_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.