m. HISTORY. 409 



Huygens writes in 1637, that it was impossible to find artificers in that 

 country having skill enough to make telescopes of sufficient power to observe 

 the satellites of Jupiter. 



The principal discoveries made by Galileo by means of his telescope in 

 chronological order, are as follows : 



He first of all observed the cause of the lunar spots, recognising in them 

 the effect of shade produced by the heights which must exist upon the Moon, 

 and thus proving that it was covered with mountains and table-lands ; and 

 he pointed out the way for determining their heights. He likewise perceived 

 that the secondary light transmitted to us from the Moon proceeds from the 

 reflection of the solar rays from the suface of the earth to that of the Moon. 



Directing his telescope towards the Milky Way he was the first to certify the 

 fact that it was composed of myriads of stars. 



On the 3rd of January 1610, he discovered that three secondary planets 

 revolve round Jupiter, and on the 13th of the same month he found the fourth. 

 He named these satellites the " Stelle Medicee." He showed that .they, 

 in revolving round Jupiter, underwent eclipses, just as our moon does. He 

 determined the length of their rotations, remarking that by their means it 

 is possible to observe 1,000 eclipses a year, which would be of great utility 

 for finding the longitude of any place. 



In the month of August 1610, whilst observing Saturn, he noticed that it 

 appeared to be continually accompanied by two planets which touched it at 

 the extremities of the same diameter, so that he called it incorporeal. 



In the same month he first saw the spots on the Sun, determined the 

 direction of their motion, and, with happy intuition, compared them to 

 terrestrial clouds. 



On the 30th September 1610 he found that Venus changed figure like the 

 Moon, so he called it horned (lafalcata). 



He likewise observed Mars and its phases, and reasoned admirably upon 

 the comets. 



Taking his stand upon these astronomical observations, Galileo embraced, 

 maintained, and defended the system of Coperniciis, so that he had to undergo 

 the cruel persecutions of Borne, which compelled him to utter that recantp.<- 

 tion, at the end of which he broke out into the now celebrated exclamation, 

 " Eppur si muove .'" 



1816. Telescope of Torricelli, with an object-glass of 

 50mm. in diameter; eye-glass, plano-convex, 22mm. in diameter. 

 The Royal Institute of " Studii Supcriori" Florence. 



Torricelli was the first who set himself the task of solving the optical 

 problem of finding the proper shape to be given to the surfaces of the 

 glasses used for telescopes. After many months of study and hard work, as 

 he himself relates, he succeeded in his object and the result was fully con- 

 firmed by experience. He did not make his lenses on metallic forms or 

 moulds, but fixed them on pieces of slate with a cold mixture, in order to 

 obviate the defects caused by the change of shape which the glasses undergo 

 when warmed. To him we also owe the microscope formed of one lens, or 

 rather of a little glass ball, which worked with a lamp. 



1818. Day Telescope of Divini, from S. Severino. The 

 diameter of the object-glass is 70mm. ; the diameter of the first 

 eye-glass and of the second lens, double-convex, 33mm. The 

 third lens is wanting. 



The Royal Institute of" Studii Superiori" Florence. 



Divini constructed telescopes between the years 1646 and 1668, and even 

 made some of the length of 72 palmi Komani. 



