80 cosmos. 



Christian Huygens, indeed, in 1655, discovered the first 

 satellite of Saturn, Titan (the sixth in distance from the 

 centre of the planet), with a twelve-feet telescope ; he sub- 

 sequently however examined the heavens with instruments of 

 a greater focal length, even of 122 feet ; but the three object- 

 glasses in the possession of the Royal Society of London, 

 whose focal lengths are respectively 123, 170, and 210 feet, 

 and which were constructed by Constantino Huygens, brother 

 of the great astronomer, were only tested by the latter, as 

 he expressly states, 26 upon terrestrial objects. Auzout, who 

 in 1663 constructed colossal telescopes without tubes, and 

 therefore without a solid connexion between the object- 

 glass and the eye-piece, completed an object glass, which, 

 with a focal length of 320 feet, magnified 600 times. 27 

 The most useful application of these object-glasses, mounted 

 on poles, was that which led Dominic Cassini, between the 

 years 1671 and 1684, to the successive discoveries of the 

 eighth, fifth, fourth, and third satellites of Saturn. He made 

 use of object-glasses that had been ground by Borelli, Cam- 

 pani, and Hartsoeker. Those of the latter had a focal length 

 of 266 feet. 



During the many years I passed at the Paris Observatory, 

 I frequently had in my hands the instruments made by 

 Campani, which were in such great repute during the reign 

 of Louis XIV; and when we consider the faint light of 

 Saturn's satellites, and the difficulty of managing instruments, 



26 The remarkable artistical skill of Constantin Huygens, 

 who was private secretary to King William the Third, has 

 only recently been presented in its proper light by Uyten- 

 brock in the " Oratio de fratribus Christiano atque Constantino 

 Hugenio, artis dioptrics cultoribus," 1838; and by Prof. 

 Kaiser, the learned director of the Observatory at Leyden 

 (in Schumacher's Astron. Nachr., no. 592, s. 246). 



27 See Arago, in the Annuaire pour 1844, p. 381. 



