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 demonstrated the existence of the little bodies, still less did they yield such a volume of accurate knowledge as would enable us to determine their movements, so as to say when they would again be likely to be visible. No astronomer ever seriously entertained the notion that there was such a system of attendants revolving around Jupiter until their existence had been demonstrated once and for all by the telescope of Galileo.

There can be no doubt that the moons of Jupiter are in themselves quite bright enough to be ordinarily seen by the unaided eye were it not for a single circumstance. They lie too close to the great planet. At first sight it might seem that the very fact that they are placed in the brilliant illumination which Jupiter radiates should rather tend to make them more easily discerned. The nearer an object is held to a source of light the better it can generally be seen. Does it not therefore appear somewhat paradoxical to say that the reason we are generally unable to see the moons of Jupiter with the unaided eye, is because they lie so close to a lustrous globe? Ought not that to be the very reason why they should be seen with all the greater facility? This is a point which may require a few words of explanation because it is intimately connected with the recent great discovery.

We hold a book near a candle when we want to read, because under the circumstances supposed the only light available comes from the candle. The type has no other illumination, and the nearer it is to the candle the clearer the printing appears. But this is not at all analogous to the case of Jupiter and his satellites. We cannot think of Jupiter as the candle, and a satellite as a page of the book. If such were indeed the analogy, then the nearer