XX o , NOTES. 



reduit considerablement la longueur. Cela a suffi pour qu'un satellite con- 

 veuablement ecarte de la planete soit devenu visible. Le fait a etc constate 

 par tous les jeunes astronomes de PObsejvatoire" (Arago, in the Comptes 

 reudus, T. xv. p. 751). I may instance, as a remarkable example of the 

 keen sight and great sensibility of the retina in particular individuals 

 who s<je Jupiter's satellites with the naked eye, a deceased master tailor of the 

 name of Sehon, in Breslau, respecting whom the learned and active Director 

 of the Observatory of that place, Herr von Boguslawski, has given ma 

 interesting communications. " After being assured by repeated trials, since 

 1820, that in clear moonless nights Schon, with the naked eye, could assign 

 correctly the position of Jupiter's satellites, even of more than one at a time, 

 on speaking to him of the rays and tails of light which seemed to prevent 

 others from doing the same, he expressed his astonishment at it ; and from 

 the animated discussion which arose between him and the bystanders 

 respecting the difficulty of seeing the satellites with the naked eye, I could 

 not but infer that the planets and the fixed stars always appeared to him as 

 luminous points, free from rays. He saw the third satellite best, and he could 

 also see the first at its widest elongation ; but he never saw the second or 

 fourth. "When the atmosphere was not quite favourable, the satellites 

 appeared to him only as faint streaks of light. Small fixed stars, perhaps on 

 account of their scintillating and less tranquil light, were never confounded by 

 him with the satellites. Some years before his death, Schon complained to 

 me that his eyes, as they grew older, could no longer reach Jupiter's satellites, 

 and that now, even when the atmosphere was quite clear, their place was 

 only marked to him by faint streaks." The above account agrees perfectly 

 with what has long been known respecting the relative brightness of Jupiter's 

 satellites ; for, in individuals whose organs have so high a degree of perfection 

 and sensibility, probably brightness and the quality of light are more 

 influential than distance from the central planet. Schon never saw the second 

 and fourth satellites : the second is the smallest of all ; the fourth is, indeed, 

 next to the third, the largest, and also the most distant, but periodically it is dark 

 in colour, and at ordinary times it has the faintest light of any of the satellites. 

 Of the third and first, which have been seen best and most often with the 

 unassisted eye, the third is the largest of all, usually the brightest, and of a 

 very decided yellow colour ; but the first sometimes exceeds in the intensity of 

 its bright yellow light the brightness of the third, which is much larger 

 (Miidler, Astron. 1846, S. 231234 und 439). How, by relations of 

 refraction in the visual organ itself, distant luminous points may appear as 



