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.
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 refraction are discussed for the first and only time in any extant work of antiquity,[504] provided the Optics has really come down in its present form from the time of Ptolemy. His authorship has been questioned because the subject of refraction is not mentioned in the Almagest, although even astronomical refraction is discussed in the Optics.[505] De Morgan also objects that the author of the Optics is inferior to Ptolemy in knowledge of geometry.[506] Possibly a work by Ptolemy has received medieval additions, either Arabic or Latin, in the version now extant; maybe the entire fifth book is such a supplement. That works which were not Ptolemy’s might be attributed to him in the middle ages is seen from the case of Hero’s Catoptrica, the Latin translation of which from the Greek is entitled in the manuscripts Ptolemaei de speculis.[507]
Medieval translations of Almagest.
If there is, as in other parallel cases, the possibility that the medieval period passed off recent discoveries of its own under the authoritative name of Ptolemy, there also is the certainty that it made Ptolemy’s genuine works very much its own. This may be illustrated by the case of the Almagest. On the verge of the medieval period the work was commented upon by Pappus and Theon at Alexandria in the fourth, and by Proclus in the fifth century. The Latin translation by Boethius is not extant, but the book was in great repute among the Arabs, was translated at Bagdad early in the ninth century and revised later in the same century by Tabit ben Corra. During the twelfth century it was translated into Latin both from the Greek and the Arabic. The translation most familiar in the middle ages was that completed at Toledo in 1175 by the famous translator, Gerard of Cremona. There has recently been discovered, however, by Professors Haskins and Lockwood[508] a Sicilian translation made direct from the Greek text some ten or twelve years before Gerard’s translation. There are two manuscripts of this Sicilian translation in the Vatican and one at Florence, showing that it had at least some Italian currency. Gerard’s reputation and his many other astronomical and astrological translations probably account for the greater prevalence of his version, or possibly the theological opposition to natural science of which the anonymous Sicilian translator speaks in his preface had some effect in preventing the spread of his version.
The Tetrabiblos or Quadripartitum.
Of Ptolemy’s genuine works the most germane to and significant for our investigation is his Tetrabiblos, Quadripartitum, or four books on the control of human life by the stars. It seems to have been translated into Latin by Plato of Tivoli in the first half of the twelfth century[509] before Almagest or Geography appeared in Latin. In the middle of the thirteenth century Egidius de Tebaldis, a Lombard of the city of Parma, further translated the commentary of Haly Heben Rodan upon the Quadripartitum.[510] In the early Latin editions[511] the text is that of the medieval translation; in the few editions giving a Greek text there is a different Latin version translated directly from this Greek text.[512]
A genuine reflection of Ptolemy’s approval of astrology.
In the Tetrabiblos the art of astrology receives sanction and exposition from perhaps the ablest mathematician and closest scientific observer of the day or at least from one who seemed so to succeeding generations. Hence from that time on astrology was able to take shelter from any criticism under the aegis of his authority. Not that it lacked other exponents and defenders of great name and ability. Naturally the authenticity of the Tetrabiblos has been questioned by modern admirers of Hellenic philosophy and science who would keep the reputations of the great men of the past free from all smudge of superstition. But Franz Boll has shown that it is by Ptolemy by a close comparison of it with his other works.[513] The astrological Centiloquium or Karpos, and other treatises on divination and astrological images ascribed to Ptolemy in medieval Latin manuscripts are probably spurious, but there is no doubt of his belief in astrology. German research as usual regards its favorite Posidonius as the ultimate source of much of the Tetrabiblos, but this is not a matter of much consequence for our present investigation.
Validity of astrology.