as to the instruments used or as to the observations made. We owe, however, to him the first systematic description of the constellations (see below, § 42), though it was probably based, to a large extent, on rough observations borrowed from his Greek predecessors or from the Egyptians. He was also an accomplished mathematician, and skilled in various other branches of learning. Shortly afterwards Callippus (§ 20) further developed Eudoxus’s scheme of revolving spheres by adding, for reasons not known to us, two spheres each for the sun and moon and one each for Venus, Mercury, and Mars, thus bringing the total number up to 34. 27. We have a tolerably full account of the astronomical views of _Aristotle_ (384-322 B.C.), both by means of incidental references, and by two treatises—the _Meteorologica_ and the _De Coelo_—though another book of his, dealing specially with the subject, has unfortunately been lost. He adopted the planetary scheme of Eudoxus and Callippus, but imagined on “metaphysical grounds” that the spheres would have certain disturbing effects on one another, and to counteract these found it necessary to add 22 fresh spheres, making 56 in all. At the same time he treated the spheres as material bodies, thus converting an ingenious and beautiful geometrical scheme into a confused mechanism.[14] Aristotle’s spheres were, however, not adopted by the leading Greek astronomers who succeeded him, the systems of Hipparchus and Ptolemy being geometrical schemes based on ideas more like those of Eudoxus. [Illustration: FIG. 8.—The phases of the moon.] [Illustration: FIG. 9.—The phases of the moon.]