Galileo (1918)/Chapter 3
CHAPTER III.—GALILEO'S TELESCOPE.
We now come to the most important epoch in Galileo's career. In 1609, being at Venice, he heard for the first time of the invention of a telescope the previous year in Flanders, and at once determined to think the matter out to see whether he could find the principle of construction. He soon succeeded in fitting two lenses in a tube, so that objects viewed through it were magnified, and his second attempt was still more satisfactory. His success was speedily reported to the Venetian Council, who summoned him to exhibit his spy-glass, which was already much more efficient than the Flanders one, besides which it showed objects upright whereas the first telescopes inverted them. The Doge wished to possess one, and Galileo promptly offered to present the one with which the Council had been so pleased, and when he had taken it to the palace he was complimented on the efficient manner in which he had served the State for seventeen years, and informed that instead of waiting until his third term of office had expired, the Council had determined to raise his salary at once to nearly double what it had been at the last augmentation, and to confirm the appointment for life at the new salary of 1000 florins, or about £220.
It is considered by some that Galileo's discovery is greatly discounted by the previous success in Holland, but as the latter was due to a pure accident, while Galileo's was by design, others will be inclined to give him the greater credit, reserving an even higher meed of praise for one who deliberately invents a contrivance of great importance without any previous hint as to its possibility. The aperture of the Doge's telescope was about 13 inches. It was the second instrument of its kind, but has been lost. Its magnifying power was 3 diameters, and it showed objects 22 miles distant on the earth. On his return to Padua he made a third telescope with a power of 8 diameters, soon followed by a fourth with a power of 20 diameters. This instrument he turned towards the sky, observing the moon and Jupiter, and soon found opportunity on a visit to Florence to show it to his old pupil Cosmo, now Grand Duke, since the death of Ferdinand early in the year. It is interesting to note that three weeks before Ferdinand's death Galileo had cast his horoscope at the request of the Grand Duchess, and promised many more years of life.
Early in 1610 came the fifth telescope, magnifying more than 30 diameters. With this Galileo began his series of astronomical discoveries, except that of the appearance of the moon, begun with the fourth telescope, and shown by its means to the young Grand Duke the previous year. In his tract, the "Sidereus Nuncius" (or Sidereal Messenger), printed at Venice in the same year, he announced the first fruits of his search with the new instrument; first the diversified surface of the moon, with many markings not visible to the naked eye; mountains four or five miles high, and the "earthshine," which he attributed to its true cause; next, the great number of stars revealed by the telescope in clusters, including the true structure of the Milky Way. The planets, moreover, which in his fourth telescope were seen to differ from stars, in showing discs instead of points of light, yielded fresh surprises to the fifth telescope. It was on January 7, 1610, that with this instrument directed towards Jupiter Galileo first saw three bright objects close to the planet. Repeated observation proved that these, with a fourth, which must have been on the first night hidden by the planet, were moons revolving round Jupiter. This conclusion was reached in a very few weeks, and Galileo gave the newly discovered bodies the name of "Medicean stars" in compliment to the Grand Duke and his three brothers.
Meanwhile Galileo had fitted up a workshop for the manufacture of telescopes, finding, however, that only about one in ten of his object glasses was good enough to show the "Medicean stars". The demand for the new instrument was great, as Galileo's telescopes were much more efficient than any to be procured elsewhere, so that his workmen, who made also hundreds of his other instruments, geometrical compasses, hydrostatic balances, air thermometers, magnets, and so on, were kept busy, though Galileo ground all his lenses himself until his sight failed. It was some time before the superiority of other forms of telescope for very high magnifying power was realised. Galileo's principle still survives in field glasses, where the power is not high and when the inverting effect of the astronomical telescope is inconvenient, especially as the erecting eyepiece involves loss of light. The Grand Duke Cosmo II. asked for the telescope with which these discoveries were made, but Galileo, though he at once consented, nevertheless kept it for his own use, ostensibly only on loan. Its focal length was 51 feet and aperture 21 inches.
During the earlier part of 1610 more than a hundred telescopes, accompanied by copies of the "Sidereus Nuncius," were sent to princes and learned men of Italy, France, Flanders, and Germany, and the French Court in particular, where Marie dei Medici was queen, desired to bespeak for the King, Henry of Bourbon, the privilege of giving his name to the next striking discovery in the heavens. Many savants, however, refused to accept Galileo's discoveries, some even declining to look through a telescope, rather than risk seeing something in whose existence they would not believe. Galileo did not at this time state openly in the "Sidereus Nuncius" the conclusion to which his discoveries had driven him, namely, that the earth moves round the sun, and not the sun round the earth. He spoke of it and lectured about it, not realising how he would be affected by the furious outcry against it. Kepler, the greatest astronomer of the time, accepted everything in full confidence, though the mystic numbers in which he delighted were liable to be upset by such new discoveries. Simon Marius (Mayer) claimed priority in the discovery of Jupiter's moons, as he had previously claimed it in the case of the proportional compass, but there is no doubt that both claims were fraudulent. In the endeavour to increase his list of discoveries and comply with the earnest wish of the Italian Queen of France, Galileo next turned his attention to Saturn. He found no satellite, but under the imperfect definition of his telescope he noted that the planet appeared triple, the appearance being caused, as was discovered later, by the ring projecting on both sides of the ball. To avoid the chance of being again accused of adopting other people's discoveries, he published this one in the form of a jumble of letters, which when sorted out, would spell the words:—
"Altissimuni Planetam Tergeminum Observavi," meaning, "I have found the furthermost planet (Saturn at that time, as Uranus and Neptune were not yet discovered) to be triple," the fact of the matter being that the imperfect definition of his telescope caused the ring of Saturn, which was then open as viewed from the earth, to resemble companion stars, one each side of the central ball. These attendant objects gradually diminished in apparent size, and after two years disappeared altogether, the ring being then edgeways as seen from the earth. This occurs every fourteen years, though naturally the fact was quite unknown to Galileo, and he was very much taken aback at the vanishing of the new objects, which suggested to him a reference to the Greek myth of Saturn swallowing his children. He predicted their reappearance and observed them again as the ring widened out. Later he realised the dark space on each side of the ball between it and the ring, but, as he gave no indication of having grasped the significance of this appearance, it is probable that he did not really understand it. The true explanation was discovered some years after his death, by Huygens, with a telescope of magnifying power of 100 diameters. It is needless to emphasise the additional annoyance caused to the Aristotelians by this new evidence of change in the "immutable" heavens.
To Galileo himself the successive discoveries and the controversies they aroused brought a large increase of students and pupils from all directions, and his teaching work grew to such an extent that he could find very little time for original research. He felt that after twenty years he had had enough of this labour and was desirous of finding more leisure. He had gratefully accepted the life-appointment at Padua, but was only too ready to relinquish it when an opportunity of escape was offered. His great European reputation made it seem desirable to the Tuscan Court to attach him permanently to the Grand Ducal service, and negotiations were opened between Galileo and the Duke's Secretary of State. Galileo stated plainly how irksome he found the necessity of spending so much time on pupils on account of his family expenses, and even the amount of lecturing required, though this was not very great. He also said how many inventions he had in mind, many of them only of use to princes making war; he enumerated books in contemplation, some of them already in hand, on the structure of the universe, on local motion, on mechanics, on sound and speech, on light and colours, on the tides and other subjects, besides works on military science, manœuvres, fortification, etc. In addition to these and other proposed reprints and publications, he pointed out how much time he wanted for observation of the Medicean stars in order to construct tables of their motion, and for other astronomical pursuits. The result of the negotiations was his nomination by Cosmo II. as First Mathematician of the University of Pisa, and also Philosopher and Mathematician to the Grand Duke, but with no obligation to lecture nor to live at Pisa.