III. HISTORY. 407 



ment depending ou the mathematical principle siu30=| the aperture is thus 

 always accurately equilateral, aud concentric with the mirror or object glass. 

 The graduated base and the micrometer head give the side of the triangle, 

 whence the aperture is readily obtained. 



1810. Astro-Photometer, according to Glan's system. 



Schmidt and Ifaensch, Berlin. 



III. OBJECTS ILLUSTRATING THE HISTORY OF 

 THE TELESCOPE AND ASTRONOMICAL OBSER- 

 VATION. 



1811. Galileo's Telescope. Object-glass, 50 mm. in dia- 

 meter ; eye-glass, plano-concave, 23 mm. in diameter. It served for 

 the most important discoveries and experiments of Galileo, and 

 was constructed by himself. 



The Royal Institute of " Studii Superiori" Florence. 



1814. Galileo's Telescope. Object-glass, 38 mm. in diameter ; 

 eye-glass, double concave, 19 mm. in diameter; made by Galileo. 



The Royal Institute of 61 Studii Superiori" Florence* 



1815. Object Glass of the diameter of 40 mm., made by 

 Galileo, and with which he discovered the satellites of Jupiter 

 on the 9th of January 1610, and first saw the spots on the sun. 

 Having been broken, it was presented by Viviani to Prince Leopoldo 

 dei Medici, who placed it in a frame finely wrought in ivory, with 

 inscriptions. 



The Royal Institute of " Studii Superiori" Florence. 



Galileo's part in the invention of the telescope, and the merit due to him, 

 clearly appears from the following extract of the answer given by him, in the 

 " Saggiatore," to Padre Orazio Grassi, a Jesuit. < The share of credit that 

 " may be due to me in the invention of this instrument (the telescope) and 

 " whether I can reasonably claim it as my own offspring, I have expressed in 

 " my * Avviso Sidereo,' which I wrote in Venice. I happened to be there when 

 " the news reached me that a Dutchman had presented Count Maurice with a 

 '* glass, by means of which things far away appeared just as clearly as if they 

 " were quite close at hand, but without any detail being mentioned. Upon 

 " hearing this, I returned to Padua, where I was then living, and pondered over 

 " this problem, and the first night after my return I found it out. The following 

 " day I made the instrument. After that I immediately set to work to make 

 " a more perfect one, which being completed six days afterwards, I took to 

 " Venice, where so great a marvel attracted the attention of almost all the 

 " principal gentlemen of that republic. Finally, by the advice of one of my 

 " dearest patrons, I presented it to the prince .in presence of the college. The 

 " gratitude with which it was received, and the esteem in which it was held, 

 " are proved by the ducal letters which I have still preserved, since they bear 

 " witness to the generosity of His Serene Highness in confirming me for life in 

 " my lectureship of the studio of Padua, with double the payment of that which 

 " I had had beforehand, which in its turn was more than three times what 



