The satellites in transit present some curious phenomena. When they enter the disk, they appear intensely luminous upon its grayish border; but as they advance, they seem by degrees to lose their brightness, until they finally become undistinguishable from the luminous surface of Jupiter. It sometimes happens, however, that the first, the third and the fourth satellites, after ceasing to appear as bright spots, continue to be visible as dark spots upon the bright central portions of the planet's disk; but in these cases their disks appear smaller than the shadows they cast. Undoubtedly these satellites have extensive atmospheres, since they sometimes pass unperceived across the central parts of Jupiter, this being probably when their atmospheres are condensed into clouds, strongly reflecting light; while when these clouds are absent, we can see their actual surface, with traces of the dark spots upon them similar to those on Mars. From the variation in the brightness of these satellites, which is said to be always observed in the same part of their orbit, William Herschel was led to suppose that these bodies, like our Moon, rotate upon their axes in the same period in which they move round the planet, so that they always present the same face to Jupiter; but these conclusions have been denied. From my observations it is apparent, however, that the light reflected by them varies in intensity as well as in color. But this is rather to be attributed to the presence of an atmosphere surrounding these bodies, which when cloudy reflects more light than when clear, with corresponding changes in the color of the light. The satellites in transit are sometimes preceded or followed, according to the position of the Sun, by a round black spot having about the same size as the satellite itself. This black spot is the shadow of the satellite cast on the vapory envelope of Jupiter, similar to the shadow cast by the Moon on the Earth, during eclipses of the Sun; in fact, all the Jovian regions traversed by these shadows have the Sun totally eclipsed. Sometimes it happens that the shadow appears elliptical. This occurs either when it is observed very near the limb, or when entering upon a round, cloud-like spot. This effect is attributable to the perspective under which the shadow is seen on the spherical globe or spot. The proper motion of the satellites in the Jovian sky is much more rapid than that of the Moon in our sky. During one Jovian day of ten hours, the first satellite advances 84°; the second, 42°; the third, 20° and the fourth, 9°. The first satellite passes from New Moon to its first quarter in a little more than a Jovian day, while the fourth occupies ten such days in attaining the same phase.