"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, of the name of Jansen, had presented to prince Maurice of Nassau an optical instrument, which possessed the singular property of causing distant objects to appear nearer and larger to the observer. A few days afterwards, the truth of this report was confirmed by a letter which he received from James Badovere 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.
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. 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 Dutch optician's shop, performing the part of a philosophical toy, by exhibiting a magnified and inverted image of a distant weathercock, must have been a single lens such as we have mentioned, or an astronomical telescope consisting of two convex lenses. Upon either of these suppositions, it differed entirely 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.
The interest which the exhibition of the telescope excited at Venice did not soon subside: Serturi 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 Serturi 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 this new instrument. Galileo's tube,—or the double eye-glass, as it was then called, for Demisiano had not yet given it the appellation of a 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.
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 Gærtner, 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 show Jupiter's disc well defined.
After Galileo had completed his first instrument, which magnified only three times, he executed a larger and more accurate 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 magnified 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 satisfaction."[8]
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 reflected from their summits 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 incident 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 perfectly smooth; to cover it with mountains, and to 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 perfectly 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 various and so beautiful."