VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 277 



appears that the satellite, when it could not be seen, was nearly 22" from the 

 planet. 



We must not however conclude, that this is the given distance at which it will 

 always vanish. For instance, the same satellite, though hardly to be seen, was 

 however not quite invisible March 2, 1791. Its distance from the planet, computed 



6 x 33" 



as before, was then only - — = 19".8. The clearness of the atmosphere, and 



other favourable circumstances, must certainly have great influence in observations 

 of very faint objects ; therefore, a computation of all the observations where the 

 satellites were not seen, as well as a few others where they were seen, when pretty 

 near the apogee or perigee, will be the surest way of settling the fact. The result 

 of these computations show that both the satellites became always invisible when 

 they were near the planet : that the 1st was generally lost when it came within 18" 

 of the planet, and the 2d at the distance of about 20". In very uncommon and 

 beautiful nights, the 1st has once been seen at 13". 8, and the 2d at 17". 3 ; but at 

 no time have they been visible when nearer the planet. 



I shall now endeavour toinvestigate the cause which can render small stars and satel- 

 lites invisible at so great a distance as 18 or 10". A dense atmosphere of the planet 

 would account for the defalcation of light sufficiently, were it not proved that the 

 satellites are equally lost, whether they are in the nearest half of their orbits, or in 

 that which is farthest from us. But as a satellite cannot be eclipsed by an atmos- 

 phere that is behind it, a surmise of this kind cannot be entertained. Let us then 

 turn our view to light itself, and see whether certain affections between bright and 

 very bright objects, contrasted with others that take place between faint and very 

 faint ones, will not explain the phenomena of vanishing satellites. 



The light of Jupiter or Saturn, for instance, on account of its brilliancy, is 

 diffused, almost equally, over a space of several minutes all round these planets. 

 Their satellites also, having a great share of brightness, and moving in a sphere 

 that is strongly illuminated, cannot be much affected by their various distances 

 from the planets. The case then is, that they have much light to lose, and com- 

 paratively lose but little. The Georgian planet, on the contrary, is very faint ; and 

 the influence of its feeble light cannot extend far, with any degree of equality. 

 This enables us to see the faintest objects, even when they are only a minute or 2 

 removed from it. The satellites of this planet are very nearly the dimmest objects 

 that can be seen in the heavens ; so that they cannot bear any considerable diminu- 

 tion of their light, by a contrast with a more luminous object, without becoming 

 invisible. If then the sphere of illumination of our new planet be limited to 18 

 or 20", we may fully account for the loss of the satellites when they come within 

 its reach ; for they have very little light to lose, and lose it pretty suddenly. This 

 contrast therefore, between the condition of the Georgian satellites and those of 

 the brighter planets, seems to be sufficient to account for the phenomenon of their 

 becoming invisible. 



