In 1691, Halley became a candidate for the Savilian Professorship of Astronomy at Oxford. He was not, however, successful, for his candidature was opposed by Flamsteed, the Astronomer Royal of the time, and another was appointed. He received some consolation for this particular disappointment by the fact that, in 1696, owing to Newton"s friendly influence, he was appointed deputy Controller of the Mint at Chester, an office which he did not retain for long, as it was abolished two years later. At last, in 1703, he received what he had before vainly sought, and he was appointed to the Savilian chair.
His observations of the eclipse of the sun, which occurred in 1715, added greatly to Halley"s reputation. This phenomenon excited special attention, inasmuch as it was the first total eclipse of the sun which had been visible in London since the year 1140. Halley undertook the necessary calculations, and predicted the various circ.u.mstances with a far higher degree of precision than the official announcement. He himself observed the phenomenon from the Royal Society"s rooms, and he minutely describes the outer atmosphere of the sun, now known as the corona; without, however, offering an opinion as to whether it was a solar or a lunar appendage.
At last Halley was called to the dignified office which he of all men was most competent to fill. On February 9th, 1720, he was appointed Astronomer Royal in succession to Flamsteed. He found things at the Royal Observatory in a most unsatisfactory state. Indeed, there were no instruments, nor anything else that was movable; for such things, being the property of Flamsteed, had been removed by his widow, and though Halley attempted to purchase from that lady some of the instruments which his predecessor had employed, the unhappy personal differences which had existed between him and Flamsteed, and which, as we have already seen, prevented his election as Savilian Professor of Astronomy, proved a bar to the negotiation. Greenwich Observatory wore a very different appearance in those days, from that which the modern visitor, who is fortunate enough to gain admission, may now behold. Not only did Halley find it bereft of instruments, we learn besides that he had no a.s.sistants, and was obliged to transact the whole business of the establishment single-handed.
In 1721, however, he obtained a grant of 500 pounds from the Board of Ordnance, and accordingly a transit instrument was erected in the same year. Some time afterwards he procured an eight-foot quadrant, and with these instruments, at the age of sixty-four, he commenced a series of observations on the moon. He intended, if his life was spared, to continue his observations for a period of eighteen years, this being, as astronomers know, a very important cycle in connection with lunar movements. The special object of this vast undertaking was to improve the theory of the moon"s motion, so that it might serve more accurately to determine longitudes at sea. This self-imposed task Halley lived to carry to a successful termination, and the tables deduced from his observations, and published after his death, were adopted almost universally by astronomers, those of the French nation being the only exception.
Throughout his life Halley had been singularly free from illness of every kind, but in 1737 he had a stroke of paralysis. Notwithstanding this, however, he worked diligently at his telescope till 1739, after which his health began rapidly to give way. He died on January 14th, 1742, in the eighty-sixth year of his age, retaining his mental faculties to the end. He was buried in the cemetery of the church of Lee in Kent, in the same grave as his wife, who had died five years previously. We are informed by Admiral Smyth that Pond, a later Astronomer Royal, was afterwards laid in the same tomb.
Halley"s disposition seems to have been generous and candid, and wholly free from anything like jealousy or rancour. In person he was rather above the middle height, and slight in build; his complexion was fair, and he is said to have always spoken, as well as acted, with uncommon sprightliness. In the eloge p.r.o.nounced upon him at the Paris Academie Des Sciences, of which Halley had been made a member in 1719 it was said, "he possessed all the qualifications which were necessary to please princes who were desirous of instruction, with a great extent of knowledge and a constant presence of mind; his answers were ready, and at the same time pertinent, judicious, polite and sincere."
[PLATE: GREENWICH OBSERVATORY IN HALLEY"S TIME.]
Thus we find that Peter the Great was one of his most ardent admirers. He consulted the astronomer on matters connected with shipbuilding, and invited him to his own table. But Halley possessed n.o.bler qualifications than the capacity of pleasing Princes. He was able to excite and to retain the love and admiration of his equals.
This was due to the warmth of his attachments, the unselfishness of his devotion to his friends, and to a vein of gaiety and good-humour which pervaded all his conversation.
BRADLEY.
James Bradley was descended from an ancient family in the county of Durham. He was born in 1692 or 1693, at Sherbourne, in Gloucestershire, and was educated in the Grammar School at Northleach. From thence he proceeded in due course to Oxford, where he was admitted a commoner at Balliol College, on March 15th, 1711.
Much of his time, while an undergraduate, was pa.s.sed in Ess.e.x with his maternal uncle, the Rev. James Pound, who was a well-known man of science and a diligent observer of the stars. It was doubtless by intercourse with his uncle that young Bradley became so expert in the use of astronomical instruments, but the immortal discoveries he subsequently made show him to have been a born astronomer.
The first exhibition of Bradley"s practical skill seems to be contained in two observations which he made in 1717 and 1718. They have been published by Halley, whose acuteness had led him to perceive the extraordinary scientific talents of the young astronomer. Another ill.u.s.tration of the sagacity which Bradley manifested, even at the very commencement of his astronomical career, is contained in a remark of Halley"s, who says: "Dr. Pound and his nephew, Mr. Bradley, did, myself being present, in the last opposition of the sun and Mars this way demonstrate the extreme minuteness of the sun"s parallax, and that it was not more than twelve seconds nor less than nine seconds." To make the significance of this plain, it should be observed that the determination of the sun"s parallax is equivalent to the determination of the distance from the earth to the sun. At the time of which we are now writing, this very important unit of celestial measurement was only very imperfectly known, and the observations of Pound and Bradley may be interpreted to mean that, from their observations, they had come to the conclusion that the distance from the earth to the sun must be more than 94 millions of miles, and less than 125 millions. We now, of course, know that they were not exactly right, for the true distance of the sun is about 93 millions of miles. We cannot, however, but think that it was a very remarkable approach for the veteran astronomer and his brilliant nephew to make towards the determination of a magnitude which did not become accurately known till fifty years later.
Among the earliest parts of astronomical work to which Bradley"s attention was directed, were the eclipses of Jupiter"s satellites.
These phenomena are specially attractive inasmuch as they can be so readily observed, and Bradley found it extremely interesting to calculate the times at which the eclipses should take place, and then to compare his observations with the predicted times. From the success that he met with in this work, and from his other labours, Bradley"s reputation as an astronomer increased so greatly that on November the 6th, 1718, he was elected a Fellow of the Royal Society.
Up to this time the astronomical investigations of Bradley had been more those of an amateur than of a professional astronomer, and as it did not at first seem likely that scientific work would lead to any permanent provision, it became necessary for the youthful astronomer to choose a profession. It had been all along intended that he should enter the Church, though for some reason which is not told us, he did not take orders as soon as his age would have ent.i.tled him to do so. In 1719, however, the Bishop of Hereford offered Bradley the Vicarage of Bridstow, near Ross, in Monmouthshire, and on July 25th, 1720, he having then taken priest"s orders, was duly inst.i.tuted in his vicarage. In the beginning of the next year, Bradley had some addition to his income from the proceeds of a Welsh living, which, being a sinecure, he was able to hold with his appointment at Bridstow. It appears, however, that his clerical occupations were not very exacting in their demands upon his time, for he was still able to pay long and often-repeated visits to his uncle at Wandsworth, who, being himself a clergyman, seems to have received occasional a.s.sistance in his ministerial duties from his astronomical nephew.
The time, however, soon arrived when Bradley was able to make a choice between continuing to exercise his profession as a divine, or devoting himself to a scientific career. The Savilian Professorship of Astronomy in the University of Oxford became vacant by the death of Dr. John Keill. The statutes forbade that the Savilian Professor should also hold a clerical appointment, and Mr. Pound would certainly have been elected to the professorship had he consented to surrender his preferments in the Church. But Pound was unwilling to sacrifice his clerical position, and though two or three other candidates appeared in the field, yet the talents of Bradley were so conspicuous that he was duly elected, his willingness to resign the clerical profession having been first ascertained.
There can be no doubt that, with such influential friends as Bradley possessed, he would have made great advances had he adhered to his profession as a divine. Bishop Hoadly, indeed, with other marks of favour, had already made the astronomer his chaplain. The engrossing nature of Bradley"s interest in astronomy decided him, however, to sacrifice all other prospects in comparison with the opening afforded by the Savilian Professorship. It was not that Bradley found himself devoid of interest in clerical matters, but he felt that the true scope for such abilities as he possessed would be better found in the discharge of the scientific duties of the Oxford chair than in the spiritual charge of a parish. On April the 26th, 1722, Bradley read his inaugural lecture in that new position on which he was destined to confer such l.u.s.tre.
It must, of course, be remembered that in those early days the art of constructing the astronomical telescope was very imperfectly understood. The only known method for getting over the peculiar difficulties presented in the construction of the refracting telescope, was to have it of the most portentous length. In fact, Bradley made several of his observations with an instrument of two hundred and twelve feet focus. In such a case, no tube could be used, and the object gla.s.s was merely fixed at the top of a high pole. Notwithstanding the inconvenience and awkwardness of such an instrument, Bradley by its means succeeded in making many careful measurements. He observed, for example, the transit of Mercury over the sun"s disc, on October 9th, 1723; he also observed the dimensions of the planet Venus, while a comet which Halley discovered on October the 9th, 1723, was a.s.siduously observed at Wanstead up to the middle of the ensuing month. The first of Bradley"s remarkable contributions to the "Philosophical Transactions" relates to this comet, and the extraordinary amount of work that he went through in connection therewith may be seen from an examination of his book of Calculations which is still extant.
The time was now approaching when Bradley was to make the first of those two great discoveries by which his name has acquired a l.u.s.tre that has placed him in the very foremost rank of astronomical discoverers. As has been often the case in the history of science, the first of these great successes was attained while he was pursuing a research intended for a wholly different purpose. It had long been recognised that as the earth describes a vast orbit, nearly two hundred million miles in diameter, in its annual journey round the sun, the apparent places of the stars should alter, to some extent, in correspondence with the changes in the earth"s position. The nearer the star the greater the shift in its apparent place on the heavens, which must arise from the fact that it was seen from different positions in the earth"s...o...b..t. It had been pointed out that these apparent changes in the places of the stars, due to the movement of the earth, would provide the means of measuring the distances of the stars. As, however, these distances are enormously great in comparison with the orbit which the earth describes around the sun, the attempt to determine the distances of the stars by the shift in their positions had hitherto proved ineffectual. Bradley determined to enter on this research once again; he thought that by using instruments of greater power, and by making measurements of increased delicacy, he would be able to perceive and to measure displacements which had proved so small as to elude the skill of the other astronomers who had previously made efforts in the same direction. In order to simplify the investigation as much as possible, Bradley devoted his attention to one particular star, Beta Draconis, which happened to pa.s.s near his zenith. The object of choosing a star in this position was to avoid the difficulties which would be introduced by refraction had the star occupied any other place in the heavens than that directly overhead.
We are still able to identify the very spot on which the telescope stood which was used in this memorable research. It was erected at the house then occupied by Molyneux, on the western extremity of Kew Green. The focal length was 24 feet 3 inches, and the eye-gla.s.s was 3 and a half feet above the ground floor. The instrument was first set up on November 26th, 1725. If there had been any appreciable disturbance in the place of Beta Draconis in consequence of the movement of the earth around the sun, the star must appear to have the smallest lat.i.tude when in conjunction with the sun, and the greatest when in opposition. The star pa.s.sed the meridian at noon in December, and its position was particularly noticed by Molyneux on the third of that month. Any perceptible displacement by parallax--for so the apparent change in position, due to the earth"s motion, is called--would would have made the star shift towards the north. Bradley, however, when observing it on the 17th, was surprised to find that the apparent place of the star, so far from shifting towards the north, as they had perhaps hoped it would, was found to lie a little more to the south than when it was observed before. He took extreme care to be sure that there was no mistake in his observation, and, true astronomer as he was, he scrutinized with the utmost minuteness all the circ.u.mstances of the adjustment of his instruments. Still the star went to the south, and it continued so advancing in the same direction until the following March, by which time it had moved no less than twenty seconds south from the place which it occupied when the first observation was made. After a brief pause, in which no apparent movement was perceptible, the star by the middle of April appeared to be returning to the north. Early in June it reached the same distance from the zenith which it had in December. By September the star was as much as thirty-nine seconds more to the north than it had been in March, then it returned towards the south, regaining in December the same situation which it had occupied twelve months before.
This movement of the star being directly opposite to the movements which would have been the consequence of parallax, seemed to show that even if the star had any parallax its effects upon the apparent place were entirely masked by a much larger motion of a totally different description. Various attempts were made to account for the phenomenon, but they were not successful. Bradley accordingly determined to investigate the whole subject in a more thorough manner. One of his objects was to try whether the same movements which he had observed in one star were in any similar degree possessed by other stars. For this purpose he set up a new instrument at Wanstead, and there he commenced a most diligent scrutiny of the apparent places of several stars which pa.s.sed at different distances from the zenith. He found in the course of this research that other stars exhibited movements of a similar description to those which had already proved so perplexing. For a long time the cause of these apparent movements seemed a mystery. At last, however, the explanation of these remarkable phenomena dawned upon him, and his great discovery was made.
One day when Bradley was out sailing he happened to remark that every time the boat was laid on a different tack the vane at the top of the boat"s mast shifted a little, as if there had been a slight change in the direction of the wind. After he had noticed this three or four times he made a remark to the sailors to the effect that it was very strange the wind should always happen to change just at the moment when the boat was going about. The sailors, however, said there had been no change in the wind, but that the alteration in the vane was due to the fact that the boat"s course had been altered. In fact, the position of the vane was determined both by the course of the boat and the direction of the wind, and if either of these were altered there would be a corresponding change in the direction of the vane. This meant, of course, that the observer in the boat which was moving along would feel the wind coming from a point different from that in which the wind appeared to be blowing when the boat was at rest, or when it was sailing in some different direction. Bradley"s sagacity saw in this observation the clue to the Difficulty which had so long troubled him.
It had been discovered before the time of Bradley that the pa.s.sage of light through s.p.a.ce is not an instantaneous phenomenon. Light requires time for its journey. Galileo surmised that the sun may have reached the horizon before we see it there, and it was indeed sufficiently obvious that a physical action, like the transmission of light, could hardly take place without requiring some lapse of time.
The speed with which light actually travelled was, however, so rapid that its determination eluded all the means of experimenting which were available in those days. The penetration of Roemer had previously detected irregularities in the observed times of the eclipses of Jupiter"s satellites, which were undoubtedly due to the interval which light required for stretching across the interplanetary s.p.a.ces. Bradley argued that as light can only travel with a certain speed, it may in a measure be regarded like the wind, which he noticed in the boat. If the observer were at rest, that is to say, if the earth were a stationary object, the direction in which the light actually does come would be different from that in which it appears to come when the earth is in motion. It is true that the earth travels but eighteen miles a second, while the velocity with which light is borne along attains to as much as 180,000 miles a second. The velocity of light is thus ten thousand times greater than the speed of the earth. But even though the wind blew ten thousand times faster than the speed with which the boat was sailing there would still be some change, though no doubt a very small change, in the position of the vane when the boat was in progress from the position it would have if the boat were at rest. It therefore occurred to this most acute of astronomers that when the telescope was pointed towards a star so as to place it apparently in the centre of the field of view, yet it was not generally the true position of the star. It was not, in fact, the position in which the star would have been observed had the earth been at rest. Provided with this suggestion, he explained the apparent movements of the stars by the principle known as the "aberration of light." Every circ.u.mstance was accounted for as a consequence of the relative movements of the earth and of the light from the star. This beautiful discovery not only established in the most forcible manner the nature of the movement of light; not only did it ill.u.s.trate the truth of the Copernican theory which a.s.serted that the earth revolved around the sun, but it was also of the utmost importance in the improvement of practical astronomy. Every observer now knows that, generally speaking, the position which the star appears to have is not exactly the position in which the star does actually lie. The observer is, however, able, by the application of the principles which Bradley so clearly laid down, to apply to an observation the correction which is necessary to obtain from it the true place in which the object is actually situated. This memorable achievement at once conferred on Bradley the highest astronomical fame. He tested his discovery in every way, but only to confirm its truth in the most complete manner.
Halley, the Astronomer Royal, died on the 14th, January, 1742, and Bradley was immediately pointed out as his successor. He was accordingly appointed Astronomer Royal in February, 1742. On first taking up his abode at Greenwich he was unable to conduct his observations owing to the wretched condition in which he found the instruments. He devoted himself, however, a.s.siduously to their repair, and his first transit observation is recorded on the 25th July, 1742. He worked with such energy that on one day it appears that 255 transit observations were taken by himself alone, and in September, 1747, he had completed the series of observations which established his second great discovery, the nutation of the earth"s axis. The way in which he was led to the detection of the nutation is strikingly ill.u.s.trative of the extreme care with which Bradley conducted his observations. He found that in the course of a twelve-month, when the star had completed the movement which was due to aberration, it did not return exactly to the same position which it had previously occupied. At first he thought this must be due to some instrumental error, but after closer examination and repeated study of the effect as manifested by many different stars, he came to the conclusion that its origin must be sought in some quite different source. The fact is that a certain change takes place in the apparent position of the stars which is not due to the movement of the star itself, but is rather to be attributed to changes in the points from which the star"s positions are measured.
We may explain the matter in this way. As the earth is not a sphere, but has protuberant parts at the equator, the attraction of the moon exercises on those protuberant parts a pulling effect which continually changes the direction of the earth"s axis, and consequently the position of the pole must be in a state of incessant fluctuation. The pole to which the earth"s axis points on the sky is, therefore, slowly changing. At present it happens to lie near the Pole Star, but it will not always remain there. It describes a circle around the pole of the Ecliptic, requiring about 25,000 years for a complete circuit. In the course of its progress the pole will gradually pa.s.s now near one star and now near another, so that many stars will in the lapse of ages discharge the various functions which the present Pole Star does for us. In about 12,000 years, for instance, the pole will have come near the bright star, Vega. This movement of the pole had been known for ages. But what Bradley discovered was that the pole, instead of describing an uniform movement as had been previously supposed, followed a sinuous course now on one side and now on the other of its mean place. This he traced to the fluctuations of the moon"s...o...b..t, which undergoes a continuous change in a period of nineteen years. Thus the efficiency with which the moon acts on the protuberant ma.s.s of the earth varies, and thus the pole is caused to oscillate.
This subtle discovery, if perhaps in some ways less impressive than Bradley"s earlier achievements of the detection of the aberration of light, is regarded by astronomers as testifying even in a higher degree to his astonishing care and skill as an observer, and justly ent.i.tles him to a unique place among the astronomers whose discoveries have been effected by consummate practical skill in the use of astronomical instruments.
Of Bradley"s private or domestic life there is but little to tell. In 1744, soon after he became Astronomer Royal, he married a daughter of Samuel Peach, of Chalford, in Gloucestershire. There was but one child, a daughter, who became the wife of her cousin, Rev. Samuel Peach, rector of Compton, Beauchamp, in Berkshire.
Bradley"s last two years of life were clouded by a melancholy depression of spirits, due to an apprehension that he should survive his rational faculties. It seems, however, that the ill he dreaded never came upon him, for he retained his mental powers to the close.
He died on 13th July, 1762, aged seventy, and was buried at Michinghamton.
WILLIAM HERSCHEL.
William Herschel, one of the greatest astronomers that has ever lived, was born at Hanover, on the 15th November, 1738. His father, Isaac Herschel, was a man evidently of considerable ability, whose life was devoted to the study and practice of music, by which he earned a somewhat precarious maintenance. He had but few worldly goods to leave to his children, but he more than compensated for this by bequeathing to them a splendid inheritance of genius. Touches of genius were, indeed, liberally scattered among the members of Isaac"s large family, and in the case of his forth child, William, and of a sister several years younger, it was united with that determined perseverance and rigid adherence to principle which enabled genius to fulfil its perfect work.
A faithful chronicler has given us an interesting account of the way in which Isaac Herschel educated his sons; the narrative is taken from the recollections of one who, at the time we are speaking of, was an unnoticed little girl five or six years old. She writes:--
"My brothers were often introduced as solo performers and a.s.sistants in the orchestra at the Court, and I remember that I was frequently prevented from going to sleep by the lively criticisms on music on coming from a concert. Often I would keep myself awake that I might listen to their animating remarks, for it made me so happy to see them so happy. But generally their conversation would branch out on philosophical subjects, when my brother William and my father often argued with such warmth that my mother"s interference became necessary, when the names--Euler, Leibnitz, and Newton--sounded rather too loud for the repose of her little ones, who had to be at school by seven in the morning." The child whose reminiscences are here given became afterwards the famous Caroline Herschel. The narrative of her life, by Mrs. John Herschel, is a most interesting book, not only for the account it contains of the remarkable woman herself, but also because it provides the best picture we have of the great astronomer to whom Caroline devoted her life.
This modest family circle was, in a measure, dispersed at the outbreak of the Seven Years" War in 1756. The French proceeded to invade Hanover, which, it will be remembered, belonged at this time to the British dominions. Young William Herschel had already obtained the position of a regular performer in the regimental band of the Hanoverian Guards, and it was his fortune to obtain some experience of actual warfare in the disastrous battle of Hastenbeck.
He was not wounded, but he had to spend the night after the battle in a ditch, and his meditations on the occasion convinced him that soldiering was not the profession exactly adapted to his tastes. We need not attempt to conceal the fact that he left his regiment by the very simple but somewhat risky process of desertion. He had, it would seem, to adopt disguises to effect his escape. At all events, by some means he succeeded in eluding detection and reached England in safety. It is interesting to have learned on good authority that many years after this offence was committed it was solemnly forgiven. When Herschel had become the famous astronomer, and as such visited King George at Windsor, the King at their first meeting handed to him his pardon for deserting from the army, written out in due form by his Majesty himself.
It seems that the young musician must have had some difficulty in providing for his maintenance during the first few years of his abode in England. It was not until he had reached the age of twenty-two that he succeeded in obtaining any regular appointment. He was then made Instructor of Music to the Durham Militia. Shortly afterwards, his talents being more widely recognised, he was appointed as organist at the parish church at Halifax, and his prospects in life now being fairly favourable, and the Seven Years" War being over, he ventured to pay a visit to Hanover to see his father. We can imagine the delight with which old Isaac Herschel welcomed his promising son, as well as his parental pride when a concert was given at which some of William"s compositions were performed. If the father was so intensely gratified on this occasion, what would his feelings have been could he have lived to witness his son"s future career? But this pleasure was not to be his, for he died many years before William became an astronomer.
In 1766, about a couple of years after his return to England from This visit to his old home, we find that Herschel had received a further promotion to be organist in the Octagon Chapel, at Bath.
Bath was then, as now, a highly fashionable resort, and many notable personages patronised the rising musician. Herschel had other points in his favour besides his professional skill; his appearance was good, his address was prepossessing, and even his nationality was a distinct advantage, inasmuch as he was a Hanoverian in the reign of King George the Third. On Sundays he played the organ, to the great delight of the congregation, and on week-days he was occupied by giving lessons to private pupils, and in preparation for public performances. He thus came to be busily employed, and seems to have been in the enjoyment of comfortable means.
[PLATE: 7, NEW KING STREET, BATH, WHERE HERSCHEL LIVED.]
From his earliest youth Herschel had been endowed with that invaluable characteristic, an eager curiosity for knowledge. He was naturally desirous of perfecting himself in the theory of music, and thus he was led to study mathematics. When he had once tasted the charms of mathematics, he saw vast regions of knowledge unfolded before him, and in this way he was induced to direct his attention to astronomy. More and more this pursuit seems to have engrossed his attention, until at last it had become an absorbing pa.s.sion. Herschel was, however, still obliged, by the exigency of procuring a livelihood, to give up the best part of his time to his profession as a musician; but his heart was eagerly fixed on another science, and every spare moment was steadily devoted to astronomy. For many years, however, he continued to labour at his original calling, nor was it until he had attained middle age and become the most celebrated astronomer of the time, that he was enabled to concentrate his attention exclusively on his favourite pursuit.
It was with quite a small telescope which had been lent him by a friend that Herschel commenced his career as an observer. However, he speedily discovered that to see all he wanted to see, a telescope of far greater power would be necessary, and he determined to obtain this more powerful instrument by actually making it with his own hands. At first it may seem scarcely likely that one whose occupation had previously been the study and practice of music should meet with success in so technical an operation as the construction of a telescope. It may, however, be mentioned that the kind of instrument which Herschel designed to construct was formed on a very different principle from the refracting telescopes with which we are ordinarily familiar. His telescope was to be what is termed a reflector. In this type of instrument the optical power is obtained by the use of a mirror at the bottom of the tube, and the astronomer looks down through the tube TOWARDS HIS MIRROR and views the reflection of the stars with its aid. Its efficiency as a telescope depends entirely on the accuracy with which the requisite form has been imparted to the mirror. The surface has to be hollowed out a little, and this has to be done so truly that the slightest deviation from good workmanship in this essential particular would be fatal to efficient performance of the telescope.
[PLATE: WILLIAM HERSCHEL.]
The mirror that Herschel employed was composed of a mixture of two parts of copper to one of tin; the alloy thus obtained is an intensely hard material, very difficult to cast into the proper shape, and very difficult to work afterwards. It possesses, however, when polished, a l.u.s.tre hardly inferior to that of silver itself.
Herschel has recorded hardly any particulars as to the actual process by which he cast and figured his reflectors. We are however, told that in later years, after his telescopes had become famous, he made a considerable sum of money by the manufacture and sale of great instruments. Perhaps this may be the reason why he never found it expedient to publish any very explicit details as to the means by which his remarkable successes were obtained.
[PLATE: CAROLINE HERSCHEL.]
Since Herschel"s time many other astronomers, notably the late Earl of Rosse, have experimented in the same direction, and succeeded in making telescopes certainly far greater, and probably more perfect, than any which Herschel appears to have constructed. The details of these later methods are now well known, and have been extensively practised. Many amateurs have thus been able to make telescopes by following the instructions so clearly laid down by Lord Rosse and the other authorities. Indeed, it would seem that any one who has a little mechanical skill and a good deal of patience ought now to experience no great difficulty in constructing a telescope quite as powerful as that which first brought Herschel into fame. I should, however, mention that in these modern days the material generally used for the mirror is of a more tractable description than the metallic substance which was employed by Herschel and by Lord Rosse.
A reflecting telescope of the present day would not be fitted with a mirror composed of that alloy known as speculum metal, whose composition I have already mentioned. It has been found more advantageous to employ a gla.s.s mirror carefully figured and polished, just as a metallic mirror would have been, and then to impart to the polished gla.s.s surface a fine coating of silver laid down by a chemical process. The silver-on-gla.s.s mirrors are so much lighter and so much easier to construct that the more old-fashioned metallic mirrors may be said to have fallen into almost total disuse. In one respect however, the metallic mirror may still claim the advantage that, with reasonable care, its surface will last bright and untarnished for a much longer period than can the silver film on the gla.s.s. However, the operation of re-silvering a gla.s.s has now become such a simple one that the advantage this indicates is not relatively so great as might at first be supposed.
[PLATE: STREET VIEW, HERSCHEL HOUSE, SLOUGH.]
Some years elapsed after Herschel"s attention had been first directed to astronomy, before he reaped the reward of his exertions in the possession of a telescope which would adequately reveal some of the glories of the heavens. It was in 1774, when the astronomer was thirty-six years old, that he obtained his first glimpse of the stars with an instrument of his own construction. Night after night, as soon as his musical labours were ended, his telescopes were brought out, sometimes into the small back garden of his house at Bath, and sometimes into the street in front of his hall-door. It was characteristic of him that he was always endeavouring to improve his apparatus. He was incessantly making fresh mirrors, or trying new lenses, or combinations of lenses to act as eye-pieces, or projecting alterations in the mounting by which the telescope was supported.