The mediæval scientific opinion was based on the theory of Ptolemy and his school of Alexandrian astronomers, who lived in the second century A. D., some details to the theory having been added by the Arabians. Ptolemy says, “The world is divided into two vast regions; the one ethereal, the other elementary. The ethereal region begins with the first mover, which accomplishes its journey from east to west in twenty-four hours; ten skies participate in this motion, and their totality comprises the double crystalline heaven, the firmament and the seven planets.” (See diagram.) The mediæval man of science thought that, inasmuch as he was upon the earth, he was therefore standing at the centre of things. Directly above him was the cavity of the sky, ruled by the moon; and below the moon were the four elements,――fire, air, water, and earth. This region was the realm of imperfection. But above the moon the scientist saw a series of nine other heavens, each with an orderly revolution of its own; and beyond all is God. The universe was therefore to Ptolemy a great but a limited sphere, consisting of ten spheres one inside another (like the rings of an onion). Each planet moved with the motion of its own heaven (or sphere), which was sometimes called “crystalline” because it was transparent. The movements of the heavenly bodies, each in its own revolving heaven, were contained in the whole sphere, which revolved with a motion of its own. By ascribing other movements to the planets within their respective heavens, the mediæval astronomers were able to predict every conjunction and eclipse to the minute. These separate movements of the planets were called epicycles, the form of which is shown in the diagram on the opposite page. Illustration: PTOLEMAIC COSMOGRAPHY A diagram showing the division of the universe into the ten spheres or heavens (From the private library of Professor R. W. Willson of Harvard University)