66 COSMOS. 



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- 



of some persons to be dilated by rays of only one or two 

 minutes, it is not impossible 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 object-glass and the eye-piece 

 had nearly the same focus, and which therefore did not mag- 

 nify. This instrument does not entirely destroy the diverging 

 rays, although it considerably reduces their length. 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 Observatory." 

 (Arago in the Comptes rendus,tom. xv. 1842, p. 751.) 



As a remarkable 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 Schon, who died at 

 Breslau in 1837, and with reference to whom I have re- 

 ceived some interesting communications from the learred 

 and active director of the Breslau Observatory, Von Boguj- 

 lawski. " After having (since 1820) convinced ourselves, by 

 several rigid tests, that in serene moonless nights Schon was 

 able correctly to indicate the position of several of Jupiter's 

 satellites at the same time, we spoke to him of the emana- 

 tions and tails which appeared to prevent others from seeing 

 so clearly as he did, when he expressed his astonishment at 

 these obstructing radiations. From the animated discussions 

 between himself and the bystanders 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 



