then undertaken by Berge, who proceeded to finish the circle quite as deliberately as his predecessor. After four years Berge promised the instrument in the following August, but it did not come. Two years later (1806) the professor complains that he can get no answer from Berge. In 1807, it is stated that Berge will send the telescope in a month. He did not; but in the next year (1808), about twenty-three years after the great circle was ordered, it was erected at Dunsink, where it is still to be seen.

The following circ.u.mstances have been authenticated by the signatures of Provosts, Proctors, Bursars, and other College dignitaries:--In 1793 the Board ordered two of the clocks at the observatory to be sent to Mr. Crosthwaite for repairs. Seven years later, in 1800, Mr.

Crosthwaite was asked if the clocks were ready. This impatience was clearly unreasonable, for even in four more years, 1804, we find the two clocks were still in hand. Two years later, in 1806, the Board determined to take vigorous action by asking the Bursar to call upon Crosthwaite. This evidently produced some effect, for in the following year, 1807, the Professor had no doubt that the clocks would be speedily returned. After eight years more, in 1815, one of the clocks was still being repaired, and so it was in 1816, which is the last record we have of these interesting time-pieces. Astronomers are, however, accustomed to deal with such stupendous periods in their calculations, that even the time taken to repair a clock seems but small in comparison.

The long tenure of the chair of Astronomy by Brinkley is divided into two nearly equal periods by the year in which the great circle was erected. Brinkley was eighteen years waiting for his telescope, and he had eighteen years more in which to use it. During the first of these periods Brinkley devoted himself to mathematical research; during the latter he became a celebrated astronomer. Brinkley's mathematical labours procured for their author some reputation as a mathematician. They appear to be works of considerable mathematical elegance, but not indicating any great power of original thought.

Perhaps it has been prejudicial to Brinkley's fame in this direction, that he was immediately followed in his chair by so mighty a genius as William Rowan Hamilton.

After the great circle had been at last erected, Brinkley was able to begin his astronomical work in earnest. Nor was there much time to lose. He was already forty-five years old, a year older than was Herschel when he commenced his immortal career at Slough. Stimulated by the consciousness of having the command of an instrument of unique perfection, Brinkley loftily attempted the very highest cla.s.s of astronomical research. He resolved to measure anew with his own eye and with his own hand the constants of aberration and of nutation. He also strove to solve that great problem of the universe, the discovery of the distance of a fixed star.

These were n.o.ble problems, and they were n.o.bly attacked. But to appraise with justice this work of Brinkley, done seventy years ago, we must not apply to it the same criterion as we would think right to apply to similar work were it done now. We do not any longer use Brinkley's constant of aberration, nor do we now think that Brinkley's determinations of the star distances were reliable. But, nevertheless, his investigations exercised a marked influence on the progress of science; they stimulated the study of the principles on which exact measurements were to be conducted.

Brinkley had another profession in addition to that of an astronomer. He was a divine. When a man endeavours to pursue two distinct occupations concurrently, it will be equally easy to explain why his career should be successful, or why it should be the reverse. If he succeeds, he will, of course, exemplify the wisdom of having two strings to his bow. Should he fail, it is, of course, because he has attempted to sit on two stools at once. In Brinkley's case, his two professions must be likened to the two strings rather than to the two stools. It is true that his practical experience of his clerical life was very slender. He had made no attempt to combine the routine of a parish with his labours in the observatory.

Nor do we a.s.sociate a special eminence in any department of religious work with his name. If, however, we are to measure Brinkley's merits as a divine by the ecclesiastical preferment which he received, his services to theology must have rivalled his services to astronomy.

Having been raised step by step in the Church, he was at last appointed to the See of Cloyne, in 1826, as the successor of Bishop Berkeley.

Now, though it was permissible for the Archdeacon to be also the Andrews Professor, yet when the Archdeacon became a Bishop, it was understood that he should transfer his residence from the observatory to the palace. The chair of Astronomy accordingly became vacant.

Brinkley's subsequent career seems to have been devoted entirely to ecclesiastical matters, and for the last ten years of his life he did not contribute a paper to any scientific society. Arago, after a characteristic lament that Brinkley should have forsaken the pursuit of science for the temporal and spiritual attractions of a bishopric, pays a tribute to the conscientiousness of the quondam astronomer, who would not even allow a telescope to be brought into the palace lest his mind should be distracted from his sacred duties.

The good bishop died on the 13th September, 1835. He was buried in the chapel of Trinity College, and a fine monument to his memory is a familiar object at the foot of the n.o.ble old staircase of the library.

The best memorial of Brinkley is his admirable book on the "Elements of Plane Astronomy." It pa.s.sed through many editions in his lifetime, and even at the present day the same work, revised first by Dr. Luby, and more recently by the Rev. Dr. Stubbs and Dr. Brunnow, has a large and well-merited circulation.

JOHN HERSCHEL.

This ill.u.s.trious son of an ill.u.s.trious father was born at Slough, near Windsor, on the 7th March, 1792. He was the only child of Sir William Herschel, who had married somewhat late in life, as we have already mentioned.

[PLATE: ASTRONOMETER MADE BY SIR J. HERSCHEL to compare the light of certain stars by the intervention of the moon.]

The surroundings among which the young astronomer was reared afforded him an excellent training for that career on which he was to enter, and in which he was destined to attain a fame only less brilliant than that of his father. The circ.u.mstances of his youth permitted him to enjoy one great advantage which was denied to the elder Herschel. He was able, from his childhood, to devote himself almost exclusively to intellectual pursuits. William Herschel, in the early part of his career, had only been able to s.n.a.t.c.h occasional hours for study from his busy life as a professional musician. But the son, having been born with a taste for the student's life, was fortunate enough to have been endowed with the leisure and the means to enjoy it from the commencement. His early years have been so well described by the late Professor Pritchard in the "Report of the Council of the Royal Astronomical Society for 1872," that I venture to make an extract here:--

"A few traits of John Herschel's boyhood, mentioned by himself in his maturer life, have been treasured up by those who were dear to him, and the record of some of them may satisfy a curiosity as pardonable as inevitable, which craves to learn through what early steps great men or great nations become ill.u.s.trious. His home was singular, and singularly calculated to nurture into greatness any child born as John Herschel was with natural gifts, capable of wide development. At the head of the house there was the aged, observant, reticent philosopher, and rarely far away his devoted sister, Caroline Herschel, whose labours and whose fame are still cognisable as a beneficent satellite to the brighter light of her ill.u.s.trious brother. It was in the companions.h.i.+p of these remarkable persons, and under the shadow of his father's wonderful telescope, that John Herschel pa.s.sed his boyish years. He saw them, in silent but ceaseless industry, busied about things which had no apparent concern with the world outside the walls of that well-known house, but which, at a later period of his life, he, with an unrivalled eloquence, taught his countrymen to appreciate as foremost among those living influences which but satisfy and elevate the n.o.blest instincts of our nature. What sort of intercourse pa.s.sed between the father and the boy may be gathered from an incident or two which he narrated as having impressed themselves permanently on the memory of his youth.

He once asked his father what he thought was the oldest of all things. The father replied, after the Socratic method, by putting another question: 'And what do you yourself suppose is the oldest of all things?' The boy was not successful in his answers, thereon the old astronomer took up a small stone from the garden walk: 'There, my child, there is the oldest of all the things that I certainly know.'

On another occasion his father is said to have asked the boy, 'What sort of things, do you think, are most alike?' The delicate, blue-eyed boy, after a short pause, replied, 'The leaves of the same tree are most like each other.' 'Gather, then, a handful of leaves of that tree,' rejoined the philosopher, 'and choose two that are alike.' The boy failed; but he hid the lesson in his heart, and his thoughts were revealed after many days. These incidents may be trifles; nor should we record them here had not John Herschel himself, though singularly reticent about his personal emotions, recorded them as having made a strong impression on his mind. Beyond all doubt we can trace therein, first, that grasp and grouping of many things in one, implied in the stone as the oldest of things; and, secondly, that fine and subtle discrimination of each thing out of many like things as forming the main features which characterized the habit of our venerated friend's philosophy."

John Herschel entered St. John's College, Cambridge, when he was seventeen years of age. His university career abundantly fulfilled his father's eager desire, that his only son should develop a capacity for the pursuit of science. After obtaining many lesser distinctions, he finally came out as Senior Wrangler in 1813. It was, indeed, a notable year in the mathematical annals of the University. Second on that list, in which Herschel's name was first, appeared that of the ill.u.s.trious Peac.o.c.k, afterwards Dean of Ely, who remained throughout life one of Herschel's most intimate friends.

Almost immediately after taking his degree, Herschel gave evidence of possessing a special apt.i.tude for original scientific investigation.

He sent to the Royal Society a mathematical paper which was published in the PHILOSOPHICAL TRANSACTIONS. Doubtless the splendour that attached to the name he bore a.s.sisted him in procuring early recognition of his own great powers. Certain it is that he was made a Fellow of the Royal Society at the unprecedentedly early age of twenty-one. Even after this remarkable encouragement to adopt a scientific career as the business of his life, it does not seem that John Herschel at first contemplated devoting himself exclusively to science. He commenced to prepare for the profession of the Law by entering as a student at the Middle Temple, and reading with a practising barrister.

But a lawyer John Herschel was not destined to become. Circ.u.mstances brought him into a.s.sociation with some leading scientific men. He presently discovered that his inclinations tended more and more in the direction of purely scientific pursuits. Thus it came to pa.s.s that the original intention as to the calling which he should follow was gradually abandoned. Fortunately for science Herschel found its pursuit so attractive that he was led, as his father had been before him, to give up his whole life to the advancement of knowledge. Nor was it unnatural that a Senior Wrangler, who had once tasted the delights of mathematical research, should have been tempted to devote much time to this fascinating pursuit. By the time John Herschel was twenty-nine he had published so much mathematical work, and his researches were considered to possess so much merit, that the Royal Society awarded him the Copley Medal, which was the highest distinction it was capable of conferring.

At the death of his father in 1822, John Herschel, with his tastes already formed for a scientific career, found himself in the possession of ample means. To him also pa.s.sed all his father's great telescopes and apparatus. These material aids, together with a dutiful sense of filial obligation, decided him to make practical astronomy the main work of his life. He decided to continue to its completion that great survey of the heavens which had already been inaugurated, and, indeed, to a large extent accomplished, by his father.

The first systematic piece of practical astronomical work which John Herschel undertook was connected with the measurement of what are known as "Double Stars." It should be observed, that there are in the heavens a number of instances in which two stars are seen in very close a.s.sociation. In the case of those objects to which the expression "Double Stars" is generally applied, the two luminous points are so close together that even though they might each be quite bright enough to be visible to the unaided eye, yet their proximity is such that they cannot be distinguished as two separate objects without optical aid. The two stars seem fused together into one. In the telescope, however, the bodies may be discerned separately, though they are frequently so close together that it taxes the utmost power of the instrument to indicate the division between them.

The appearance presented by a double star might arise from the circ.u.mstance that the two stars, though really separated from each other by prodigious distances, happened to lie nearly in the same line of vision, as seen from our point of view. No doubt, many of the so-called double stars could be accounted for on this supposition. Indeed, in the early days when but few double stars were known, and when telescopes were not powerful enough to exhibit the numerous close doubles which have since been brought to light, there seems to have been a tendency to regard all double stars as merely such perspective effects. It was not at first suggested that there could be any physical connection between the components of each pair. The appearance presented was regarded as merely due to the circ.u.mstance that the line joining the two bodies happened to pa.s.s near the earth.

[PLATE: SIR JOHN HERSCHEL.]

In the early part of his career, Sir William Herschel seems to have entertained the view then generally held by other astronomers with regard to the nature of these stellar pairs. The great observer thought that the double stars could therefore be made to afford a means of solving that problem in which so many of the observers of the skies had been engaged, namely, the determination of the distances of the stars from the earth. Herschel saw that the displacement of the earth in its annual movement round the sun would produce an apparent s.h.i.+ft in the place of the nearer of the two stars relatively to the other, supposed to be much more remote. If this s.h.i.+ft could be measured, then the distance of the nearer of the stars could be estimated with some degree of precision.

As has not unfrequently happened in the history of science, an effect was perceived of a very different nature from that which had been antic.i.p.ated. If the relative places of the two stars had been apparently deranged merely in consequence of the motion of the earth, then the phenomenon would be an annual one. After the lapse of a year the two stars would have regained their original relative positions. This was the effect for which William Herschel was looking. In certain of the so called double stars, he, no doubt, did find a movement. He detected the remarkable fact that both the apparent distance and the relative positions of the two bodies were changing. But what was his surprise to observe that these alterations were not of an annually periodic character. It became evident then that in some cases one of the component stars was actually revolving around the other, in an orbit which required many years for its completion. Here was indeed a remarkable discovery. It was clearly impossible to suppose that movements of this kind could be mere apparent displacements, arising from the annual s.h.i.+ft in our point of view, in consequence of the revolution of the earth.

Herschel's discovery established the interesting fact that, in certain of these double stars, or binary stars, as these particular objects are more expressively designated, there is an actual orbital revolution of a character similar to that which the earth performs around the sun. Thus it was demonstrated that in these particular double stars the nearness of the two components was not merely apparent. The objects must actually lie close together at a distance which is small in comparison with the distance at which either of them is separated from the earth. The fact that the heavens contain pairs of twin suns in mutual revolution was thus brought to light.

In consequence of this beautiful discovery, the attention of astronomers was directed to the subject of double stars with a degree of interest which these objects had never before excited. It was therefore not unnatural that John Herschel should have been attracted to this branch of astronomical work. Admiration for his father's discovery alone might have suggested that the son should strive to develop this territory newly opened up to research. But it also happened that the mathematical talents of the younger Herschel inclined his inquiries in the same direction. He saw clearly that, when sufficient observations of any particular binary star had been acc.u.mulated, it would then be within the power of the mathematician to elicit from those observations the shape and the position in s.p.a.ce of the path which each of the revolving stars described around the other. Indeed, in some cases he would be able to perform the astonis.h.i.+ng feat of determining from his calculations the weight of these distant suns, and thus be enabled to compare them with the ma.s.s of our own sun.

[PLATE: NEBULA IN SOUTHERN HEMISPHERE, drawn by Sir John Herschel.]

But this work must follow the observations, it could not precede them. The first step was therefore to observe and to measure with the utmost care the positions and distances of those particular double stars which appear to offer the greatest promise in this particular research. In 1821, Herschel and a friend of his, Mr.

James South, agreed to work together with this object. South was a medical man with an ardent devotion to science, and possessed of considerable wealth. He procured the best astronomical instruments that money could obtain, and became a most enthusiastic astronomer and a practical observer of tremendous energy.

South and John Herschel worked together for two years in the observation and measurement of the double stars discovered by Sir William Herschel. In the course of this time their a.s.siduity was rewarded by the acc.u.mulation of so great a ma.s.s of careful measurements that when published, they formed quite a volume in the "Philosophical Transactions." The value and accuracy of the work, when estimated by standards which form proper criteria for that period, is universally recognised. It greatly promoted the progress of sidereal astronomy, and the authors were in consequence awarded medals from the Royal Society, and the Royal Astronomical Society, as well as similar testimonials from various foreign inst.i.tutions.

This work must, however, be regarded as merely introductory to the main labours of John Herschel's life. His father devoted the greater part of his years as an observer to what he called his "sweeps" of the heavens. The great reflecting telescope, twenty feet long, was moved slowly up and down through an arc of about two degrees towards and from the pole, while the celestial panorama pa.s.sed slowly in the course of the diurnal motion before the keenly watching eye of the astronomer. Whenever a double star traversed the field Herschel described it to his sister Caroline, who, as we have already mentioned, was his invariable a.s.sistant in his midnight watches. When a nebula appeared, then he estimated its size and its brightness, he noticed whether it had a nucleus, or whether it had stars disposed in any significant manner with regard to it. He also dictated any other circ.u.mstance which he deemed worthy of record. These observations were duly committed to writing by the same faithful and indefatigable scribe, whose business it also was to take a memorandum of the exact position of the object as indicated by a dial placed in front of her desk, and connected with the telescope.

John Herschel undertook the important task of re-observing the various double stars and nebulae which had been discovered during these memorable vigils. The son, however, lacked one inestimable advantage which had been possessed by the father. John Herschel had no a.s.sistant to discharge all those duties which Caroline had so efficiently accomplished. He had, therefore, to modify the system of sweeping previously adopted in order to enable all the work both of observing and of recording to be done by himself. This, in many ways, was a great drawback to the work of the younger astronomer. The division of labour between the observer and the scribe enables a greatly increased quant.i.ty of work to be got through. It is also distinctly disadvantageous to an observer to have to use his eye at the telescope directly after he has been employing it for reading the graduations on a circle, by the light of a lamp, or for entering memoranda in a note book. Nebulae, especially, are often so excessively faint that they can only be properly observed by an eye which is in that highly sensitive condition which is obtained by long continuance in darkness. The frequent withdrawal of the eye from the dark field of the telescope, and the application of it to reading by artificial light, is very prejudicial to its use for the more delicate purpose. John Herschel, no doubt, availed himself of every precaution to mitigate the ill effects of this inconvenience as much as possible, but it must have told upon his labours as compared with those of his father.

But nevertheless John Herschel did great work during his "sweeps." He was specially particular to note all the double stars which presented themselves to his observation. Of course some little discretion must be allowed in deciding as to what degree of proximity in adjacent stars does actually bring them within the category of "double stars." Sir John set down all such objects as seemed to him likely to be of interest, and the results of his discoveries in this branch of astronomy amount to some thousands. Six or seven great memoirs in the TRANSACTIONS of the Royal Astronomical Society have been devoted to giving an account of his labours in this department of astronomy.

[PLATE: THE Cl.u.s.tER IN THE CENTAUR, drawn by Sir John Herschel.]

One of the achievements by which Sir John Herschel is best known is his invention of a method by which the orbits of binary stars could be determined. It will be observed that when one star revolves around another in consequence of the law of gravitation, the orbit described must be an ellipse. This ellipse, however, generally speaking, appears to us more or less foreshortened, for it is easily seen that only under highly exceptional circ.u.mstances would the plane in which the stars move happen to be directly square to the line of view. It therefore follows that what we observe is not exactly the track of one star around the other; it is rather the projection of that track as seen on the surface of the sky. Now it is remarkable that this apparent path is still an ellipse. Herschel contrived a very ingenious and simple method by which he could discover from the observations the size and position of the ellipse in which the revolution actually takes place. He showed how, from the study of the apparent orbit of the star, and from certain measurements which could easily be effected upon it, the determination of the true ellipse in which the movement is performed could be arrived at. In other words, Herschel solved in a beautiful manner the problem of finding the true orbits of double stars. The importance of this work may be inferred from the fact that it has served as the basis on which scores of other investigators have studied the fascinating subject of the movement of binary stars.

The labours, both in the discovery and measurement of the double stars, and in the discussion of the observations with the object of finding the orbits of such stars as are in actual revolution, received due recognition in yet another gold medal awarded by the Royal Society. An address was delivered on the occasion by the Duke of Suss.e.x (30th November, 1833), in the course of which, after stating that the medal had been conferred on Sir John Herschel, he remarks:--

"It has been said that distance of place confers the same privilege as distance of time, and I should gladly avail myself of the privilege which is thus afforded me by Sir John Herschel's separation from his country and friends, to express my admiration of his character in stronger terms than I should otherwise venture to use; for the language of panegyric, however sincerely it may flow from the heart, might be mistaken for that of flattery, if it could not thus claim somewhat of an historical character; but his great attainments in almost every department of human knowledge, his fine powers as a philosophical writer, his great services and his distinguished devotion to science, the high principles which have regulated his conduct in every relation of life, and, above all, his engaging modesty, which is the crown of all his other virtues, presenting such a model of an accomplished philosopher as can rarely be found beyond the regions of fiction, demand abler pens than mine to describe them in adequate terms, however much inclined I might feel to undertake the task."

The first few lines of the eulogium just quoted allude to Herschel's absence from England. This was not merely an episode of interest in the career of Herschel, it was the occasion of one of the greatest scientific expeditions in the whole history of astronomy.

Herschel had, as we have seen, undertaken a revision of his father's "sweeps" for new objects, in those skies which are visible from our lat.i.tudes in the northern hemisphere. He had well-nigh completed this task. Zone by zone the whole of the heavens which could be observed from Windsor had pa.s.sed under his review. He had added hundreds to the list of nebulae discovered by his father. He had announced thousands of double stars. At last, however, the great survey was accomplished. The contents of the northern hemisphere, so far at least as they could be disclosed by his telescope of twenty feet focal length, had been revealed.

[PLATE: SIR JOHN HERSCHEL'S OBSERVATORY AT FELDHAUSEN, Cape of Good Hope.]

But Herschel felt that this mighty task had to be supplemented by another of almost equal proportions, before it could be said that the twenty-foot telescope had done its work. It was only the northern half of the celestial sphere which had been fully explored. The southern half was almost virgin territory, for no other astronomer was possessed of a telescope of such power as those which the Herschels had used. It is true, of course, that as a certain margin of the southern hemisphere was visible from these lat.i.tudes, it had been more or less scrutinized by observers in northern skies. And the glimpses which had thus been obtained of the celestial objects in the southern sky, were such as to make an eager astronomer long for a closer acquaintance with the celestial wonders of the south. The most glorious object in the sidereal heavens, the Great Nebula in Orion, lies indeed in that southern hemisphere to which the younger Herschel's attention now became directed. It fortunately happens, however, for votaries of astronomy all the world over, that Nature has kindly placed her most astounding object, the great Nebula in Orion, in such a favoured position, near the equator, that from a considerable range of lat.i.tudes, both north and south, the wonders of the Nebula can be explored. There are grounds for thinking that the southern heavens contain noteworthy objects which, on the whole, are nearer to the solar system than are the noteworthy objects in the northern skies. The nearest star whose distance is known, Alpha Centauri, lies in the southern hemisphere, and so also does the most splendid cl.u.s.ter of stars.

Influenced by the desire to examine these objects, Sir John Herschel determined to take his great telescope to a station in the southern hemisphere, and thus complete his survey of the sidereal heavens. The lat.i.tude of the Cape of Good Hope is such that a suitable site could be there found for his purpose. The purity of the skies in South Africa promised to provide for the astronomer those clear nights which his delicate task of surveying the nebulae would require.