37 



of the Pull Moon. My estimate of Ms surface brightness = | 

 that of the Moon. The product of these fractions is ^ri— J 

 not far from Bond's estimate. 



If, then, we may take it as proved that solar illumination is 

 vastly insufficient to account for Jupiter's brightness, how is 

 the excess to be accounted for? Prof. Newcomb says 

 (hesitatingly) :—" A still more remarkable resemblance to 

 the sun has sometimes been suspected— nothing less, in fact, 

 than that Jupiter shines partly by his own light, It was at 

 one time supposed that he actually emitted more light than 

 fell upon him from the sun ; and if this were proved, it 

 would show conclusively that he was self-luminous." Mr. 

 Proctor favours this opinion, and indeed accepts it as a 

 necessary fact. I think we can come to no other conclusion. 

 The greatest difficulty in the way of this theory is, I think, 

 the relative brightness of Jupiter's satellites. As I suggested 

 in my former paper, they may derive a small portion of theirlight 

 from their primary, but this, of course, would be apparent; to 

 us only in the farther portion of their orbits. On this 

 question, Kewcomb says:— "If we assume that the planet 

 emits any great amount of light, we are met by the fact that 

 the satellites would shine by this light when in the shadow of the 

 planet. As these bodies totally disappear in this position, the 

 quantity of light emitted by Jupiter must be quite small." 

 If these satellites were seen in a darle shj when in shadow, 

 I, too, think they would be visible. By a rough process of 

 calculation (which it would take too long to describe), and 

 taking my own estimate of Jupiter's brightness, I find that 

 his first satellite would receive from him about 50 times the 

 light which our new Moon receives from earth shine, or 

 about J ff of our sunshine, obtained thus : — 

 Pull moonlight to sunlight (mean of several Sun. 



observers)... ■•• =550' 



Earthlight to Moon... 



Jupiter to Satellite I, at f Moon brightness 



.. = 13 



Moon. 



• = ^°_ 

 Earth. 



13 x 50 _1 



550-000 846 i~ 



(assuming equally reflective surfaces) as between Earth 

 and Moon. This is about equal to the sunshine received 

 by the satellite of Neptune. But it must be remembered 

 that when Jupiter's satellites enter his shadow, they are in 

 such close proximity to the limb of the planet that such feeble 

 illumination would be totally overpowered by his glare. 

 The evidence of the satellites appears still further hostile 



