Ptolemy based his two great works partly upon the results already attained by earlier scientists, following Hipparchus especially in astronomy and Marinus in geography. He duly acknowledged his debts to these and other writers; praised Hipparchus and recounted his discoveries; and where he corrected Marinus, did so with reason. But while Ptolemy used previous authorities, he was far from relying upon them solely. In the _Geography_ he adds a good deal concerning the orient and northern lands from the reports of Roman merchants and soldiers. His intention was to repeat briefly what the ancients had already made clear, and to devote his works chiefly to points which had remained obscure. His ideal was to rest his conclusions upon the surest possible observation; and where such materials were meager, as in the case of the _Geography_, he says so at the start. He also recognized that delicate observations should be repeated at long intervals in order to minimize the possibility of error. He devised and described some scientific instruments and conducted a long series of astronomical observations. He anteceded Comte in holding that one should adopt the simplest possible hypothesis consistent with the facts to be explained. [Sidenote: The _Optics_.] Besides some minor astronomical works and a treatise on music which seems to be largely a compilation an important work on optics is ascribed to Ptolemy.[498] It is the most experimental in method of his writings, although Alexander von Humboldt’s characterization of it as the only work in ancient literature which reveals an investigator of nature in the act of physical experimentation[499] must be regarded as an exaggeration in view of our knowledge of the writings of other Alexandrines such as Hero and Ctesibius. As in the case of some of Ptolemy’s other minor works, the Greek original is lost and also the Arabic text from which was presumably made the medieval Latin version which alone has come down to us. Yet there are at least sixteen manuscripts of this Latin version still in existence.[500] The translation was made in the twelfth century by Eugene of Palermo, admiral of Sicily, whose name is attached to other translations and who was also the author of a number of Greek poems.[501] Heller states that the _Optics_ was lost at the beginning of the seventeenth century but that manuscripts of it were rediscovered by Laplace and Delambre.[502] At any rate the first of the five books is no longer extant, although Bridges thinks that Roger Bacon was acquainted with it in the thirteenth century.[503] It dealt with the relations between the eye and light. In the second book conditions of visibility are discussed and the dependence of the apparent size of bodies upon the angle of vision. The third and fourth books deal with different kinds of mirrors, plane, convex, concave, conical, and pyramidical. Most important of all is the fifth and last book, in which dioptrics and