Observatory'; but should the student not be possessed of either of these most useful publications, he may notice that when a satellite is seen steadily approaching Jupiter on the following side, a transit is impending. The satellite will come up to the margin of the planet, looking like a brilliant little bead of light as it joins itself to it (a particularly exquisite sight), will glide across the margin, and after a longer or shorter period will become invisible, being merged in the greater brightness of the central portions of Jupiter's disc, unless it should happen to traverse one of the dark belts, in which case it may be visible throughout its entire journey. It will be followed or preceded, according to the season, by its shadow, which will generally appear as a dark circular dot. In transits which occur before opposition the shadows precede the satellites; after opposition they travel behind them. The transit of the satellite itself will in most cases be a pretty sharp test of the performance of a 3-inch telescope, or anything below that aperture; but the transit of the shadow may be readily seen with a 2-1/2-inch, probably even with a 2-inch. There are certain anomalies in the behaviour of the shadows which have never been satisfactorily explained. They have not always been seen of a truly circular form, nor always of the same degree of darkness, that of the second satellite being notably lighter in most instances than those of the others. There are few more beautiful celestial pictures than that presented by Jupiter with a satellite and its shadow in transit. The swift rotation of the great planet and the rapid motion of the shadow can be very readily observed, and the whole affords a most picturesque illustration of celestial mechanics.