NATURAL VISION". 67 



trated in a single point. The impression on the nerves is 

 weaker. A very dense starry swarm, in which scarcely any 

 of the separate stars belong even to the 7th magnitude, may, 

 on the contrary, be visible to the unaided eye in consequence 

 of the images of the many different stars crossing each other 

 upon the retina, by which every sensible point of its surface is 

 more powerfully excited, as if by one concentrated image." 17 



it was at the greatest digression, but he never saw the second 

 and the fourth alone. When the air was not in a very favour- 

 able 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 Schon com- 

 plained to me that his failing eye could no longer distinguish 

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

 in clear weather, by light fawit streaks." These circumstances 

 entirely coincide with what has been long known regarding 

 the relative lustre 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. 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 colour, and is gene- 

 rally the faintest of aU 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 colour; the latter 

 occasionally exceeds in the intensity of its clear yellow light 

 the lustre of the third, which is also much larger. (Madler, 

 Astr. 1846, s. 231-234, and 439.) Sturm and Airy, in the 

 Comptes rendus, t. xx. pp. 764-6, show how, under proper 

 conditions of refraction in the organ of vision, remote luminous 

 points may appear as light streaks. 



" *' L'image epanouie d'une etoile de 7eme grandeur 

 n'ebranle pas suflisamment la retine : elle n'y fait pas naitre 

 une sensation appreciable de lumiere. Si Viniage netait 

 point tpanouie (par des rayons divergents), la sensation 



F 2 



