VISIBILITY OF STARS. 51 



gtars fainter than those of the sixth magnitude — have been 

 able to distinguish the satelUtes of Jupiter without a tele- 

 scope. The angular distance of the third and brightest sat- 

 ellite from the center of the planet is 4' 42" ; that of the 

 fourth, which is only one sixth smaller than the largest, is 

 8' 16" ; and all Jupiter's satellites sometimes exhibit, as Ar- 

 ago maintains,* a more intense light for equal surfaces than 



* Arago, in the Annuaire pour 1842, p. 284, and in the Comptea 

 Retidus, torn, xv,, 1842, p. 750. (Schuin., Astron. Nachr., No. 702.) 

 '• I have instituted some calculations of magnitudes, in relerence to your 

 conjectures on the visibility of Jupiter's satellites," writes Dr. Galle, in 

 letters addressed to me, " but I have found, contrary to my expecta- 

 tions, that they are not of the fifth magnitude, but, at most, only of the 

 sixth, or even of the seventh magnitude. The third and brightest sat- 

 ellite alone appeared nearly equal in brightness to a neighboring star 

 of the sixth magnitude, which I could scarcely recognize with the naked 

 eye, even at some distance from Jupiter ; so that, considered in refer- 

 ence to the brightness of Jupiter, this satellite would probably be of the 

 fifth or sixth magnitude if it were isolated from the planet. The fourth 

 satellite was at its greatest elongation, but yet I could not estimate it at 

 more than the seventh magnitude. The rays of Jupiter would not pi-e- 

 vent this satellite from being seen if it were itself bnghter. From a 

 comparison of Aldebaran with the neighbonng star d Tauri, which is 

 easily recognized as a double star (at a distance of 5^ minutes), I should 

 estimate the radiation of Jupiter at five or six minutes, at least, for or- 

 dinary vision." These estimates correspond with those of Arago, who 

 is even of opinion that this false radiation may amount in the case of 

 some persons to double this quantity. The mean distances of the four 

 satellites from the center of the main planet are undoubtedly 1' 51", 

 2' 57", 4' 42", and 8' J.6". " Si nous supposons que I'image de Jupiter, 

 dans certains yeux exceptionnels, s'epanouisse seulement par des ray- 

 ons d'une ou deux minutes d'amplitude, il ne semblera pas impossible 

 que les satellites soient de terns en tems aper^us, sans avoir besoin de 

 recourir a I'artifice de I'ampiification. Pour vei'ifier cette conjecture, 

 j'ai fait construire une petite lunette dans laquelle I'objectif et I'ocu- 

 laire out a pen pres le meme foyer, et qui des lors ne grossit poiyit. 

 Cette lunette ne detruit pas enti^rement les rayons divergents, mais 

 elle en reduit considerablement la longueur. Cela a suffi pour qu'un 

 satellite convenablement ecarte de la plauete, soit deverni visible. Le 

 fait a ete constate par toua les jeunes astronomes de I'Observatoire." 

 ** If we suppose that the image of Jupiter appeal's to the eyes of some 

 persons to be dilated by rays of only one or two minutes, it is ntt im- 

 possible that the satellites may from time to time be seen without the 

 aid of magnifying glasses. In order to verify this conjecture, I caused 

 a small instrument to be constructed in which the ol)ject-glass and the 

 eye-piece had nearly the same focus, and which, therefore, didnot mag 

 nify. This insti-ument does not entirely destroy the diverijing rays, al 

 though it considerably reduces their len^jth. This method has sufficed 

 to render a satellite visible when at a sufficient distance from the planet. 

 This observation has been confirmed by all the young astronomers at 

 the Observatoiy." (Arago, in the Comptes Rendus, tom. xv., 1842, p. 

 751.) 



