On reëntering the Conciergerie, the evening before his death, Bailly spoke of the efforts that must have been made to excite the passions of the auditors, who followed the various phases of his trial. Factitious excitement is always the produce of corruption. The working classes are without money;, they then cannot have been the corruptors or direct promoters of the distressing scenes of which Bailly complained.
The implacable enemies of the former President of the National Assembly had procured for pay some auxiliaries among the turnkeys of the Conciergerie. M. Beugnot informs us that when the venerable magistrate was consigned to the gendarmes who were to conduct him to the Tribunal, "these wretches pushed him violently, sending him from one to the other like a drunken man, calling out:Hold there, Bailly! Catch, Bailly, there!and that they laughed and shouted at the grave demeanour the philosopher maintained amidst the insults of those cannibals."
To confirm my statement that these violences (in comparison with which, in truth, those of the Champ de Mars lose their virulence,) were fomented by pay, I have more than the formal declaration of our colleague's fellow prisoner. For in fact I find that no other prisoner or convict underwent such treatment; not even the man called the Admiral, when he was taken to the Conciergerie for having attempted to assassinate Collot-d'Herbois.
Besides, it is not only on indirect considerations that my decided opinion is founded relative to the intervention of rich and influential people in those scenes of indescribable barbarity on the Champ de Mars. Mérard St. Just, the intimate friend of Bailly, has alluded by his initials to a wretch who, the very day of our colleague's death, publicly boasted of having electrified the few acolytes who, together with him, insisted on the removal of the scaffold; the day after the execution, the meeting of the Jacobins reëchoed with the name of another individual of the Gros Caillou, who also claimed his share of influence in the crime.
I have progressively unrolled before you the series of events in our revolution, in which Bailly took an active part; I have scrupulously searched out the smallest circumstances of the deplorable affair on the Champ de Mars; I have followed our colleague in his proscription to the Revolutionary Tribunal, and to the foot of the scaffold. We had seen him before, surrounded by esteem, by respect, and by glory, in the bosom of our principal academies. Yet the work is not complete; several essential traits are still wanting.
I will therefore claim a few more minutes of your kind attention. The moral life of Bailly is like those masterpieces of ancient sculpture, that deserve to be studied in every point of view, and in which new beauties are continually discovered, in proportion as the contemplation is prolonged.
Nature did not endow Bailly generously with those exterior advantages that please us at first sight. He was tall and thin. His visage compressed, his eyes small and sunk, his nose regular, but of unusual length, and a very brown complexion, constituted an imposing whole, severe and almost glacial. Fortunately, it was easy to perceive through this rough bark, the inexhaustible benevolence of the good man; the kindness that always accompanies a serene mind, and even some rudiments of gayety.
Bailly early endeavoured to model his conduct on that of the Abbé de Lacaille, who directed his first steps in the career of astronomy. And therefore it will be found that in transcribing five or six lines of the very feeling eulogy that the pupil dedicated to the memory of his revered master, I shall have made known at the same time many of the characteristic traits of the panegyrist:
"He was cold and reserved towards those of whom he knew little; but gentle, simple, equable, and familiar in the intercourse of friendship. It is there that, throwing off the grave exterior which he wore in public, he gave himself up to a peaceful and amiable gayety."
The resemblance between Bailly and Lacaille goes no farther. Bailly informs us that the great astronomer proclaimed truth on all occasions, without disquieting himself as to whom it might wound. He would not consent to put vice at its ease, saying:
"If good men thus showed their indignation, bad men being known, and vice unmasked, could no longer do harm, and virtue would be more respected." This Spartan morality could not accord with Bailly's character; he admired but did not adopt it.
Tacitus took as a motto: "To say nothing false, to omit nothing true." Our colleague contented himself in society with the first half of the precept. Never did mockery, bitterness, or severity issue from his lips. His manners were a medium between those of Lacaille and the manners of another academician who had succeeded in not making a single enemy, by adopting the two axioms: "Every thing is possible, and everybody is in the right."
Crébillon obtained permission from the French Academy to make his reception discourse in verse. At the moment when that poet, then almost sixty years of age, said, speaking of himself,
"No gall has ever poisoned my pen,"
"No gall has ever poisoned my pen,"
the hall reëchoed with approbation.
I was going to apply this line by the author ofRhadamistusto our colleague, when accident offered to my sight a passage in which Lalande reproaches Bailly for having swerved from his usual character, in 1773, in a discussion that they had together on a point in the theory of Jupiter's Satellites. I set about the search for this discussion; I found the article by Bailly in a journal of that epoch, and I affirm that this dispute does not contain a word but what is in harmony with all our colleague's published writings. I return therefore to my former idea, and say of Bailly, with perfect confidence,
"No gall had ever poisoned his pen."
"No gall had ever poisoned his pen."
Diffidence is usually the trait that the biographers of studious men endeavour most to put in high relief. I dare assert, that in the common acceptation, this is pure flattery. To merit the epithet of diffident, must we think ourselves beneath the competitors of whom we are at least the equals? Must we, in examining ourselves, fail in the tact, in the intelligence, in the judgment, that nature has awarded us, and of which we make so good a use in appreciating the works of others? Oh! then, few learned men can be said to be diffident. Look at Newton: his diffidence is almost as celebrated as his genius. Well, I will extract from two of his letters, scarcely known, two paragraphs which, put side by side, will excite some surprise; the first confirms the general opinion; the second seems with equal force to contradict it. Here are the two passages:
"We are diffident in the presence of Nature."
"We may nobly feel our own strength in the face of man's works."
In my opinion, the opposition in these two passages is only apparent; it will he explained by means of a distinction which I have already slightly indicated.
Bailly's diffidence required the same distinction. When people praised him to his face on the diversity of his knowledge, our colleague did not immediately repel the compliment; but soon after, he would stop his panegyrist, and whisper in his ear with an air of mystery: "I will confide a secret to you, pray do not take advantage of it: I am only a very little less ignorant than another man."
Never did a man act more in harmony with his principles. Bailly was led to reprimand severely a man belonging to the humblest and poorest class of society. Anger does not make him forget that he speaks to a citizen, to a man. "I ask pardon," says the first magistrate of the capital, addressing himself to a rag-gatherer; "I ask your pardon, if I am angry; but your conduct is so reprehensible, that I cannot speak to you otherwise."
Bailly's friends were wont to say that he devoted too much of his patrimony to pleasure. This word was calumniously interpreted. Mérard Saint Just has given the true sense of it: "Bailly's pleasure was beneficence."
So eminent a mind could not fail to be tolerant. Such in fact Bailly constantly showed himself in politics, and what is almost equally rare, in regard to religion. In the month of June, 1791, he checked in severe terms the fury with which the multitude appeared to be excited, at the report that at the Théatines some persons had taken the Communion two or three times in one day. "The accusation is undoubtedly false," said the Mayor of Paris; "but if it were true, the public would not have a right to inquire into it. Every one should have the free choice of his religion and his creed." Nothing would have been wanting in the picture, if Bailly had taken the trouble to remark how strange it was, that these violent scruples against repeated Communions emanated from persons who probably never took the Sacrament at all.
The reports on animal magnetism, on the hospitals, on the slaughter-houses, had carried Bailly's name into regions, whence the courtiers knew very cleverly how to discard true merit.Madamethen wished to attach the illustrious academician to her person as a cabinet secretary. Bailly accepted. It was an entirely honorary title. The secretary saw the princess only once, that was on the day of his presentation.
Were more important functions reserved for him? We must suppose so; for some influential persons offered to procure Bailly a title of nobility and a decoration. This time the philosopher flatly refused, saying, in answer to the earnest negotiators: "I thank you, but he who has the honour of belonging to the three principal academies of France is sufficiently decorated, sufficiently noble in the eyes of rational men; a cordon, or a title, could add nothing to him."
The first secretary of the Academy of Sciences had, some years before, acted as Bailly did. Only he gave his refusal in such strong terms, that I could not easily believe them to have been written by the timid pen of Fontenelle, if I did not find them in a perfectly authentic document, in which he says: "Of all the titles in this world, I have never had any but of one sort, the titles of Academician, and they have not been profaned by an admixture of any others, more worldly and more ostentatious."
Bailly married, in November, 1787, an intimate friend of his mother's, already a widow, only two years younger than himself. Madame Bailly, a distant relation of the author of theMarseillaise, had an attachment for her husband that bordered on adoration. She lavished on him the most tender and affectionate attention. The success that Madame Bailly might have had in the fashionable world by her beauty, her grace, by her ineffable goodness, did not tempt her. She lived in almost absolute retirement, even when the learned academician was most in society. The Mayor's wife appeared only at one public ceremony: the day of the benediction of the colours of the sixty battalions of the National Guard by the Archbishop of Paris, she accompanied Madame de Lafayette to the Cathedral. She said: "My husband's duty is to show himself in public wherever there is any good to be done, or sound advice to be given; mine is to remain at home." This rare retiring and respectable conduct did not disarm some hideous pamphleteers. Their impudent sarcasms were continually attacking the modest wife on her domestic hearth, and troubling her peace of mind. In their logic of the tavern they fancied that an elegant and handsome woman, who avoided society, could not fail to be ignorant and stupid. Thence arose a thousand imaginary stories, ridiculous both as to their matter and form, thrown out daily to the public, more, indeed, to offend and disgust the upright magistrate than to humble his companion.
The axe that ended our colleague's life, with the same stroke, and almost as completely, crushed in Madame Bailly, after so many poignant agitations and unexampled misfortunes, all that was left of strength of mind and power of intellect. A strange incident also aggravated the sadness of Madame Bailly's situation. On a day of trouble, during her husband's lifetime, she had placed the assignats resulting from the sale of their house at Chaillot, amounting to about thirty thousand francs, in the wadding of a dress. The enfeebled memory of the unfortunate widow did not recall to her the existence of this treasure, even in the time of her greatest distress. When the age of the material which had secreted them began to reveal them to daylight, they were no longer of any value.
The widow of the author of one of the best works of the age, of the learned member of our three great academies, of the first President of the National Assembly, of the first Mayor of Paris, found herself thus reduced, by an unheard-of turn of fortune, to implore help from public pity. It was the geometer Cousin, member of this academy, who by his incessant solicitations got Madame Bailly's name inserted at the Board of Charity in his arrondissement. The support was distributed in kind. Cousin used to receive the articles at the Hôtel de Ville, where he was a Municipal Councillor, and carried them himself to the street de la Sourdière. It was, in short, in the street de la Sourdière that Madame Bailly had obtained two rooms gratis, in the house of a compassionate person, whose name I very much regret not having learnt. Does it not appear to you, Gentlemen, that the academician Cousin, who crossed the whole of Paris, with the bread under his arm and the meat and the candle, intended for the unfortunate widow of an illustrious colleague, did himself more honour than if he had come to one of the sittings bringing in his portfolio the results of some fine scientific research? Such noble actions are certainly worth good "Papers."
Affairs proceeded thus up to the revolution of the 18th Brumaire. On the 21st, the public criers were announcing everywhere, even in the street de la Sourdière, that General Bonaparte was Consul, and M. de Laplace Minister of the Interior. This name, so well known by the respectable widow, reached even the room that she inhabited, and caused her some emotion. That same evening, the new minister (this was a noble beginning, Gentlemen) asked for a pension of 2000 francs for Madame Bailly. The Consul granted the demand, adding to it this express condition, that the first half year should be paid in advance, and immediately. Early on the 22d, a carriage stopped in the street de la Sourdière; Madame de Laplace descends from it, carrying in her hand a purse filled with gold. She rushed to the staircase, runs to the humble abode, that had now for several years witnessed irremediable sorrow and severe misery; Madame Bailly was at the window: "My dear friend, what are you doing there so early?" exclaimed the wife of the minister. "Madam," replied the widow, "I heard the public crier yesterday, and I was expecting you!"
If after having, from a sense of duty, expatiated upon anarchical, odious, and sanguinary scenes, the historian of our civil discords has the good fortune to meet on his progress with an incident that gratifies the mind, raises the soul, and fills the heart with pleasing emotions, he stops there, Gentlemen, as the African traveller halts in an oasis!
William Herschel, one of the greatest astronomers that ever lived in any age or country, was born at Hanover, on the 15th of November, 1738. The name of Herschel has become too illustrious for people to neglect searching back, up the stream of time, to learn the social position of the families that have borne it. Yet the just curiosity of the learned world on this subject has not been entirely satisfied. We only know that Abraham Herschel, great-grandfather of the astronomer, resided at Mähren, whence he was expelled on account of his strong attachment to the Protestant faith; that Abraham's son Isaac was a farmer in the vicinity of Leipzig; that Isaac's eldest son, Jacob Herschel, resisted his father's earnest desire to see him devote himself to agriculture, that he determined on being a musician, and settled at Hanover.
Jacob Herschel, father of William, the astronomer, was an eminent musician; nor was he less remarkable for the good qualities of his heart and of his mind. His very limited means did not enable him to bestow a complete education on his family, consisting of six boys and four girls. But at least, by his care, his ten children all became excellent musicians. The eldest, Jacob, even acquired a rare degree of ability, which procured for him the appointment of Master of the Band in a Hanoverian regiment, which he accompanied to England. The third son, William, remained under his father's roof. Without neglecting the fine arts, he took lessons in the French language, and devoted himself to the study of metaphysics, for which he retained a taste to his latest day.
In 1759, William Herschel, then about twenty-one years old, went over to England, not with his father, as has been erroneously published, but with his brother Jacob, whose connections in that country seemed likely to favour the young man's opening prospects in life. Still, neither London nor the country towns afforded him any resource in the beginning, and the first two or three years after his expatriation were marked by some cruel privations, which, however, were nobly endured. A fortunate chance finally raised the poor Hanoverian to a better position; Lord Durham engaged him as Master of the Band in an English regiment which was quartered on the borders of Scotland. From this moment the musician Herschel acquired a reputation that spread gradually, and in the year 1765 he was appointed organist at Halifax (Yorkshire). The emoluments of this situation, together with giving private lessons both in the town and the country around, procured a degree of comfort for the young William. He availed himself of it to remedy, or rather to complete, his early education. It was then that he learnt Latin and Italian, though without any other help than a grammar and a dictionary. It was then also that he taught himself something of Greek. So great was the desire for knowledge with which he was inspired while residing at Halifax, that Herschel found means to continue his hard philological exercises, and at the same time to study deeply the learned but very obscure mathematical work on the theory of music by R. Smith. This treatise, either explicitly or implicitly, supposed the reader to possess some knowledge of algebra and of geometry, which Herschel did not possess, but of which he made himself master in a very short time.
In 1766, Herschel obtained the appointment of organist to the Octagon Chapel at Bath. This was a more lucrative post than that of Halifax, but new obligations also devolved on the able pianist. He had to play incessantly either at the Oratorios, or in the rooms at the baths, at the theatre, and in the public concerts. Then, being immersed in the most fashionable circle in England, Herschel could no longer refuse the numerous pupils who wished to be instructed in his school. It is difficult to imagine how, among so many duties, so many distractions of various kinds, Herschel could continue so many studies, which already at Halifax had required in him so much resolution, so much perseverance, and a very uncommon degree of talent. We have already seen that it was by music that Herschel was led to mathematics; mathematics in their turn led him to optics, the principal and fertile source of his illustrious career. The hour finally struck, when his theoretic knowledge was to guide the young musician into a laborious application of principles quite foreign to his habits; and the brilliant success of which, as well as their excessive hardihood, will excite reasonable astonishment.
A telescope, a simple telescope, only two English feet in length, falls into the hands of Herschel during his residence at Bath. This instrument, however imperfect, shows him a multitude of stars in the sky that the naked eye cannot discern; shows him also some of the known objects, but now under their true dimensions; reveals forms to him that the richest imaginations of antiquity had never suspected. Herschel is transported with enthusiasm. He will, without delay, have a similar instrument but of larger dimensions. The answer from London is delayed for some days: these few days appear as many centuries to him. When the answer arrives, the price that the optician demands proves to be much beyond the pecuniary resources of a mere organist. To any other man this would have been a clap of thunder. This unexpected difficulty on the contrary, inspired Herschel with fresh energy; he cannot buy a telescope, then he will construct one with his own hands. The musician of the Octagon Chapel rushes immediately into a multitude of experiments, on metallic alloys that reflect light with the greatest intensity, on the means of giving the parabolic figure to the mirrors, on the causes that in the operation of polishing affect the regularity of the figure, &c. So rare a degree of perseverance at last receives its reward. In 1774 Herschel has the happiness of being able to examine the heavens with a Newtonian telescope of five English feet focus, entirely made by himself. This success tempts him to undertake still more difficult enterprises. Other telescopes of seven, of eight, of ten, and even of twenty feet focal distance, crown his efforts. As if to answer in advance those critics who would have accused him of a superfluity of apparatus, of unnecessary luxury, in the large size of the new instruments, and his extreme minutiæ in their execution, Nature granted to the astronomical musician, on the 13th of March 1781, the unheard-of honour of commencing his career of observation with the discovery of a new planet, situated on the confines of our solar system. Dating from that moment, Herschel's reputation, no longer in his character of musician, but as a constructor of telescopes and as an astronomer, spread throughout the world. The King, George III., a great lover of science, and much inclined besides to protect and patronize both men and things of Hanoverian origin, had Herschel presented to him; he was charmed with the simple yet lucid and modest account that he gave of his repeated endeavours; he caught a glimpse of the glory that so penetrating an observer might reflect on his reign, ensured to him a pension of 300 guineas a year, and moreover a residence near Windsor Castle, first at Clay Hall and then at Slough. The visions of George III. were completely realized. We may confidently assert, relative to the little house and garden of Slough, that it is the spot of all the world where the greatest number of discoveries have been made. The name of that village will never perish; science will transmit it religiously to our latest posterity.
I will avail myself of this opportunity to rectify a mistake, of which ignorance and idleness wish to make a triumphant handle, or, at all events, to wield in their cause as an irresistible justification. It has been repeated to satiety, that at the time when Herschel entered on his astronomical career he knew nothing of mathematics. But I have already said, that during his residence at Bath, the organist of the Octagon Chapel had familiarized himself with the principles of geometry and algebra; and a still more positive proof of this is, that a difficult question on the vibration of strings loaded with small weights had been proposed for discussion in 1779: Herschel undertook to solve it, and his dissertation was inserted in several scientific collections of the year 1780.
The anecdotic life of Herschel, however, is now closed. The great astronomer will not quit his observatory any more, except to go and submit the sublime results of his laborious vigils to the Royal Society of London. These results are contained in his memoirs; they constitute one of the principal riches of the celebrated collection known under the title ofPhilosophical Transactions.
Herschel belonged to the principal Academies of Europe, and about 1816 he was named Knight of the Guelphic order of Hanover. According to the English habit, from the time of that nomination the title of Sir William took the place, in all this illustrious astronomer's memoirs, already honoured with so much celebrity, of the former appellation of Doctor William. Herschel had been named a Doctor (of laws) in the University of Oxford in 1786. This dignity, by special favour, was conferred on him without any of the obligatory formalities of examination, disputation, or pecuniary contribution, usual in that learned corporation.
I should wound the elevated sentiments that Herschel professed all his life, if I were not here to mention two indefatigable assistants that this fortunate astronomer found in his own family. The one was Alexander Herschel, endowed with a remarkable talent for mechanism, always at his brother's orders, and who enabled him to realize without delay any ideas that he had conceived;[15]the other was Miss Caroline Herschel, who deserves a still more particular and detailed mention.
Miss Caroline Lucretia Herschel went to England as soon as her brother became special astronomer to the king. She received the appellation there of Assistant Astronomer, with a moderate salary. From that moment she unreservedly devoted herself to the service of her brother, happy in contributing night and day to his rapidly increasing scientific reputation. Miss Caroline shared in all the night-watches of her brother, with her eye constantly on the clock, and the pencil in her hand; she made all the calculations without exception; she made three or four copies of all the observations in separate registers; coördinated, classed, and analyzed them. If the scientific world saw with astonishment how Herschel's works succeeded each other with unexampled rapidity during so many years, they were specially indebted for it to the ardour of Miss Caroline. Astronomy, moreover, has been directly enriched by several comets through this excellent and respectable lady. After the death of her illustrious brother, Miss Caroline retired to Hanover, to the house of Jahn Dietrich Herschel, a musician of high reputation, and the only surviving brother of the astronomer.
William Herschel died without pain on the 23d of August 1822, aged eighty-three. Good fortune and glory never altered in him the fund of infantine candour, inexhaustible benevolence, and sweetness of character, with which nature had endowed him. He preserved to the last both his brightness of mind and vigour of intellect. For some years Herschel enjoyed with delight the distinguished success of his only son,[16]Sir John Herschel. At his last hour he sunk to rest with the pleasing conviction that his beloved son, heir of a great name, would not allow it to fall into oblivion, but adorn it with fresh lustre, and that great discoveries would honour his career also. No prediction of the illustrious astronomer has been more completely verified.
The English journals gave an account of the means adopted by the family of William Herschel, for preserving the remains of the great telescope of thirty-nine English feet (twelve metres) constructed by that celebrated astronomer.
The metal tube of the instrument carrying at one end the recently cleaned mirror of four feet ten inches in diameter, has been placed horizontally in the meridian line, on solid piers of masonry, in the midst of the circle, where formerly stood the mechanism requisite for manœuvring the telescope. The first of January 1840, Sir John Herschel, his wife, their children, seven in number, and some old family servants, assembled at Slough. Exactly at noon, the party walked several times in procession round the instrument; they then entered the tube of the telescope, seated themselves on benches that had been prepared for the purpose, and sung a requiem, with English words composed by Sir John Herschel himself. After their exit, the illustrious family ranged themselves around the great tube, the opening of which was then hermetically sealed. The day concluded with a party of intimate friends.
I know not whether those persons who will only appreciate things from the peculiar point of view from which they have been accustomed to look, may think there was something strange in several of the details of the ceremony that I have just described. I affirm at least that the whole world will applaud the pious feeling which actuated Sir John Herschel; and that all the friends of science will thank him for having consecrated the humble garden where his father achieved such immortal labours, by a monument more expressive in its simplicity than pyramids or statues.
FOOTNOTES:[15]When age and infirmities obliged Alexander Herschel to give up his profession as a musician, he quitted Bath, and returned to Hanover, very generously provided by Sir William with a comfortable independence for life.[16]Sir W. Herschel had married Mary, the widow of John Pitt, Esq., possessed of a considerable jointure, and the union proved a remarkable accession of domestic happiness. This lady survived Sir William by several years. They had but this son.—Translator's Note.
[15]When age and infirmities obliged Alexander Herschel to give up his profession as a musician, he quitted Bath, and returned to Hanover, very generously provided by Sir William with a comfortable independence for life.
[15]When age and infirmities obliged Alexander Herschel to give up his profession as a musician, he quitted Bath, and returned to Hanover, very generously provided by Sir William with a comfortable independence for life.
[16]Sir W. Herschel had married Mary, the widow of John Pitt, Esq., possessed of a considerable jointure, and the union proved a remarkable accession of domestic happiness. This lady survived Sir William by several years. They had but this son.—Translator's Note.
[16]Sir W. Herschel had married Mary, the widow of John Pitt, Esq., possessed of a considerable jointure, and the union proved a remarkable accession of domestic happiness. This lady survived Sir William by several years. They had but this son.—Translator's Note.
1780.Philosophical Transactions, vol. lxx.—Astronomical Observations on the Periodical Star in the Neck of the Whale.—Astronomical Observations relative to the Lunar Mountains.1781.Phil. Trans., vol. lxxi.—Astronomical Observations on the Rotation of the Planets on their Axes, made with a View to decide whether the Daily Rotation of the Earth be always the same.—On the Comet of 1781, afterwards called theGeorgium Sidus.1782.Phil. Trans., vol. lxxii.—On the Parallax of the Fixed Stars.—Catalogue of Double Stars.—Description of a Lamp Micrometer, and the Method of using it.—Answers to the Doubts that might be raised to the high magnifying Powers used by Herschel.1783.Phil. Trans., vol. lxxiii.—Letter to Sir Joseph Banks on the Name to be given to the new Planet.—On the Diameter of the Georgium Sidus, followed by the Description of a Micrometer with luminous or dark Disks.—On the proper Motion of the Solar System, and the various Changes that have occurred among the Fixed Stars since the Time of Flamsteed.1784.Phil. Trans., vol. lxxiv.—On some remarkable Appearances in the Polar Regions of Mars, the Inclination of its Axis, the Position of its Poles, and its Spheroïdal Form.—Some Details on the real Diameter of Mars, and on its Atmosphere.—Analysis of some Observations on the Constitution of the Heavens.1785.Phil. Trans., vol. lxxv.—Catalogue of Double Stars.—On the Constitution of the Heavens.1786.Phil Trans., vol., lxxvi.—Catalogue of a Thousand Nebulæ and Clusters of Stars.—Researches on the Cause of a Defect of Definition in Vision, which has been attributed to the Smallness of the Optic Pencils.1787.Phil. Trans., vol. lxxvii.—Remarks on the new Comet.—Discovery of Two Satellites revolving round George's Planet.—On Three Volcanoes in the Moon.1788.Phil. Trans., vol. lxxviii.—On George's Planet (Uranus) and its Satellites.1789.Phil. Trans., vol. lxxix.—Observations on a Comet. Catalogue of a Second Thousand new Nebulæ and Clusters of Stars.—Some Preliminary Remarks on the Constitution of the Heavens.1790.Phil. Trans., vol. lxxx.—Discovery of Saturn's Sixth and Seventh Satellites; with Remarks on the Constitution of the Ring, on the Planet's Rotation round an Axis, on its Spheroïdal Form, and on its Atmosphere.—On Saturn's Satellites, and the Rotation of the Ring round an Axis.1791.Phil. Trans., vol. lxxxi.—On the Nebulous Stars and the Suitableness of this Epithet.1792.Phil. Trans., vol. lxxxii.—On Saturn's Ring, and the Rotation of the Planet's Fifth Satellite round an Axis.—Mixed Observations.1793.Phil. Trans., vol. lxxxiii.—Observations on the Planet Venus.1794.Phil. Trans., vol. lxxxiv.—Observations on a Quintuple Band in Saturn.—On some Peculiarities observed during the last Solar Eclipse.—On Saturn's Rotation round an Axis.1795.Phil. Trans., vol. lxxxv.—On the Nature and Physical Constitution of the Sun and Stars.—Description of a Reflecting Telescope forty feet in length.1796.Phil. Trans., vol. lxxxvi.—Method of observing the Changes that happen to the Fixed Stars; Remarks on the Stability of our Sun's Light.—Catalogue of Comparative Brightness, to determine the Permanency of the Lustre of Stars.—On the Periodical Star α Herculis, with Remarks tending to establish the Rotatory Motion of the Stars on their Axes; to which is added a second Catalogue of the Brightness of the Stars.1797.Phil. Trans., vol. lxxxvii.—A Third Catalogue of the comparative Brightness of the Stars; with an Introductory Account of an Index to Mr. Flamsteed's Observations of the Fixed Stars, contained in the Second Volume of the Historia Cœlestis to which are added several useful Results derived from that Index.—Observations of the changeable Brightness of the Satellites of Jupiter, and of the Variation in their apparent Magnitudes; with a Determination of the Time of their rotary Motions on their Axes, to which is added a Measure of the Diameter of the Second Satellite, and an Estimate of the comparative Size of the Fourth.1798.Phil. Trans., vol. lxxxviii.—On the Discovery of Four additional Satellites of the Georgium Sidus. The retrograde Motion of its old Satellites announced; and the Cause of their Disappearance at certain Distances from the Planet explained.1799.Phil. Trans., vol. lxxxix.—A Fourth Catalogue of the comparative Brightness of the Stars.1800.Phil. Trans., vol. xc.—On the Power of penetrating into Space by Telescopes, with a comparative Determination of the Extent of that Power in Natural Vision, and in Telescopes of various Sizes and Constructions; illustrated by select Observations.—Investigation of the Powers of the Prismatic Colours to heat and illuminate Objects; with Remarks that prove the different Refrangibility of radiant Heat; to which is added an Inquiry into the Method of viewing the Sun advantageously with Telescopes of large Apertures and high magnifying Powers.—Experiments on the Refrangibility of the Invisible Rays of the Sun.—Experiments on the Solar and on the Terrestrial Rays that occasion Heat; with a comparative View of the Laws to which Light and Heat, or rather the Rays which occasion them, are subject, in order to determine whether they are the same or different.1801.Phil. Trans., vol. xci.—Observations tending to investigate the Nature of the Sun, in order to find the Causes or Symptoms of its variable Emission of Light and Heat; with Remarks on the Use that may possibly be drawn from Solar Observations.—Additional Observations tending to investigate the Symptoms of the variable Emission of the Light and Heat of the Sun; with Trials to set aside darkening Glasses, by transmitting the Solar Rays through Liquids, and a few Remarks to remove Objections that might be made against some of the Arguments contained in the former paper.1802.Phil. Trans., vol. xcii.—Observations on the two lately discovered celestial Bodies (Ceres and Pallas).—Catalogue of 500 new Nebulæ and Clusters of Stars, with Remarks on the Construction of the Heavens.1803.Phil. Trans., vol. xciii.—Observations of the Transit of Mercury over the Disk of the Sun; to which is added an Investigation of the Causes which often prevent the proper Action of Mirrors.—Account of the Changes that have happened during the last Twenty-five Years in the relative Situation of Double Stars; with an Investigation of the Cause to which they are owing.1804.Phil. Trans., vol. xciv.—Continuation of an Account of the Changes that have happened in the relative Situation of Double Stars.1805.Phil. Trans., vol. xcv.—Experiments for ascertaining how far Telescopes will enable us to determine very small Angles, and to distinguish the real from the spurious Diameters of Celestial and Terrestrial Objects: with an Application of the Result of these Experiments to a Series of Observations on the Nature and Magnitude of Mr. Harding's lately discovered Star.—On the Direction and Velocity of the Motion of the Sun and Solar System.—Observation on the singular Figure of the Planet Saturn.1806.Phil. Trans., vol. xcvi.—On the Quantity and Velocity of the Solar Motion.—Observations on the Figure, the Climate, and the Atmosphere of Saturn and its Ring.1807.Phil. Trans., vol. xcvii.—Experiments for investigating the Cause of the Coloured Concentric Rings, discovered by Sir Isaac Newton between two Object-glasses laid one upon another.—Observations on the Nature of the new celestial Body discovered by Dr. Olbers, and of the Comet which was expected to appear last January in its Return from the Sun.1808.Phil. Trans., vol. xcviii.—Observations of a Comet, made with a view to investigate its Magnitude, and the Nature of its Illumination. To which is added, an Account of a new Irregularity lately perceived in the Apparent Figure of the Planet Saturn.1809.Phil. Trans., vol. xcix.—Continuation of Experiments for investigating the Cause of Coloured Concentric Rings, and other Appearances of a similar Nature.1810.Phil. Trans., vol. c.—Supplement to the First and Second Part of the Paper of Experiments for investigating the Cause of Coloured Concentric Rings between Object-glasses, and other Appearances of a similar Nature.1811.Phil. Trans., vol. ci.—Astronomical Observations relating to the Construction of the Heavens, arranged for the Purpose of a critical Examination, the Result of which appears to throw some new Light upon the Organization of the Celestial Bodies.1812.Phil. Trans., vol. cii.—Observations of a Comet, with Remarks on the Construction of its different Parts.—Observations of a Second Comet, with Remarks on its Construction.1814.Phil. Trans., vol. civ.—Astronomical Observations relating to the Sidereal Part of the Heavens, and its Connection with the Nebulous Part; arranged for the Purpose of a critical Examination.1815.Phil. Trans., vol. cv.—A Series of Observations of the Satellites of the Georgian Planet, including a Passage through the Node of their Orbits; with an Introductory Account of the Telescopic Apparatus that has been used on this Occasion, and a final Exposition of some calculated Particulars deduced from the Observations.1817.Phil. Trans., vol. cvii.—Astronomical Observations and Experiments tending to investigate the Local Arrangement of the Celestial Bodies in Space, and to determine the Extent and Condition of the Milky Way.1818.Phil. Trans., vol. cviii.—Astronomical Observations and Experiments selected for the Purpose of ascertaining the relative Distances of Clusters of Stars, and of investigating how far the Power of Telescopes may be expected to reach into Space, when directed to ambiguous Celestial Objects.1822.Memoirs of the Astronomical Society of London.—On the Positions of 145 new Double Stars.
1780.Philosophical Transactions, vol. lxx.—Astronomical Observations on the Periodical Star in the Neck of the Whale.—Astronomical Observations relative to the Lunar Mountains.
1781.Phil. Trans., vol. lxxi.—Astronomical Observations on the Rotation of the Planets on their Axes, made with a View to decide whether the Daily Rotation of the Earth be always the same.—On the Comet of 1781, afterwards called theGeorgium Sidus.
1782.Phil. Trans., vol. lxxii.—On the Parallax of the Fixed Stars.—Catalogue of Double Stars.—Description of a Lamp Micrometer, and the Method of using it.—Answers to the Doubts that might be raised to the high magnifying Powers used by Herschel.
1783.Phil. Trans., vol. lxxiii.—Letter to Sir Joseph Banks on the Name to be given to the new Planet.—On the Diameter of the Georgium Sidus, followed by the Description of a Micrometer with luminous or dark Disks.—On the proper Motion of the Solar System, and the various Changes that have occurred among the Fixed Stars since the Time of Flamsteed.
1784.Phil. Trans., vol. lxxiv.—On some remarkable Appearances in the Polar Regions of Mars, the Inclination of its Axis, the Position of its Poles, and its Spheroïdal Form.—Some Details on the real Diameter of Mars, and on its Atmosphere.—Analysis of some Observations on the Constitution of the Heavens.
1785.Phil. Trans., vol. lxxv.—Catalogue of Double Stars.—On the Constitution of the Heavens.
1786.Phil Trans., vol., lxxvi.—Catalogue of a Thousand Nebulæ and Clusters of Stars.—Researches on the Cause of a Defect of Definition in Vision, which has been attributed to the Smallness of the Optic Pencils.
1787.Phil. Trans., vol. lxxvii.—Remarks on the new Comet.—Discovery of Two Satellites revolving round George's Planet.—On Three Volcanoes in the Moon.
1788.Phil. Trans., vol. lxxviii.—On George's Planet (Uranus) and its Satellites.
1789.Phil. Trans., vol. lxxix.—Observations on a Comet. Catalogue of a Second Thousand new Nebulæ and Clusters of Stars.—Some Preliminary Remarks on the Constitution of the Heavens.
1790.Phil. Trans., vol. lxxx.—Discovery of Saturn's Sixth and Seventh Satellites; with Remarks on the Constitution of the Ring, on the Planet's Rotation round an Axis, on its Spheroïdal Form, and on its Atmosphere.—On Saturn's Satellites, and the Rotation of the Ring round an Axis.
1791.Phil. Trans., vol. lxxxi.—On the Nebulous Stars and the Suitableness of this Epithet.
1792.Phil. Trans., vol. lxxxii.—On Saturn's Ring, and the Rotation of the Planet's Fifth Satellite round an Axis.—Mixed Observations.
1793.Phil. Trans., vol. lxxxiii.—Observations on the Planet Venus.
1794.Phil. Trans., vol. lxxxiv.—Observations on a Quintuple Band in Saturn.—On some Peculiarities observed during the last Solar Eclipse.—On Saturn's Rotation round an Axis.
1795.Phil. Trans., vol. lxxxv.—On the Nature and Physical Constitution of the Sun and Stars.—Description of a Reflecting Telescope forty feet in length.
1796.Phil. Trans., vol. lxxxvi.—Method of observing the Changes that happen to the Fixed Stars; Remarks on the Stability of our Sun's Light.—Catalogue of Comparative Brightness, to determine the Permanency of the Lustre of Stars.—On the Periodical Star α Herculis, with Remarks tending to establish the Rotatory Motion of the Stars on their Axes; to which is added a second Catalogue of the Brightness of the Stars.
1797.Phil. Trans., vol. lxxxvii.—A Third Catalogue of the comparative Brightness of the Stars; with an Introductory Account of an Index to Mr. Flamsteed's Observations of the Fixed Stars, contained in the Second Volume of the Historia Cœlestis to which are added several useful Results derived from that Index.—Observations of the changeable Brightness of the Satellites of Jupiter, and of the Variation in their apparent Magnitudes; with a Determination of the Time of their rotary Motions on their Axes, to which is added a Measure of the Diameter of the Second Satellite, and an Estimate of the comparative Size of the Fourth.
1798.Phil. Trans., vol. lxxxviii.—On the Discovery of Four additional Satellites of the Georgium Sidus. The retrograde Motion of its old Satellites announced; and the Cause of their Disappearance at certain Distances from the Planet explained.
1799.Phil. Trans., vol. lxxxix.—A Fourth Catalogue of the comparative Brightness of the Stars.
1800.Phil. Trans., vol. xc.—On the Power of penetrating into Space by Telescopes, with a comparative Determination of the Extent of that Power in Natural Vision, and in Telescopes of various Sizes and Constructions; illustrated by select Observations.—Investigation of the Powers of the Prismatic Colours to heat and illuminate Objects; with Remarks that prove the different Refrangibility of radiant Heat; to which is added an Inquiry into the Method of viewing the Sun advantageously with Telescopes of large Apertures and high magnifying Powers.—Experiments on the Refrangibility of the Invisible Rays of the Sun.—Experiments on the Solar and on the Terrestrial Rays that occasion Heat; with a comparative View of the Laws to which Light and Heat, or rather the Rays which occasion them, are subject, in order to determine whether they are the same or different.
1801.Phil. Trans., vol. xci.—Observations tending to investigate the Nature of the Sun, in order to find the Causes or Symptoms of its variable Emission of Light and Heat; with Remarks on the Use that may possibly be drawn from Solar Observations.—Additional Observations tending to investigate the Symptoms of the variable Emission of the Light and Heat of the Sun; with Trials to set aside darkening Glasses, by transmitting the Solar Rays through Liquids, and a few Remarks to remove Objections that might be made against some of the Arguments contained in the former paper.
1802.Phil. Trans., vol. xcii.—Observations on the two lately discovered celestial Bodies (Ceres and Pallas).—Catalogue of 500 new Nebulæ and Clusters of Stars, with Remarks on the Construction of the Heavens.
1803.Phil. Trans., vol. xciii.—Observations of the Transit of Mercury over the Disk of the Sun; to which is added an Investigation of the Causes which often prevent the proper Action of Mirrors.—Account of the Changes that have happened during the last Twenty-five Years in the relative Situation of Double Stars; with an Investigation of the Cause to which they are owing.
1804.Phil. Trans., vol. xciv.—Continuation of an Account of the Changes that have happened in the relative Situation of Double Stars.
1805.Phil. Trans., vol. xcv.—Experiments for ascertaining how far Telescopes will enable us to determine very small Angles, and to distinguish the real from the spurious Diameters of Celestial and Terrestrial Objects: with an Application of the Result of these Experiments to a Series of Observations on the Nature and Magnitude of Mr. Harding's lately discovered Star.—On the Direction and Velocity of the Motion of the Sun and Solar System.—Observation on the singular Figure of the Planet Saturn.
1806.Phil. Trans., vol. xcvi.—On the Quantity and Velocity of the Solar Motion.—Observations on the Figure, the Climate, and the Atmosphere of Saturn and its Ring.
1807.Phil. Trans., vol. xcvii.—Experiments for investigating the Cause of the Coloured Concentric Rings, discovered by Sir Isaac Newton between two Object-glasses laid one upon another.—Observations on the Nature of the new celestial Body discovered by Dr. Olbers, and of the Comet which was expected to appear last January in its Return from the Sun.
1808.Phil. Trans., vol. xcviii.—Observations of a Comet, made with a view to investigate its Magnitude, and the Nature of its Illumination. To which is added, an Account of a new Irregularity lately perceived in the Apparent Figure of the Planet Saturn.
1809.Phil. Trans., vol. xcix.—Continuation of Experiments for investigating the Cause of Coloured Concentric Rings, and other Appearances of a similar Nature.
1810.Phil. Trans., vol. c.—Supplement to the First and Second Part of the Paper of Experiments for investigating the Cause of Coloured Concentric Rings between Object-glasses, and other Appearances of a similar Nature.
1811.Phil. Trans., vol. ci.—Astronomical Observations relating to the Construction of the Heavens, arranged for the Purpose of a critical Examination, the Result of which appears to throw some new Light upon the Organization of the Celestial Bodies.
1812.Phil. Trans., vol. cii.—Observations of a Comet, with Remarks on the Construction of its different Parts.—Observations of a Second Comet, with Remarks on its Construction.
1814.Phil. Trans., vol. civ.—Astronomical Observations relating to the Sidereal Part of the Heavens, and its Connection with the Nebulous Part; arranged for the Purpose of a critical Examination.
1815.Phil. Trans., vol. cv.—A Series of Observations of the Satellites of the Georgian Planet, including a Passage through the Node of their Orbits; with an Introductory Account of the Telescopic Apparatus that has been used on this Occasion, and a final Exposition of some calculated Particulars deduced from the Observations.
1817.Phil. Trans., vol. cvii.—Astronomical Observations and Experiments tending to investigate the Local Arrangement of the Celestial Bodies in Space, and to determine the Extent and Condition of the Milky Way.
1818.Phil. Trans., vol. cviii.—Astronomical Observations and Experiments selected for the Purpose of ascertaining the relative Distances of Clusters of Stars, and of investigating how far the Power of Telescopes may be expected to reach into Space, when directed to ambiguous Celestial Objects.
1822.Memoirs of the Astronomical Society of London.—On the Positions of 145 new Double Stars.
The chronological and detailed analysis of so many labours would throw us into numerous repetitions. A systematic order will be preferable; it will more distinctly fix the eminent place that Herschel will never cease to occupy in the small group of our contemporary men of genius, whilst his name will reëcho to the most distant posterity. The variety and splendour of Herschel's labours vie with their extent. The more we study them, the more we must admire them. It is with great men, as it is with great movements in the arts, we cannot understand them without studying them under various points of view.
Let us here again make a general reflection. The memoirs of Herschel are, for the greater part, pure and simple extracts from his inexhaustible journals of observations at Slough, accompanied by a few remarks. Such a table would not suit historical details. In these respects the author has left almost every thing to his biographers to do for him. And they must impose on themselves the task of assigning to the great astronomer's predecessors the portion that legitimately belongs to them, out of the mass of discoveries, which the public (we must say) has got into an erroneous habit of referring too exclusively to Herschel.
At one time I thought of adding a note to the analysis of each of the illustrious observer's memoirs, containing a detailed indication of the improvements or corrections that the progressive march of science has brought on. But in order to avoid an exorbitant length in this biography, I have been obliged to give up my project. In general I shall content myself with pointing out what belongs to Herschel, referring to myTreatise on Popular Astronomyfor the historical details. The life of Herschel had the rare advantage of forming an epoch in an extensive branch of astronomy; it would require us almost to write a special treatise on astronomy, to show thoroughly the importance of all the researches that are due to him.
FOOTNOTE:[17]These titles are copied direct from the Philosophical Transactions, instead of being retranslated.—Translator's Note.
[17]These titles are copied direct from the Philosophical Transactions, instead of being retranslated.—Translator's Note.
[17]These titles are copied direct from the Philosophical Transactions, instead of being retranslated.—Translator's Note.
The improvements that Herschel made in the construction and management of telescopes have contributed so directly to the discoveries with which that observer enriched astronomy, that we cannot hesitate to bring them forward at once.
I read the following passage in a Memoir by Lalande, printed in 1783, and forming part of the preface to vol. viii. of theEphemerides of the Celestial Motions.
"Each time that Herschel undertakes to polish a mirror (of a telescope), he condemns himself to ten, or twelve, or even fourteen hours' constant work. He does not quit his workshop for a minute, not even to eat, but receives from the hands of his sister that nourishment without which one could not undergo such prolonged fatigue. Nothing in the world would induce Herschel to abandon his work; for, according to him, it would be to spoil it."
The advantages that Herschel found in 1783, 1784, and 1785, in employing telescopes of twenty feet and with large apertures, made him wish to construct much larger still. The expense would be considerable; King George III. provided for it. The work, begun about the close of 1785, was finished in August, 1789. This instrument had an iron cylindrical tube, thirty-nine feet four inches English in length, and four feet ten inches in diameter. Such dimensions are enormous compared with those of telescopes made till then. They will appear but small, however, to persons who have heard the report of a pretended ball given in the Slough telescope. The propagators of this popular rumour had confounded the astronomer Herschel with the brewer Meux, and a cylinder in which a man of the smallest stature could scarcely stand upright, with certain wooden vats, as large as a house, in which beer is made and kept in London.
Herschel's telescope, forty English feet[18]in length, allowed of the realization of an idea, the advantages of which would not be sufficiently appreciated if I did not here recall to mind some facts.
In any telescope, whether refracting or reflecting, there are two principal parts: the part that forms the aërial images of the distant objects, and the small lens by the aid of which these images are enlarged just as if they consisted of radiating matter. When the image is produced by means of a lenticular glass, the place it occupies will be found in the prolongation of the line that extends from the object to the centre of the lens. The astronomer, furnished with an eye-piece, and wishing to examine that image, must necessarily place himselfbeyondthe point where the rays that form it have crossed each other;beyond, let us carefully remark, meansfarther offfrom the object-glass. The observer's head, his body, cannot then injure the formation or the brightness of the image, however small may be the distance from which we have to study it. But it is no longer thus with the image formed by means of reflection. For the image is now placed between the object and the reflecting mirror; and when the astronomer approaches in order to examine it, he inevitably intercepts, if not the totality, at least a very considerable portion of the luminous rays, which would otherwise have contributed to give it great splendour. It will now be understood, why in optical instruments where the images of distant objects are formed by the reflection of light, it has been necessary to carry the images, by the aid of a second reflection, out of the tube that contains and sustains the principal mirror. When the small mirror, on the surface of which the second reflection is effected, is plane, and inclined at an angle of 45° to the axis of the telescope; when the image is reflected laterally, through an opening made near the edge of the tube and furnished with an eye-piece; when, in a word, the astronomer looks definitively in a direction perpendicular to the line described by the luminous rays coming from the object and falling on the centre of the great mirror, then the telescope is calledNewtonian. But in theGregoriantelescope, the image formed by the principal mirror falls on a second mirror, which is very small, slightly curved, and parallel to the first. The small mirror reflects the first image and throws it beyond the large mirror, through an opening made in the middle of that principal mirror.
Both in the one and in the other of these two telescopes, the small mirror interposed between the object and the great mirror forms relative to the latter a sort of screen which prevents its entire surface from contributing towards forming the image. The small mirror, also, in regard to intensity, gives some trouble.
Let us suppose, in order to clear up our ideas, that the material of which the two mirrors are made, reflects only half of the incident light. In the course of the first reflection, the immense quantity of rays that the aperture of the telescope had received, may be considered as reduced to half. Nor is the diminution less on the small mirror. Now, half of half is a quarter. Therefore the instrument will send to the eye of the observer only a quarter of the incident light that its aperture had received. These two causes of diminished light not existing in a refracting telescope, it would give, under parity of dimensions, four times more[19]light than a Newtonian or Gregorian telescope gives.
Herschel did away with the small mirror in his large telescope. The large mirror is not mathematically centred in the large tube that contains it, but is placed rather obliquely in it. This slight obliquity causes the images to be formed not in the axis of the tube, but very near its circumference, or outer mouth, we may call it. The observer may therefore look at them there direct, merely by means of an eye-piece. A small portion of the astronomer's head, it is true, then encroaches on the tube; it forms a screen, and interrupts some incident rays. Still, in a large telescope, the loss does not amount to half by a great deal; which it would inevitably do if the small mirror were there.
Those telescopes, in which the observer, placed at the anterior extremity of the tube, looks direct into the tube and turns his back to the objects, were called by Herschelfront view telescopes. In vol. lxxvi. of thePhilosophical Transactionshe says, that the idea of this construction occurred to him in 1776, and that he then applied it unsuccessfully to a ten-foot telescope; that during the year 1784, he again made a fruitless trial of it in a twenty-foot telescope. Yet I find that on the 7th of September 1784, he recurred to afront viewin observing some nebulæ and groups of stars. However discordant these dates may be, we cannot without injustice neglect to remark, that a front view telescope was already described in 1732, in volume vi. of the collection entitledMachines and Inventions approved by the Academy of Sciences. The author of this innovation is Jaques Lemaire, who has been unduly confounded with the English Jesuit, Christopher Maire, assistant to Boscovitch, in measuring the meridian comprised between Rome and Rimini. Jaques Lemaire having only telescopes of moderate dimensions in view, was obliged, in order not to sacrifice any of the light, to place the great mirror so obliquely, that the image formed by its surface should fall entirely outside the tube of the instrument. So great a degree of inclination would certainly deform the objects. Thefront viewconstruction is admissible only in very large telescopes.
I find in theTransactionsfor 1803, that in solar observations, Herschel sometimes employed telescopes, the great mirror of which was made of glass. It was a telescope of this sort that he used for observing the transit of Mercury on the 9th of November, 1802. It was seven English feet long, and six inches and three tenths in diameter.
Practical astronomers know how much the mounting of a telescope contributes to produce correct observations. The difficulty of a solid yet very movable mounting, increases rapidly with the dimensions and weight of an instrument. We may then conceive that Herschel had to surmount many obstacles, to mount a telescope suitably, of which the mirror alone weighed upwards of 1000 kilogrammes (a ton). But he solved this problem to his entire satisfaction by the aid of a combination of spars, of pulleys, and of ropes, of all which a correct idea may be formed by referring to the woodcut we have given in ourTreatise on Popular Astronomy(vol. i.). This great apparatus, and the entirely different stands that Herschel imagined for telescopes of smaller dimensions, assign to that illustrious observer a distinguished place amongst the most ingenious mechanics of our age.
Persons in general, I may even say the greater part of astronomers, know not what was the effect that the great forty-foot telescope had in the labours and discoveries of Herschel. Still, we are not less mistaken when we fancy that the observer of Slough always used this telescope, than in maintaining with Baron von Zach (seeMonatliche Correspondenz, January, 1802), that the colossal instrument was of no use at all, that it did not contribute to any one discovery, that it must be considered as a mere object of curiosity. These assertions are distinctly contradicted by Herschel's own words. In the volume ofPhilosophical Transactionsfor the year 1795 (p. 350), I read for example: "On the 28th of August 1789, having directed my telescope (of forty feet) to the heavens, I discovered the sixth satellite of Saturn, and I perceived the spots on that planet, better than I had been able to do before." (See also, relative to this sixth satellite, thePhilosophical Transactionsfor 1790, p. 10.) In that same volume of 1790, p. 11, I find: "The great light of my forty-foot telescope was then so useful, that on the 17th of September 1789, I remarked the seventh satellite, then situated at its greatest western elongation."
The 10th of October, 1791, Herschel saw the ring of Saturn and the fourth satellite, looking in at the mirror of his forty-foot telescope, with his naked eye, without any sort of eye-piece.
Let us acknowledge the true motives that prevented Herschel from oftener using his telescope of forty feet. Notwithstanding the excellence of the mechanism, the manœuvring of that instrument required the constant aid of two labourers, and that of another person charged with noting the time at the clock. During some nights when the variation of temperature was considerable, this telescope, on account of its great mass, was always behindhand with the atmosphere in thermometric changes, which was very injurious to the distinctness of the images.
Herschel found that in England, there are not above a hundred hours in a year during which the heavens can be advantageously observed with a telescope of forty feet, furnished with a magnifying power of a thousand. This remark led the celebrated astronomer to the conclusion, that, to take a complete survey of the heavens with his large instrument, though each successive field should remain only for an instant under inspection, would not require less than eight hundred years.
Herschel explains in a very natural way the rare occurrence of the circumstances in which it is possible to make good use of a telescope of forty feet, and of very large aperture.
A telescope does not magnify real objects only, but magnifies also the apparent irregularities arising from atmospheric refractions; now, all other things being equal, these irregularities of refraction must be so much the stronger, so much the more frequent, as the stratum of air is thicker through which the rays have passed to go and form the image.
Astronomers experienced extreme surprise, when in 1782, they learned that Herschel had applied linear magnifying powers of a thousand, of twelve hundred, of two thousand two hundred, of two thousand six hundred, and even of six thousand times, to a reflecting telescope of seven feet in length. The Royal Society of London experienced this surprise, and officially requested Herschel to give publicity to the means he had adopted for ascertaining such amounts of magnifying power in his telescopes. Such was the object of a memoir that he inserted in vol. lxxii. of thePhilosophical Transactions; and it dissipated all doubts. No one will be surprised that magnifying powers, which it would seem ought to have shown the Lunar mountains, as the chain of Mont Blanc is seen from Maçon, from Lyons, and even from Geneva, were not easily believed in. They did not know that Herschel had never used magnifying powers of three thousand, and six thousand times, except in observing brilliant stars; they had not remembered that light reflected by planetary bodies, is too feeble to continue distinct under the same degree of magnifying power as the actual light of the fixed stars does.
Opticians had given up, more from theory than from careful experiments, attempting high magnifying powers, even for reflecting telescopes. They thought that the image of a small circle cannot be distinct, cannot be sharp at the edges, unless the pencil of rays coming from the object in nearly parallel lines, and which enters the eye after having passed through the eye-piece, be sufficiently broad. This being once granted, the inference followed, that an image ceases to be well defined, when it does not strike at least two of the nervous filaments of the retina with which that organ is supposed to be overspread. These gratuitous circumstances, grafted on each other, vanished in presence of Herschel's observations. After having put himself on his guard against the effects of diffraction, that is to say, against the scattering that light undergoes when it passes the terminal angles of bodies, the illustrious astronomer proved, in 1786, that objects can be seen well defined by means of pencils of light whose diameter does not equal five tenths of a millimetre.
Herschel looked on the almost unanimous opinion of the double lens eye-piece being preferable to the single lens eye-piece, as a very injurious prejudice in science. For experience proved to him, notwithstanding all theoretic deductions, that with equal magnifying powers, in reflecting telescopes at least (and this restriction is of some consequence), the images were brighter and better defined with single than with double eye-pieces. On one occasion, this latter eye-piece would not show him the bands of Saturn, whilst by the aid of a single lens they were perfectly visible. Herschel said: "The double eye-piece must be left to amateurs and to those who, for some particular object, require a large field of vision." (Philosophical Transactions, 1782, pages 94 and 95.)
It is not only relative to the comparative merit of single or double eye-pieces that Herschel differs from the general opinions of opticians; he thinks, moreover, that he has proved by decisive experiments, that concave eye-pieces (like that used by Galileo) surpass the convex eye-piece by a great deal, both as regards clearness and definition.
Herschel assigns the date of 1776 to the experiments which he made to decide this question. (Philosophical Transactions, year 1815, p. 297.) Plano-concave and double concave lenses produced similar effects. In what did these lenses differ from the double convex lenses? In one particular only: the latter received the rays reflected by the large mirror of the telescope, after their union at the focus, whereas the concave lenses received the same rays before that union. When the observer made use of a convex lens, the rays that went to the back of the eye to form an image on the retina, had crossed each other before in the air; but no crossing of this kind took place when the observer used a concave lens. Holding the double advantage of this latter sort of lens over the other, as quite proved, one would be inclined, like Herschel, to admit, "that a certain mechanical effect, injurious to clearness and definition, would accompany the focal crossing of the rays of light."[20]
This idea of the crossing of the rays suggested an experiment to the ingenious astronomer, the result of which deserves to be recorded.
A telescope of ten English feet was directed towards an advertisement covered with very small printing, and placed at a sufficient distance. The convex lens of the eye-piece was carried not by a tube properly so called, but by four rigid fine wires placed at right angles. This arrangement left the focus open in almost every direction. A concave mirror was then placed so that it threw a very condensed image of the sun laterally on the very spot where the image of the advertisement was formed. The solar rays, after having crossed each other, finding nothing on their route, went on and lost themselves in space. A screen, however, allowed the rays to be intercepted at will before they united.
This done, having applied the eye to the eye-piece and directed all his attention to the telescopic image of the advertisement, Herschel did not perceive that the taking away and then replacing the screen made the least change in the brightness or definition of the letters. It was therefore of no consequence, in the one instance as well as in the other, whether the immense quantity of solar rays crossed each other at the very place where,in another direction, the rays united that formed the image of the letters. I have marked in Italics the words that especially show in what this curious experiment differs from the previous experiments, and yet does not entirely contradict them. In this instance the rays of various origin, those coming from the advertisement and from the sun, crossed each other respectively in almost rectangular directions; during the comparative examination of the stars with convex and with concave eye-pieces, the rays that seemed to have a mutual influence, had a common origin and crossed each other at very acute angles. There seems to be nothing, then, in the difference of the results at which we need to be much surprised.
Herschel increased the catalogue, already so extensive, of the mysteries of vision, when he explained in what manner we must endeavour to distinguish separately the two members of certain double stars very close to each other. He said if you wish to assure yourself that η Coronæ is a double star, first direct your telescope to α Geminorum, to ζ Aquarii, to μ Draconis, to ρ Herculis, to α Piscium, to ε Lyræ. Look at those stars for a long time, so as to acquire the habit of observing such objects. Then pass on to ξ Ursæ majoris, where the closeness of the two members is still greater. In a third essay select ι Bootis (marked 44 by Flamsteed andiin Harris's maps)[21], the star that precedes α Orionis,nof the same constellation, and you will then be prepared for the more difficult observation of η Coronæ. Indeed η Coronæ is a sort of miniature of ι Bootis, which may itself be considered as a miniature of α Gem. (Philosophical Transactions, 1782, p. 100.)
As soon as Piazzi, Olbers, and Harding had discovered three of the numerous telescopic planets now known, Herschel proposed to himself to determine their real magnitudes; but telescopes not having then been applied to the measurement of excessively small angles, it became requisite, in order to avoid any illusion, to try some experiments adapted to giving a scale of the powers of those instruments. Such was the labour of that indefatigable astronomer, of which I am going to give a compressed abridgment.
The author relates first, that in 1774, he endeavoured to ascertain experimentally, with the naked eye and at the distance of distinct vision, what angle a circle must subtend to be distinguished by its form from a square of similar dimensions. The angle was never smaller than 2' 17"; therefore at its maximum it was about one fourteenth of the angle subtended by the diameter of the moon.
Herschel did not say, either of what nature the circles and squares of paper were that he used, nor on what background they were projected. It is a lacuna to be regretted, for in those phenomena the intensity of light must be an important feature. However it may have been, the scrupulous observer not daring to extend to telescopic vision what he had discovered relative to vision with the naked eye, he undertook to do away with all doubt, by direct observations.
On examining some pins' heads placed at a distance in the open air, with a three-foot telescope, Herschel could easily discern that those bodies were round, when the subtended angles became, after their enlargement, 2' 19". This is almost exactly the result obtained with the naked eye.
When the globules were darker; when, instead of pins' heads, small globules of sealing-wax were used, their spherical form did not begin to be distinctly visible till the moment when the subtended magnified angles, that is, the moment when the natural angle multiplied by the magnifying power, amounted to five minutes.
In a subsequent series of experiments, some globules of silver placed very far from the observer, allowed their globular form to be perceived, even when the magnified angle remained below two minutes.
Under equality of subtended angle, then, the telescopic vision with strong magnifying powers showed itself superior to the naked eye vision. This result is not unimportant.
If we take notice of the magnifying powers used by Herschel in these laborious researches, powers that often exceeded five hundred times, it will appear to be established that the telescopes possessed by modern astronomers, may serve to verify the round form of distant objects, the form of celestial bodies even when the diameters of those bodies do not subtend naturally (to the naked eye), angles of above three tenths of a second: and 500, multiplied by three tenths of a second, give 2' 30".
Refracting telescopes were still ill understood instruments, the result of chance, devoid of certain theory, when they already served to reveal brilliant astronomical phenomena. Their theory, in as far as it depended on geometry and optics, made rapid progress. These two early phases of the problem leave but little more to be wished for; it is not so with a third phase, hitherto a good deal neglected, connected with physiology, and with the action of light on the nervous system. Therefore, we should search in vain in old treatises on optics and on astronomy, for a strict and complete discussion on the comparative effect that the size and intensity of the images, that the magnifying power and the aperture of a telescope may have, by night and by day, on the visibility of the faintest stars. This lacuna Herschel tried to fill up in 1799; such was the aim of the memoir entitled,On the space-penetrating Power of Telescopes.
This memoir contains excellent things; still, it is far from exhausting the subject. The author, for instance, entirely overlooks the observations made by day. I also find, that the hypothetical part of the discussion is not perhaps so distinctly separated from the rigorous part as it might be; that disputable numbers, though given with a degree of precision down to the smallest decimals, do not look well as terms of comparison with some results which; on the contrary, rest on observations bearing mathematical evidence.
Whatever may be thought of these remarks, the astronomer or the physicist who would like again to undertake the question of visibility with telescopes, will find some important facts in Herschel's memoir, and some ingenious observations, well adapted to serve them as guides.