62 cosmos. 



we find Sir William Herschel, in his investigations on the 

 magnitude of the apparent diameters of Arcturus (0"2 within 

 the nebula) and of Vega Lyrae, using a power of 6500. Since 

 the middle of the seventeenth century, constant attempts 

 have been made to increase the focal length of the telescope. 

 Christian Huygens, indeed, in 1655, discovered the first sat- 

 ellite of Saturn, Titan (the sixth in distance from the center 

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

 ly, 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 Constantin Huygens, brother 

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

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

 in 1663 constructed colossal telescopes without tubes, and 

 therefore without a solid connection between the object-glass 

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

 focal length of 320 feet, magnified 600 times. f 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 ob- 

 ject-glasses that had been ground by Borelli, Campani, 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 Cam- 

 pani, 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, worked 

 by strings only, $ we can not sufficiently admire the skill and 

 the untiring perseverance of the observer. 



* 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 Uytenbrock in the " Oratio de fratribus 

 Christiano atque Constantino Hugenio, artis dioptricae cultoribus," 1838; 

 and by Prof. Kaiser, the learned director of the Observatory at Leyden 

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



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



t " Nous avons plac6 ces grands verres, tantdt sur un grand mat, tan- 

 tot sur la tour de bois venue de Marly ; eiifin nous les avons mis dans 

 un tuyau mont6 sur un support en forme d'6chelle a trois faces, ce qui 

 a eu (dans la d6couverte des satellites de Saturne) le succes que nous 

 en avious esperfc." " We sometimes mounted these great instruments 

 on a high pole," says Dominique Cassini, " and sometimes on the wood- 



