ON INSTRUMENTS FROM ITAL Y. 109 



telescopes of rare perfection. He had devised a more applicable 

 method for whetting and cleaning the lens, of which he was the first to 

 calculate the curve previously. Here is one of Torricelli's telescopes. 

 This other one was made by Eustachio Divini di S. Severino, who, 

 between 1646 and 1668, constructed telescopes of the extraordinary 

 length of seventy-two " palmi romani." And Viviani, likewise, made 

 telescopes in a masterly fashion. It is important to remember also 

 that Cesare Marsilli, a member of the Accademia de' Lincei, devised a 

 method of making telescopes by substituting a concave mirror, for the 

 objective an idea to which he alluded in a letter to Galileo in July, 

 1626 ; but which he never carried out, probably in consequence of the 

 difficulty in obtaining the necessary mirrors. In his time, no man 

 made more excellent telescopes than Giuseppe Campani, a Roman ; 

 there exist in many places in Europe beautiful ones made by him, some 

 of which were even of the length of 210 palmi romani. This one of 

 medium size is by Campani. 



To return, once more, to Galileo. I am able to offer for your admira- 

 tion all that remain of his venerable optical instruments, the two 

 telescopes, and the broken objective, with which he made his most 

 important astronomical discoveries ; and these were described in a 

 masterly way in his " Nuncius Sidereus," and in his letters and 

 dialogues on the systems of the universe. Before all comes the dis- 

 covery of the mountains in the moon ; he showed the manner of calcu- 

 lating their heights, and proved that some of them are higher than 

 some of our terrestrial ones. Then he explained that the lunar disc 

 appears to us to shine but feebly in the various phases of the new moon, 

 in consequence of the reflection of the earth, and how the movements 

 of the moon influence the flux and reflux of the sea. He was the first 

 to show that the milky way consists of a mass of innumerable stars ; 

 and, on the 7th of January, 1610, having his telescope fixed on Jupiter, 

 he made the discovery that three small planets revolve round it ; and 

 on the 1 3th of the same month he observed the fourth satellite. 

 Following up these researches in Rome, he determined the times of 

 their conversions, described a figure of their movements, certified that 

 they undergo eclipses, like the moon, and discovered that their progress 

 is extremely fast, inasmuch as the slowest completes a revolution round 

 Jupiter in little more than sixteen days ; and that by their means, more 



