The actinic power of Jupiter’s light is very remarkable. It surpasses that of moonlight nine times, and that of Mars twenty-four times. Dr. Lohse further ascertained that the Southern Hemisphere is twice as chemically effective as the Northern. This superiority is doubtless connected with the greater physical agitation of the same region. A series of photographs of Jupiter, taken in 1891 with the great Lick refractor, were the first of any value for purposes of investigation. Jupiter’s satellites were the first trophies of telescopic observation. They are, indeed, bright enough for naked-eye perception, could they be removed from the disk which obscures them with its excessive splendor; and the first and third have actually been seen, in despite of the glare, by a few persons with phenomenally good eyesight. The mythological titles of the Galilean group—Io, Europa, Ganymede, and Calypso (proceeding from within outward)—have been superseded by prosaic numbers. The Jovian family presents an animated and attractive spectacle. The smallest of its original members (No. II) is almost exactly the size of our moon; the largest (No. III), with its diameter of 3,550 miles, considerably exceeds the modest proportions of Mercury. Satellite I revolves in 42½ hours at the same average distance from Jupiter’s surface that our moon does from that of the earth. No. II has a period of 3 days 13 hours, and its distance from Jupiter’s centre is 415,000 miles. Both these orbits are sensibly circular; and Nos. III and IV travel in ellipses of very small eccentricity, the one at a mean distance of 664,000, the other at 1,167,000 miles, in periods respectively of 7 days 4 hours, and 16 days 16½ hours. All four revolve strictly in the plane of Jupiter’s equator. They constitute a system bound together by peculiar dynamical relations, in consequence of which they can never be all either eclipsed or seen aligned at one side of their primary at the same time. They can all, however, be simultaneously hidden behind it, or in its shadow; although this moonless condition is looked out for as a telescopic rarity.