52 COSMOS. 



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 which were used, since the ear- 

 liest ages of mankind, and especially among the Egyptians, 

 as pictorial representations to indicate the shining orbs of 

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

 (These lines are regarded by Hassenfratz as caustics on the 

 crystalline lens : intersections des deux caustiques.} 



" The image of the star which we see with the naked eye 

 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 vision, 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 

 Schbn, 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 Observatory, Von Boguslawski. " After having 

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

 moonless nights Schbn 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 appeared to prevent others from seeing so 

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

 structing 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 Schbii 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 the 

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

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

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

 and less constant light of the former. Some years before his death 

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

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

 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 on 

 persons of such great perfection and sensibility of vision. Schbii 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 generally 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 occasionally exceeds in the intensity of its clear yellow light the 

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

 s. 231-234, and 439.) Sturm and Airy, in the Complex Rendut, t. xx., 

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

 of vision, remote luminous poin'a may appear as light streaks. 



