Sir W. Herschel, in the years 1794 to 1796, found that the satellites revolve on their axes in the same time that they revolve about the planet. He was led to this conclusion by a study of the variations in the light emitted by the satellites in different parts of their orbits, and described I. as “of a very intense bright, white, and shining light,—brighter than II. or IV. (not larger). IV. inclines to red, and nearly as bright as II. The latter is of a dull ash-colour. III. is very white.” Modern observers have selected II. as relatively the most highly reflective, while IV. is the least. Spots exist on the surfaces of these objects, and probably occasion many of the differences observed. The eclipses, occultations, and transits of the satellites afford a very fertile and attractive series of phenomena for telescopic review. The exact times of occurrence are tabulated in the ‘Nautical Almanack’ and asterisks are affixed to such as are visible in this country. Prior to the date of opposition of Jupiter the eclipses occur of course on the W. side of the disk, while after opposition they take place on the E. side. The durations are as follow for the several satellites:—I. = 2^h 20^m, II. = 2^h 56^m, III. = 3^h 43^m, IV. = 4^h 56^m. In reference to III. and IV. the entire phenomenon may be generally observed; but this is not so in regard to II., as the emersions are frequently effected behind the planet. Only the immersions of I. are visible before opposition, from the same cause; for the satellite enters the cone of shadow close to the planet’s limb, and only comes out of it when the globe of the planet is interposed in the line of sight. In such cases the satellite emerges soon after from the limb of Jupiter; so that its obscuration has been compounded of two separate phenomena, viz. an eclipse and an occultation. After opposition this satellite is first occulted and then eclipsed. IV. sometimes escapes eclipse altogether, by passing above or below the shadow. The motion of light was discovered, and its velocity determined, by means of the eclipses of Jupiter’s satellites. These phenomena are also useful in ascertaining longitudes. A spectator on Jupiter himself would see a vast number of solar and lunar eclipses—about 4500 of each—during the Jovian year of 4332·6 days, because the three inner satellites exhibit these phenomena at every revolution, their orbits being very slightly inclined to Jupiter’s equator, and the latter being but little inclined to the plane of the ecliptic.