Chapter 16

_Cosmo, Grand Duke of Tuscany, invites Galileo to Pisa--Galileo visits Venice in 1609, where he first hears of the Telescope--He invents and constructs one, which excites a great sensation--Discovers Mountains in the Moon, and Forty Stars in the Pleiades--Discovers Jupiter's Satellites in 1610--Effect of this discovery on Kepler--Manner in which these discoveries were received--Galileo appointed Mathematician to Cosmo--Mayer claims the discovery of the Satellites of Jupiter--Harriot observes them in England in October 1610._

In the preceding chapter we have brought down the history of Galileo's labours to that auspicious year in which he first directed the telescope to the heavens. No sooner was that noble instrument placed in his hands, than Providence released him from his professional toils, and supplied him with the fullest leisure and the amplest means for pursuing and completing the grandest discoveries.

Although he had quitted the service and the domains of his munificent patron, the Grand Duke of Tuscany, yet he maintained his connection with the family, by visiting Florence during his academic vacations, and giving mathematical instruction to the younger branches of that distinguished house. Cosmo, who had been one of his pupils, now succeeded his father Ferdinand; and having his mind early imbued with a love of knowledge, which had become hereditary in his family, he felt that the residence of Galileo within his dominions, and still more his introduction into his household, would do honour to their common country, and reflect a lustre upon his own name. In the year 1609, accordingly, Cosmo made proposals to Galileo to return to his original situation at Pisa. These overtures were gratefully received; and in the arrangements which Galileo on this occasion suggested, as well as in the manner in which they were urged, we obtain some insight into his temper and character. He informs the correspondent through whom Cosmo's offer was conveyed, that his salary of 520 florins at Padua would be increased to as many crowns at his re-election, and that he could enlarge his income to any extent he pleased, by giving private lectures and receiving pupils. His public duties, he stated, occupied him only sixty half-hours in the year; but his studies suffered such interruptions from his domestic pupils and private lectures, that his most ardent wish was to be relieved from them, in order that he might have sufficient rest and leisure, before the close of his life, to finish and publish those great works which he had projected. In the event, therefore, of his returning to Pisa, he hoped that it would be the first object of his serene highness to give him leisure to complete his works without the drudgery of lecturing. He expresses his anxiety to gain his bread by his writings, and he promises to dedicate them to his serene master. He enumerates, among these books, two on the system of the universe, three on local motion, three books of mechanics, two on the demonstration of principles, and one of problems; besides treatises on sound and speech, on light and colours, on the tides, on the composition of continuous quantity, on the motions of animals, and on the military art. On the subject of his salary, he makes the following curious observations:--

"I say nothing," says he, "on the amount of my salary; being convinced that, as I am to live upon it, the graciousness of his highness would not deprive me of any of those comforts, of which, however, I feel the want of less than many others; and, therefore, I say nothing more on the subject. Finally, on the title and profession of my service, I should wish that, to the title of mathematician, his highness would add that of philosopher, as I profess to have studied a greater number of years in philosophy, than months in pure mathematics; and how I have profited by it, and if I can or ought to deserve this title, I may let their highnesses see, as often as it shall please them to give me an opportunity of discussing such subjects in their presence with those who are most esteemed in this knowledge."

During the progress of this negotiation, Galileo went to Venice, on a visit to a friend, in the month of April or May 1609. Here he learned, from common rumour, that a Dutchman had presented to prince Maurice of Nassau an optical instrument, which possessed the singular property of causing distant objects to appear nearer the observer. This Dutchman was Hans or John Lippershey, who, as has been clearly proved by the late Professor Moll of Utrecht,[8] was in the possession of a telescope made by himself so early as 2d October 1608. A few days afterwards, the truth of this report was confirmed by a letter which Galileo received from James Badorere at Paris, and he immediately applied himself to the consideration of the subject. On the first night after his return to Padua, he found, in the doctrines of refraction, the principle which he sought. He placed at the ends of a leaden tube two spectacle glasses, both of which were plain on one side, while one of them had its other side convex, and the other its second side concave, and having applied his eye to the concave glass, he saw objects pretty large and pretty near him. This little instrument, which magnified only three times, he carried in triumph to Venice, where it excited the most intense interest. Crowds of the principal citizens flocked to his house to see the magical toy; and after nearly a month had been spent in gratifying this epidemical curiosity, Galileo was led to understand from Leonardo Deodati, the Doge of Venice, that the senate would be highly gratified by obtaining possession of so extraordinary an instrument. Galileo instantly complied with the wishes of his patrons, who acknowledged the present by a mandate conferring upon him for life his professorship at Padua, and generously raising his salary from 520 to 1000 florins.[9]

[8] On the First Invention of Telescopes.--_Journ. R. Instit._, 1831., vol i., p. 496.

[9] Viviani _Vita del' Galileo_, p. 69.

Although we cannot doubt the veracity of Galileo, when he affirms that he had never seen any of the Dutch telescopes, yet it is expressly stated by Fuccarius, that one of these instruments had at this time been brought to Florence; and Sirturus assures us that a Frenchman, calling himself a partner of the Dutch inventor, came to Milan in May 1609, and offered a telescope to the Count de Fuentes. In a letter from Lorenzo Pignoria to Paolo Gualdo, dated from Padua, on the 31st of August 1609, it is expressly said, that, at the re-election of the professors, Galileo had contrived to obtain 1000 florins for life, which was alleged to be on account of an eye-glass like the one which was sent from Flanders to the Cardinal Borghese.

In a memoir so brief and general as the present, it would be out of place to discuss the history of this extraordinary invention. We have no hesitation in asserting that a method of magnifying distant objects was known to Baptista Porta and others; but it seems to be equally certain that an _instrument_ for producing these effects was first constructed in Holland, and that it was from that kingdom that Galileo derived the knowledge of its existence. In considering the contending claims, which have been urged with all the ardour and partiality of national feeling, it has been generally overlooked, _that a single convex lens_, whose focal length exceeds the distance at which we examine minute objects, performs the part of a telescope, when an eye, placed behind it, sees distinctly the inverted image which it forms. A lens, twenty feet in focal length, will in this manner magnify twenty times; and it was by the same principle that Sir William Herschel discovered a new satellite of Saturn, by using only the mirror of his forty-feet telescope. The instrument presented to Prince Maurice, and which the Marquis Spinola found in the shop of John Lippershey, the spectacle maker of Middleburg, must have been an astronomical telescope consisting of two convex lenses. Upon this supposition, it differed from that which Galileo constructed; and the Italian philosopher will be justly entitled to the honour of having invented that form of the telescope which still bears his name, while we must accord to the Dutch optician the honour of having previously invented the astronomical telescope.

The interest which the exhibition of the telescope excited at Venice did not soon subside: Sirturi[10] describes it as amounting almost to phrensy. When he himself had succeeded in making one of these instruments, he ascended the tower of St Mark, where he might use it without molestation. He was recognised, however, by a crowd in the street; and such was the eagerness of their curiosity, that they took possession of the wondrous tube, and detained the impatient philosopher for several hours, till they had successively witnessed its effects. Desirous of obtaining the same gratification for their friends, they endeavoured to learn the name of the inn at which he lodged; but Sirturi fortunately overheard their inquiries, and quitted Venice early next morning, in order to avoid a second visitation of this new school of philosophers. The opticians speedily availed themselves of the new instrument. Galileo's tube,--or the double eye-glass, or the cylinder, or the trunk, as it was then called, for Demisiano had not yet given it the appellation of _telescope_,--was manufactured in great quantities, and in a very superior manner. The instruments were purchased merely as philosophical toys, and were carried by travellers into every corner of Europe.

[10] De Telescopio.

The art of grinding and polishing lenses was at this time very imperfect. Galileo, and those whom he instructed, were alone capable of making tolerable instruments. It appears, from the testimony of Gassendi and Gaertner, that, in 1634, a good telescope could not be procured in Paris, Venice, or Amsterdam; and that, even in 1637, there was not one in Holland which could shew Jupiter's disc well defined.

After Galileo had completed his first instrument, which magnified only _three_ times, he executed a larger and a better one, with a power of about _eight_. "At length," as he himself remarks, "sparing neither labour nor expense," he constructed an instrument so excellent, that it bore a magnifying power of more than _thirty_ times.

The first celestial object to which Galileo applied his telescope was the moon, which, to use his own words, appeared as near as if it had been distant only two semidiameters of the earth. He then directed it to the planets and the fixed stars, which he frequently observed with "incredible delight."[11]

[11] Incredibili animi jucunditate.

The observations which he made upon the moon possessed a high degree of interest. The general resemblance of its surface to that of our own globe naturally fixed his attention; and he was soon able to trace, in almost every part of the lunar disc, ranges of mountains, deep hollows, and other inequalities, which reverberated from their summits and margins the rays of the rising sun, while the intervening hollows were still buried in darkness. The dark and luminous spaces he regarded as indicating seas and continents, which reflected, in different degrees, the incidental light of the sun; and he ascribed the phosphorescence, as it has been improperly called, or the secondary light, which is seen on the dark limb of the moon in her first and last quarters, to the reflection of the sun's light from the earth.

These discoveries were ill received by the followers of Aristotle. According to their preconceived opinions, the moon was perfectly spherical, and absolutely smooth; and to cover it with mountains, and scoop it out into valleys, was an act of impiety which defaced the regular forms which Nature herself had imprinted. It was in vain that Galileo appealed to the evidence of observation, and to the actual surface of our own globe. The very irregularities on the moon were, in his opinion, the proof of divine wisdom; and had its surface been absolutely smooth, it would have been "but a vast unblessed desert, void of animals, of plants, of cities, and of men--the abode of silence and inaction--senseless, lifeless, soulless, and stripped of all those ornaments which now render it so varied and so beautiful."

In examining the fixed stars, and comparing them with the planets, Galileo observed a remarkable difference in the appearance of their discs. All the planets appeared with round globular discs like the moon; whereas the fixed stars never exhibited any disc at all, but resembled lucid points sending forth twinkling rays. Stars of all magnitudes he found to have the same appearance; those of the fifth and sixth magnitude having the same character, when seen through a telescope, as Sirius, the largest of the stars, when seen by the naked eye. Upon directing his telescope to nebulae and clusters of stars, he was delighted to find that they consisted of great numbers of stars which could not be recognised by unassisted vision. He counted no fewer than _forty_ in the cluster called the _Pleiades_, or _Seven Stars_; and he has given us drawings of this constellation, as well as of the belt and sword of Orion, and of the nebula of Praesepe. In the great nebula of the Milky Way, he descried crowds of minute stars; and he concluded that this singular portion of the heavens derived its whiteness from still smaller stars, which his telescope was unable to separate.

Important and interesting as these discoveries were, they were thrown into the shade by those to which he was led during an accurate examination of the planets with a more powerful telescope. On the 7th of January 1610, at one o'clock in the morning, when he directed his telescope to Jupiter, he observed three stars near the body of the planet, two being to the east and one to the west of him. They were all in a straight line, and parallel to the ecliptic, and appeared brighter than other stars of the same magnitude. Believing them to be fixed stars, he paid no great attention to their distances from Jupiter and from one another. On the 8th of January, however, when, from some cause or other,[12] he had been led to observe the stars again, he found a very different arrangement of them: all the three were on the west side of Jupiter, _nearer one another than before_, and almost at equal distances. Though he had not turned his attention to the extraordinary fact of the mutual approach of the stars, yet he began to consider how Jupiter could be found to the east of the three stars, when but the day before he had been to the west of two of them. The only explanation which he could give of this fact was, that the motion of Jupiter was _direct_, contrary to astronomical calculations, and that he had got before these two stars by his own motion.

[12] Nescio quo fato ductus.

In this dilemma between the testimony of his senses and the results of calculation, he waited for the following night with the utmost anxiety; but his hopes were disappointed, for the heavens were wholly veiled in clouds. On the 10th, two only of the stars appeared, and both on the east of the planet. As it was obviously impossible that Jupiter could have advanced from west to east on the 8th of January, and from east to west on the 10th, Galileo was forced to conclude that the phenomenon which he had observed arose from the motion of the stars, and he set himself to observe diligently their change of place. On the 11th, there were still only two stars, and both to the east of Jupiter; but the more eastern star was now _twice as large as the other one_, though on the preceding night they had been perfectly equal. This fact threw a new light upon Galileo's difficulties, and he immediately drew the conclusion, which he considered to be indubitable, "_that there were in the heavens three stars which revolved round Jupiter, in the same manner as Venus and Mercury revolve round the sun_." On the 12th of January, he again observed them in new positions, and of different magnitudes; and, on the 13th, he discovered a fourth star, which completed the _four_ secondary planets with which Jupiter is surrounded.

Galileo continued his observations on these bodies every clear night till the 22d of March, and studied their motions in reference to fixed stars that were at the same time within the field of his telescope. Having thus clearly established that the four new stars were satellites or moons, which revolved round Jupiter in the same manner as the moon revolves round our own globe, he drew up an account of his discovery, in which he gave to the four new bodies the names of the _Medicean Stars_, in honour of his patron, Cosmo de Medici, Grand Duke of Tuscany. This work, under the title of "Nuncius Sidereus," or the "Sidereal Messenger," was dedicated to the same prince; and the dedication bears the date of the 24th of March, only two days after he concluded his observations.

The importance of this great discovery was instantly felt by the enemies as well as by the friends of the Copernican system. The planets had hitherto been distinguished from the fixed stars only by their relative change of place, but the telescope proved them to be bodies so near to our own globe as to exhibit well-defined discs, while the fixed stars retained, even when magnified, the minuteness of remote and lucid points. The system of Jupiter, illuminated by four moons performing their revolutions in different and regular periods, exhibited to the proud reason of man the comparative insignificance of the globe he inhabits, and proclaimed in impressive language that that globe was not the centre of the universe.

The reception which these discoveries met with from Kepler is highly interesting, and characteristic of the genius of that great man. He was one day sitting idle, and thinking of Galileo, when his friend Wachenfels stopped his carriage at his door, to communicate to him the intelligence. "Such a fit of wonder," says he, "seized me at a report which seemed to be so very absurd, and I was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or I of listening. On our parting, I immediately began to think how there could be any addition to the number of the planets without overturning my 'Cosmographic Mystery,' according to which Euclid's five regular solids do not allow more than six planets round the sun.... I am so far from disbelieving the existence of the four circumjovial planets, that I long for a telescope, to anticipate you, if possible, in discovering _two_ round Mars, as the proportion seems to require, _six_ or _eight_ round Saturn, and perhaps _one_ each round Mercury and Venus."

In a very different spirit did the Aristotelians receive the "Sidereal Messenger" of Galileo. The principal professor of philosophy at Padua resisted Galileo's repeated and urgent entreaties to look at the moon and planets through his telescope; and he even laboured to convince the Grand Duke that the satellites of Jupiter could not possibly exist. Sizzi, an astronomer of Florence, maintained that as there were only _seven_ apertures in the head--_two_ eyes, _two_ ears, _two_ nostrils, and _one_ mouth--and as there were only _seven_ metals, and _seven_ days in the week, so there could be only _seven_ planets. He seems, however, to have admitted the visibility of the four satellites through the telescope; but he argues, that as they are invisible to the naked eye, they can exercise no influence on the earth; and being useless, they do not therefore exist.

A _protege_ of Kepler's, of the name of Horky, wrote a volume against Galileo's discovery, after having declared, "that he would never concede his four new planets to that Italian from Padua, even if he should die for it." This resolute Aristotelian was at no loss for arguments. He asserted that he had examined the heavens _through Galileo's own glass_, and that no such thing as a satellite existed round Jupiter. He affirmed, that he did not more surely know that he had a soul in his body, than that reflected rays are the sole cause of Galileo's erroneous observations; and that the only use of the new planets was to gratify Galileo's thirst for gold, and afford to himself a subject of discussion.

When Horky first presented himself to Kepler, after the publication of this work, the opinion of his patron was announced to him by a burst of indignation which overwhelmed the astonished author. Horky supplicated mercy for his offence; and, as Kepler himself informed Galileo, he took him again into favour, on the condition that Kepler was to show him Jupiter's satellites, and that Horky was not only to see them, but to admit their existence.

When the spirit of philosophy had thus left the individuals who bore so unworthily her sacred name, it was fortunate for science that it found a refuge among princes. Notwithstanding the reiterated logic of his philosophical professor at Padua, Cosmo de Medici preferred the testimony of his senses to the syllogisms of his instructor. He observed the new planets several times, along with Galileo, at Pisa; and when he parted with him, he gave him a present worth more than 1000 florins, and concluded that liberal arrangement to which we have already referred.

As philosopher and principal mathematician to the Grand Duke of Tuscany, Galileo now took up his residence at Florence, with a salary of 1000 florins. No official duties, excepting that of lecturing occasionally to sovereign princes, were attached to this appointment; and it was expressly stipulated that he should enjoy the most perfect leisure to complete his treatises on the constitution of the universe, on mechanics, and on local motion. The resignation of his professorship in the university of Padua, which was the necessary consequence of his new appointment, created much dissatisfaction: but though many of his former friends refused at first to hold any communication with him, this excitement gradually subsided; and the Venetian senate at last appreciated the feelings, as well as the motives, which induced a stranger to accept of promotion in his native land.

While Galileo was enjoying the reward and the fame of his great discovery, a new species of enmity was roused against him. Simon Mayer, an astronomer of no character, pretended that he had discovered the satellites of Jupiter before Galileo, and that his first observation was made on the 29th of December, 1609. Other astronomers announced the discovery of new satellites: Scheiner reckoned five, Rheita nine, and others found even so many as twelve: these satellites, however, were found to be only fixed stars. The names of _Vladislavian_, _Agrippine_, _Uranodavian_, and _Ferdinandotertian_, which were hastily given to these common telescopic stars, soon disappeared from the page of science, and even the splendid telescopes of modern times have not been able to add another gem to the diadem of Jupiter.

A modern astronomer of no mean celebrity has, even in the present day, endeavoured to rob Galileo of this staple article of his reputation. From a careless examination of the papers of our celebrated countryman, Thomas Harriot, which Baron Zach had made in 1784, at Petworth, the seat of Lord Egremont, this astronomer has asserted[13] that Harriot first observed the satellites of Jupiter on the 16th of January, 1610; and continued his observations till the 25th of February, 1612. Baron Zach adds the following extraordinary conclusion:--"Galileo pretends to have discovered them on the 7th of January, 1610; so that it is not improbable that Harriot was likewise the first discoverer of these attendants of Jupiter." In a communication which I received from Dr Robertson, of Oxford, in 1822,[14] he informed me that he had examined a portion of Harriot's papers, entitled, "De Jovialibus Planetis;" and that it appears, from two pages of these papers, _that Harriot first observed Jupiter's satellites on the 17th of October, 1610_. These observations are accompanied with rough drawings of the positions of the satellites, and rough calculations of their periodical revolutions. My friend, Professor Rigaud,[15] who has very recently examined the Harriot MSS., has confirmed the accuracy of Dr Robertson's observations, and has thus restored to Galileo the honour of being the first and the sole discoverer of these secondary planets.

[13] Berlin Ephemeris, 1788.

[14] Edin. Phil. Journ. vol. vi. p. 313.

[15] Life and Correspondence of Dr Bradley, Oxford, 1832, p. 533, See also his Supplement. Oxford, 1833, p. 17.