liance from Jupiter's own lustre, it is obvious that the further it be away the more will its image stand out on that part of the retina where there is no temporary diminution of sensibility, and the better it will be seen. Such is the reason why the moons of Jupiter cannot be seen with the unaided eye except under conditions that need not be again referred to. Were objects of no greater brilliance quite aloof from such a bright orb as one of the great planets, they would easily be discerned without optical aid. In the ordinary language of the astronomer, one of Jupiter's satellites would be reckoned as bright as a star of the fourth magnitude.
Since the discovery of the satellites of Mars by Professor Asaph Hall at Washington, in 1877, there has been no event in the astronomical world which has possessed the same interest as that of which we are now speaking. From one point of view it might appear that the announcement of an addition of another satellite to Jupiter's system was no very significant matter after all. No doubt the new attendant of the great planet is a very trifling object, as far as dimensions are concerned. It is not nearly so large as many of those minor planets the discoveries of which are constantly being announced. How comes it that people are talking about and thinking about Jupiter's fifth satellite, while there are thousands or rather millions of stars lying unnoticed through space? and yet any one of these stars is perhaps a million times as big as this little satellite, besides being a sun, which may presumably be a source of light and heat to planets which circulate around it. What is the ground for so much excitement about the discovery of an object which is probably among the most minute, if not itself actually the most
