Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V01, Page 658 of the 1911 Encyclopedia Britannica.
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ALHAZEN (ABU ALI AL-HASAN IBN ALHASAN), Arabian mathematician of the 11th century, was born at Basra and died at Cairo in 1038. He is to be distinguished from another Alhazen who translated Ptolemy's Almagest in the loth century. Having boasted that he could construct a machine for regulating the inundations of the Nile, he was summoned to Egypt by the caliph Hakim; but, aware of the impracticability of his scheme, and fearing the caliph's anger, he feigned madness until Hakim's death in 1021. Alhazen was, nevertheless, a diligent and successful student, being the first great discoverer in optics after the time of Ptolemy. According to Giovanni Battista della Porta, he first explained the apparent increase of heavenly bodies near the horizon, although Bacon gives the credit of this discovery to Ptolemy. He taught, previous to the Polish physicist Witelo, that vision does not result from the emission of rays from the eye, and wrote also on the refraction of light, especially on atmospheric refraction, showing, e.g. the cause of morning and evening twilight. He` solved the problem of finding the point in a convex mirror at which a ray coming from one given point shall be reflected to another given point. His treatise on optics was translated into Latin by Witelo (1270), and afterwards published by F. Risner in 1572, with the title Opticae thesaurus Alhazeni libri VII., cum ejusdem libro de crepusculis et nubium ascensionibus. This work enjoyed a great reputation during the middle ages. Works on geometrical subjects were found in the Bibliotheque nationale de Paris in 1834 by E. A. Sedillot; other manuscripts are pre-served in the Bodleian library at Oxford and in the library of Leiden. See Casiri, Bibl. Arab. Hisp. Escur. ; J. E. Montucla, Histoire des mathematiques (1758); and E. A. Sedillot, Materiaux pour l'histoire des sciences mathematiques.

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