The idea of the author is to tell about these things in plain language, but with as much scientific accuracy as plain language will permit, showing the wonder that is in them without getting away from the facts. Most of them have hitherto been discussed only in technical form, and in treatises that the general public seldom sees and never reads. Among the topics touched upon are: The strange unfixedness of the “fixed stars,” the vast migrations of the suns and worlds constituting the universe. The slow passing out of existence of those collocations of stars which for thousands of years have formed famous “constellations,” preserving the memory of mythological heroes and heroines, and perhaps of otherwise unrecorded history. The tendency of stars to assemble in immense clouds, swarms, and clusters. The existence in some of the richest regions of the universe of absolutely black, starless gaps, deeps, or holes, as if one were looking out of a window into the murkiest night. The marvelous phenomena of new, or temporary, stars, which appear as suddenly as conflagrations, and often turn into something else as eccentric as themselves. The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,” or “tress,” nebulæ. The strange surroundings of the sun, only seen in particular circumstances, but evidently playing a constant part in the daily phenomena of the solar system. The mystery of the Zodiacal Light and the Gegenschein. The extraordinary transformations undergone by comets and their tails. The prodigies of meteorites and masses of stone and metal fallen from the sky. The cataclysms that have wrecked the moon. The problem of life and intelligence on the planet Mars. The problematical origin and fate of the asteroids. The strange phenomena of the auroral lights. An attempt has been made to develop these topics in an orderly way, showing their connection, so that the reader may obtain a broad general view of the chief mysteries and problems of astronomy, and an idea of the immense field of discovery which still lies, almost unexplored, before it. CURIOSITIES OF THE SKY I The Windows of Absolute Night To most minds mystery is more fascinating than science. But when science itself leads straight up to the borders of mystery and there comes to a dead stop, saying, “At present I can no longer see my way,” the force of the charm is redoubled. On the other hand, the illimitable is no less potent in mystery than the invisible, whence the dramatic effect of Keats’ “stout Cortez” staring at the boundless Pacific while all his men look at each other with a wild surmise, “silent upon a peak in Darien.” It is with similar feelings that the astronomer regards certain places where from the peaks of the universe his vision seems to range out into endless empty space. He sees there the shore of his little isthmus, and, beyond, unexplored immensity.