52 C0SM03. 



Jupiter himself; occasionally, however; as shown by recent 

 observations, they appear like gray spots on the planet. The 

 rays or tails, which to our eyes appear to radiate from the 

 planets and fixed stars, and wliich were used, since the ear- 

 liest ages of mankind, and especially among the Egyptians, 

 as pictorial representations to indicate the sliining orbs of 

 lieaven, are at least from five to six minutes in length. 

 (These lines are regarded by Hassenfratz as caustics on the 

 crystalline lens : intersections cles deux coAistiques.) 



*' The image of the star which we see with the naked eya 

 is magnified by diverging rays, in consequence of which it 

 occupies a larger space on the retina than if it were concen- 



As a remakable instance of acute %nsion, and of the great sensibility 

 of the retina in some individuals who are able to see Jupiter's satellites 

 with the naked eye, I may instance the case of a master tailor, named 

 Schon, who died at Breslau in 1837, and with reference to whom I have 

 received some interesting communications from the learned and active 

 director of the Breslau Observatoiy, Von Boguslawski. ** After having 

 (since 1820) convinced ourselves, by several rigid tests, that in serene 

 moonless nights Schon was able correctly to indicate the position of sev- 

 eral of Jupiter's satellites at the same time, we spoke to him of the em 

 anations and tails which appeai'ed to prevent others from seeing so 

 clearly as he did, when he expressed his astonishment at these ob* 

 iti'ucting radiations. From the animated discussions between himself 

 and the by-standers regarding the difficulty of seeing the satellites with 

 the naked eye, the conclusion was obvious, that the planet and fixed 

 stars must always appear to Schon like luminous points having no rays. 

 He saw the third satellite the best, and the first very plainly when it 

 was at the greatest digression, but he never saw the second and tho 

 fourth alone. When the air was not in a veiy favorable condition, the 

 satellites appeared to him like faint streaks of light. He never mistook 

 email fixed stars for satellites, probably on account of the scintillating 

 and less constant light of the fonner. Some yeai's before his death 

 Schon complained to me that his failing eye could no longer distinguish 

 Jupiter's satellites, whose position was only indicated, even in cleai' 

 weather, by light faint streaks." These circumstances entirely coin- 

 cide with what has been long known regarding the relative luster of 

 Jupiter's satellites, for the brightness and quality of the light probably 

 exert a greater influence than mere distance from the main planet ou 

 persons of such great perfection and sensibihty of vision. Schon never 

 saw the second nor the fourth satellite. The former is the smallest of 

 all ; the latter, although the largest after the third and the most remote, 

 is periodically obscured by a dark color, and is genei'ally the faintest 

 of all the satellites. Of the third and the first, which were best and 

 most frequently seen by the naked eye, the former, which is the largest 

 of all; is usually the brightest, and of a very decided yellow color ; the 

 latter occasiona.ly exceeds in the intensity of its clear yellow light the 

 luster of the third, which is also much larger. (Madler, Astr., 1846, 

 6. 231-234, and 439.) Sturm and Airy, inUie Comptes Rendns, t. xx., 

 p. 764-6, show how, under proper conditions of refraction in the orgui; 

 of vision, remote lum'uous j>uin'd mov anpcar as light streaks. 



