unaccustomed way up. By using a concave glass instead of a convex,
and placing it so as to prevent any image being formed, except on
the retina direct, this inconvenience is avoided.
[Illustration: FIG. 38.--View of the half-moon in small telescope. The
darker regions, or plains, used to be called "seas."]
Such a thing as Galileo made may now be bought at a toy-shop for I
suppose half a crown, and yet what a potentiality lay in that "glazed
optic tube," as Milton called it. Away he went with it to Venice and
showed it to the Signoria, to their great astonishment. "Many noblemen
and senators," says Galileo, "though of advanced age, mounted to the top
of one of the highest towers to watch the ships, which were visible
through my glass two hours before they were seen entering the harbour,
for it makes a thing fifty miles off as near and clear as if it were
only five." Among the people too the instrument excited the greatest
astonishment and interest, so that he was nearly mobbed. The Senate
hinted to him that a present of the instrument would not be
unacceptable, so Galileo took the hint and made another for them.
[Illustration: FIG. 39.--Portion of the lunar surface more highly
magnified, showing the shadows of a mountain range, deep pits, and other
details.]
They immediately doubled his salary at Padua, making it 1000 florins,
and confirmed him in the enjoyment of it for life.
He now eagerly began the construction of a larger and better instrument.
Grinding the lenses with his own hands with consummate skill, he
succeeded in making a telescope magnifying thirty times. Thus equipped
he was ready to begin a survey of the heavens.
[Illustration: FIG. 40.--Another portion of the lunar surface, showing a
so-called crater or vast lava pool and other evidences of ancient heat
unmodified by water.]
The first object he carefully examined was naturally the moon. He found
there everything at first sight very like the earth, mountains and
valleys, craters and plains, rocks, and apparently seas. You may imagine
the hostility excited among the Aristotelian philosophers, especially no
doubt those he had left behind at Pisa, on the ground of his spoiling
the pure, smooth, crystalline, celestial face of the moon as they had
thought it, and making it harsh and rugged and like so vile and ignoble
a body as the earth.
[Illustration: FIG. 41.--Lunar landscape showing earth. The earth would
be a stationary object in the moon's sky: its only apparent motion being
a slow oscillation as of a pendulum (the result of the moon's
libration).]
He went further, however, into heterodoxy than this--he not only made
the moon like the earth, but he made the earth shine like the moon. The
visibility of "the old moon in the new moon's arms" he explained by
earth-shine. Leonardo had given the same explanation a century before.
Now one of the many stock arguments against Copernican theory of the
earth being a planet like the rest was that the earth was dull and dark
and did not shine. Galileo argued that it shone just as much as the moon
does, and in fact rather more--especially if it be covered with clouds.
One reason of the peculiar brilliancy of Venus is that she is a very
cloudy planet.[8] Seen from the moon the earth would look exactly as the
moon does to us, only a little brighter and sixteen times as big (four
times the diameter).
[Illustration: FIG. 42.--Galileo's method of estimating the height of
lunar mountain.
_AB'BC_ is the illuminated half of the moon. _SA_ is a solar ray just
catching the peak of the mountain _M_. Then by geometry, as _MN_ is to
_MA_, so is _MA_ to _MB'_; whence the height of the mountain, _MN_, can
be determined. The earth and spectator are supposed to be somewhere in
the direction _BA_ produced, _i.e._ towards the top of the page.]
Galileo made a very good estimate of the height of lunar mountains,
of which many are five miles high and some as much as seven. He did
this simply by measuring from the half-moon's straight edge the
distance at which their peaks caught the rising or setting sun. The
above simple diagram shows that as this distance is to the diameter
of the moon, so is the height of the sun-tipped mountain to the
aforesaid distance.
Wherever Galileo turned his telescope new stars appeared. The Milky Way,
which had so puzzled the ancients, was found to be composed of stars.
Stars that appeared single to the eye were some of them found to be
double; and at intervals were found hazy nebulous wisps, some of which
seemed to be star clusters, while others seemed only a fleecy cloud.
[Illustration: FIG. 43.--Some clusters and nebulae.]
[Illustration: FIG. 44.--Jupiter's satellites, showing the stages of
their discovery.]
Now we come to his most brilliant, at least his most sensational,
discovery. Examining Jupiter minutely on January 7, 1610, he noticed
three little stars near it, which he noted down as fixing its then
position. On the following night Jupiter had moved to the other side of
the three stars. This was natural enough, but was it moving the right
way? On examination it appeared not. Was it possible the tables were
wrong? The next evening was cloudy, and he had to curb his feverish
impatience. On the 10th there were only two, and those on the other
side. On the 11th two again, but one bigger than the other. On the 12th
the three re-appeared, and on the 13th there were four. No more
appeared.
Jupiter then had moons like the earth, four of them in fact, and they
revolved round him in periods which were soon determined.
The reason why they were not all visible at first, and why their
visibility so rapidly changes, is because they revolve round him
almost in the plane of our vision, so that sometimes they are in
front and sometimes behind him, while again at other times they
plunge into his shadow and are thus eclipsed from the light of the
sun which enables us to see them. A large modern telescope will
show the moons when in front of Jupiter, but small telescopes will
only show them when clear of the disk and shadow. Often all four
can be thus seen, but three or two is a very common amount of
visibility. Quite a small telescope, such as a ship's telescope, if
held steadily, suffices to show the satellites of Jupiter, and very
interesting objects they are. They are of habitable size, and may
be important worlds for all we know to the contrary.
The news of the discovery soon spread and excited the greatest interest
and astonishment. Many of course refused to believe it. Some there were
who having been shown them refused to believe their eyes, and asserted
that although the telescope acted well enough for terrestrial objects,
it was altogether false and illusory when applied to the heavens. Others
took the safer ground of refusing to look through the glass. One of
these who would not look at the satellites happened to die soon
afterwards. "I hope," says Galileo, "that he saw them on his way to
heaven."
The way in which Kepler received the news is characteristic, though by
adding four to the supposed number of planets it might have seemed to
upset his notions about the five regular solids.
He says,[9] "I was sitting idle at home thinking of you, most
excellent Galileo, and your letters, when the news was brought me
of the discovery of four planets by the help of the double
eye-glass. Wachenfels stopped his carriage at my door to tell me,
when such a fit of wonder seized me at a report which seemed 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 us both, confounded as we were by
such a novelty, we were hardly capable, he of speaking, or I of
listening....
"On our separating, I immediately fell to thinking how there could
be any addition to the number of planets without overturning my
_Mysterium Cosmographicon_, published thirteen years ago, according
to which Euclid's five regular solids do not allow more than six
planets round the sun.
"But 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 me to require) six or eight round Saturn, and one each round
Mercury and Venus."
[Illustration: FIG. 45.--Eclipses of Jupiter's satellites. The diagram
shows the first (_i.e._ the nearest) moon in Jupiter's shadow, the
second as passing between earth and Jupiter, and appearing to transit
his disk, the third as on the verge of entering his shadow, and the
fourth quite plainly and separately visible.]
As an illustration of the opposite school, I will take the following
extract from Francesco Sizzi, a Florentine astronomer, who argues
against the discovery thus:--
"There are seven windows in the head, two nostrils, two eyes, two
ears, and a mouth; so in the heavens there are two favourable
stars, two unpropitious, two luminaries, and Mercury alone
undecided and indifferent. From which and many other similar
phenomena of nature, such as the seven metals, &c., which it were
tedious to enumerate, we gather that the number of planets is
necessarily seven.
"Moreover, the satellites are invisible to the naked eye, and
therefore can have no influence on the earth, and therefore would
be useless, and therefore do not exist.
"Besides, the Jews and other ancient nations as well as modern
Europeans have adopted the division of the week into seven days,
and have named them from the seven planets: now if we increase the
number of the planets this whole system falls to the ground."
To these arguments Galileo replied that whatever their force might be as
a reason for believing beforehand that no more than seven planets would
be discovered, they hardly seemed of sufficient weight to destroy the
new ones when actually seen.
Writing to Kepler at this time, Galileo ejaculates:
"Oh, my dear Kepler, how I wish that we could have one hearty laugh
together! Here, at Padua, is the principal professor of philosophy
whom I have repeatedly and urgently requested to look at the moon
and planets through my glass, which he pertinaciously refuses to
do. Why are you not here? What shouts of laughter we should have at
this glorious folly! And to hear the professor of philosophy at
Pisa labouring before the grand duke with logical arguments, as if
with magical incantations, to charm the new planets out of the
sky."
A young German _protege_ of Kepler, Martin Horkey, was travelling in
Italy, and meeting Galileo at Bologna was favoured with a view through
his telescope. But supposing that Kepler must necessarily be jealous of
such great discoveries, and thinking to please him, he writes, "I cannot
tell what to think about these observations. They are stupendous, they
are wonderful, but whether they are true or false I cannot tell." He
concludes, "I will never concede his four new planets to that Italian
from Padua though I die for it." So he published a pamphlet asserting
that reflected rays and optical illusions were the sole cause of the
appearance, and that the only use of the imaginary planets was to
gratify Galileo's thirst for gold and notoriety.
When after this performance he paid a visit to his old instructor
Kepler, he got a reception which astonished him. However, he pleaded so
hard to be forgiven that Kepler restored him to partial favour, on this
condition, that he was to look again at the satellites, and this time to
see them and own that they were there.
By degrees the enemies of Galileo were compelled to confess to the truth
of the discovery, and the next step was to outdo him. Scheiner counted
five, Rheiter nine, and others went as high as twelve. Some of these
were imaginary, some were fixed stars, and four satellites only are
known to this day.[10]
Here, close to the summit of his greatness, we must leave him for a
time. A few steps more and he will be on the brow of the hill; a short
piece of table-land, and then the descent begins.
LECTURE V
GALILEO AND THE INQUISITION
One sinister event occurred while Galileo was at Padua, some time before
the era we have now arrived at, before the invention of the
telescope--two years indeed after he had first gone to Padua; an event
not directly concerning Galileo, but which I must mention because it
must have shadowed his life both at the time and long afterwards. It was
the execution of Giordano Bruno for heresy. This eminent philosopher had
travelled largely, had lived some time in England, had acquired new and
heterodox views on a variety of subjects, and did not hesitate to
propound them even after he had returned to Italy.
The Copernican doctrine of the motion of the earth was one of his
obnoxious heresies. Being persecuted to some extent by the Church, Bruno
took refuge in Venice--a free republic almost independent of the
Papacy--where he felt himself safe. Galileo was at Padua hard by: the
University of Padua was under the government of the Senate of Venice:
the two men must in all probability have met.
Well, the Inquisition at Rome sent messengers to Venice with a demand
for the extradition of Bruno--they wanted him at Rome to try him for
heresy.
In a moment of miserable weakness the Venetian republic gave him up, and
Bruno was taken to Rome. There he was tried, and cast into the dungeons
for six years, and because he entirely refused to recant, was at length
delivered over to the secular arm and burned at the stake on 16th
February, Anno Domini 1600.
This event could not but have cast a gloom over the mind of lovers and
expounders of truth, and the lesson probably sank deep into Galileo's
soul.
In dealing with these historic events will you allow me to repudiate
once for all the slightest sectarian bias or meaning. I have nothing to
do with Catholic or Protestant as such. I have nothing to do with the
Church of Rome as such. I am dealing with the history of science. But
historically at one period science and the Church came into conflict. It
was not specially one Church rather than another--it was the Church in
general, the only one that then existed in those countries.
Historically, I say, they came into conflict, and historically the
Church was the conqueror. It got its way; and science, in the persons of
Bruno, Galileo, and several others, was vanquished.
Such being the facts, there is no help but to mention them in dealing
with the history of science.
Doubtless _now_ the Church regards it as an unhappy victory, and gladly
would ignore this painful struggle. This, however, is impossible. With
their creed the Churchmen of that day could act in no other way. They
were bound to prosecute heresy, and they were bound to conquer in the
struggle or be themselves shattered.
But let me insist on the fact that no one accuses the ecclesiastical
courts of crime or evil motives. They attacked heresy after their
manner, as the civil courts attacked witchcraft after _their_ manner.
Both erred grievously, but both acted with the best intentions.
We must remember, moreover, that his doctrines were scientifically
heterodox, and the University Professors of that day were probably quite
as ready to condemn them as the Church was. To realise the position we
must think of some subjects which _to-day_ are scientifically
heterodox, and of the customary attitude adopted towards them by
persons of widely differing creeds.
If it be contended now, as it is, that the ecclesiastics treated Galileo
well, I admit it freely: they treated him as well as they possibly
could. They overcame him, and he recanted; but if he had not recanted,
if he had persisted in his heresy, they would--well, they would still
have treated his soul well, but they would have set fire to his body.
Their mistake consisted not in cruelty, but in supposing themselves the
arbiters of eternal truth; and by no amount of slurring and glossing
over facts can they evade the responsibility assumed by them on account
of this mistaken attitude.
I am not here attacking the dogma of Papal Infallibility: it is
historically, I believe, quite unaffected by the controversy respecting
the motion of the earth, no Papal edict _ex cathedra_ having been
promulgated on the subject.
We left Galileo standing at his telescope and beginning his survey of
the heavens. We followed him indeed through a few of his first great
discoveries--the discovery of the mountains and other variety of surface
in the moon, of the nebulae and a multitude of faint stars, and lastly of
the four satellites of Jupiter.
This latter discovery made an immense sensation, and contributed its
share to his removal from Padua, which quickly followed it, as I shall
shortly narrate; but first I think it will be best to continue our
survey of his astronomical discoveries without regard to the place
whence they were made.
Before the end of the year Galileo had made another discovery--this time
on Saturn. But to guard against the host of plagiarists and impostors,
he published it in the form of an anagram, which, at the request of the
Emperor Rudolph (a request probably inspired by Kepler), he interpreted;
it ran thus: The furthest planet is triple.
Very soon after he found that Venus was changing from a full moon to a
half moon appearance. He announced this also by an anagram, and waited
till it should become a crescent, which it did.
This was a dreadful blow to the anti-Copernicans, for it removed the
last lingering difficulty to the reception of the Copernican doctrine.
[Illustration: FIG. 46.--Old drawings of Saturn by different observers,
with the imperfect instruments of that day. The first is Galileo's idea
of what he saw.]
Copernicus had predicted, indeed, a hundred years before, that, if ever
our powers of sight were sufficiently enhanced, Venus and Mercury would
be seen to have phases like the moon. And now Galileo with his
telescope verifies the prediction to the letter.
Here was a triumph for the grand old monk, and a bitter morsel for his
opponents.
Castelli writes: "This must now convince the most obstinate." But
Galileo, with more experience, replies:--"You almost make me laugh
by saying that these clear observations are sufficient to convince
the most obstinate; it seems you have yet to learn that long ago
the observations were enough to convince those who are capable of
reasoning, and those who wish to learn the truth; but that to
convince the obstinate, and those who care for nothing beyond the
vain applause of the senseless vulgar, not even the testimony of
the stars would suffice, were they to descend on earth to speak for
themselves. Let us, then, endeavour to procure some knowledge for
ourselves, and rest contented with this sole satisfaction; but of
advancing in popular opinion, or of gaining the assent of the
book-philosophers, let us abandon both the hope and the desire."
[Illustration: FIG. 47.--Phases of Venus. Showing also its apparent
variations in size by reason of its varying distance from the earth.
When fully illuminated it is necessarily most distant. It looks
brightest to us when a broad crescent.]
What a year's work it had been!
In twelve months observational astronomy had made such a bound as it has
never made before or since.
Why did not others make any of these observations? Because no one could
make telescopes like Galileo.
He gathered pupils round him however, and taught them how to work the
lenses, so that gradually these instruments penetrated Europe, and
astronomers everywhere verified his splendid discoveries.
But still he worked on, and by March in the very next year, he saw
something still more hateful to the Aristotelian philosophers, viz.
spots on the sun.
[Illustration: FIG. 48.]
If anything was pure and perfect it was the sun, they said. Was this
impostor going to blacken its face too?
Well, there they were. They slowly formed and changed, and by moving all
together showed him that the sun rotated about once a month.
Before taking leave of Galileo's astronomical researches, I must
mention an observation made at the end of 1612, that the apparent
triplicity of Saturn (Fig. 46) had vanished.
[Illustration: FIG. 49.--A portion of the sun's disk as seen in a
powerful modern telescope.]
"Looking on Saturn within these few days, I found it solitary,
without the assistance of its accustomed stars, and in short
perfectly round and defined, like Jupiter, and such it still
remains. Now what can be said of so strange a metamorphosis? Are
perhaps the two smaller stars consumed like spots on the sun? Have
they suddenly vanished and fled? Or has Saturn devoured his own
children? Or was the appearance indeed fraud and illusion, with
which the glasses have so long time mocked me and so many others
who have so often observed with me? Now perhaps the time is come to
revive the withering hopes of those, who, guided by more profound
contemplations, have fathomed all the fallacies of the new
observations and recognized their impossibility! I cannot resolve
what to say in a chance so strange, so new, so unexpected. The
shortness of time, the unexampled occurrence, the weakness of my
intellect, the terror of being mistaken, have greatly confounded
me."
However, he plucked up courage, and conjectured that the two attendants
would reappear, by revolving round the planet.
[Illustration: FIG. 50.--Saturn and his rings, as seen under the most
favourable circumstances.]
The real reason of their disappearance is well known to us now. The
plane of Saturn's rings oscillates slowly about our line of sight, and
so we sometimes see them edgeways and sometimes with a moderate amount
of obliquity. The rings are so thin that, when turned precisely
edgeways, they become invisible. The two imaginary attendants were the
most conspicuous portions of the ring, subsequently called _ansae_.
I have thought it better not to interrupt this catalogue of brilliant
discoveries by any biographical details; but we must now retrace our
steps to the years 1609 and 1610, the era of the invention of the
telescope.
By this time Galileo had been eighteen years at Padua, and like many
another man in like case, was getting rather tired of continual
lecturing. Moreover, he felt so full of ideas that he longed to have a
better opportunity of following them up, and more time for thinking them
out.
Now in the holidays he had been accustomed to return to his family home
at Pisa, and there to come a good deal into contact with the Grand-Ducal
House of Tuscany. Young Cosmo di Medici became in fact his pupil, and
arrived at man's estate with the highest opinion of the philosopher.
This young man had now come to the throne as Cosmo II., and to him
Galileo wrote saying how much he should like more time and leisure, how
full he was of discoveries if he only had the chance of a reasonable
income without the necessity of consuming so large a portion of his time
in elementary teaching, and practically asking to be removed to some
position in the Court. Nothing was done for a time, but negotiations
proceeded, and soon after the discovery of Jupiter's satellites Cosmo
wrote making a generous offer, which Galileo gladly and enthusiastically
accepted, and at once left Padua for Florence. All his subsequent
discoveries date from Florence.
Thus closed his brilliant and happy career as a professor at the
University of Padua. He had been treated well: his pay had become larger
than that of any Professor of Mathematics up to that time; and, as you
know, immediately after his invention of the telescope the Venetian
Senate, in a fit of enthusiasm, had doubled it and secured it to him for
life wherever he was. To throw up his chair and leave the place the very
next year scarcely seems a strictly honourable procedure. It was legal
enough no doubt, and it is easy for small men to criticize a great one,
but nevertheless I think we must admit that it is a step such as a man
with a keen sense of honour would hardly have taken.
One quite feels and sympathizes with the temptation. Not emolument, but
leisure; freedom from harassing engagements and constant teaching, and
liberty to prosecute his studies day and night without interference:
this was the golden prospect before him. He yielded, but one cannot help
wishing he had not.
As it turned out it was a false step--the first false step of his public
career. When made it was irretrievable, and it led to great misery.