THE FIXED IDEA . OF ASTRONOMICAL THEORY, We may in thought pursue a train of hypotheses and suppositions, but they do not thereby acquire reality still, in a normal condition of the human intellect, it is impossible to conceive that any thing can exist and not exist at the same time. BY AUGUST TISCHNER, LEIPZIG GUSTAV FOCK 1885. v Claudius Ptolemaeus. Terrae stator, soils motor. Nicolaus Copernicus. Terrae motor, soils stator. Wiliam Herschel, Terrae, solisque motor. DEDICATED TO ALL CANDID FRIENDS OF RATIONAL ASTRONOMY. 736353 Sir LS dorit doubt that you look in science for nothing but truth, and admit, that theory is but an attempt to explain inexplorable , great Nature, and that theory can serve but for a resting-point , a point of departiire for further explorations, and that science, if raised to dogma, is brought to a standstill, and any further success is rendered impossible. The history of science proves, too, that at every time there have been stated theories which in the course of time must be variously altered and even replaced by others quite opposed to them. It was as an explanation of thephenomena of heaven, that the theory of a fixed earth in the centre of the universe was stated-, this theory was found to be un- tenable, when the movement of the earth was recognized, and now theory based itself on a fixed sun. But after- wards it was discovered, that even the sun is not fixed, but transposes himself in space. The conviction, too, was arrived at, that not only the sun moves in mundane space, but also, that the stars, too, are not fixed, but in motion. In short, in the universe there is no immovable