ALHAZEN (full name, Abu Ali al-Hasan Ibn Al-hasan), a mathematician of the 11th century, was born at Bassora, and died at Cairo in 1038. He is to be distinguished from another Alhazen who translated Ptolemy’s Almagest in the 10th century. Alhazen having boasted that he could construct a machine for regulating the inundations of the Nile, was summoned to Egypt by the caliph Hakem; but, aware of the impracticability of his scheme, and fearing the caliph’s anger, he feigned madness until Hakem’s death in 1021. Alhazen was, nevertheless, a diligent and successful student, being the first great discoverer in optics after the time of Ptolemy. His researches were prosecuted under the greatest disadvantages, as he was compelled to eke out a livelihood by copying his own works and selling them. To him, and not to Ptolemy, is due the explanation of the apparent increase of heavenly bodies near the horizon. He correctly attributed the phenomenon to the fact that the eye compares these bodies with intervening terrestrial objects. He taught, previous to Vitello, 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. As a writer, Alhazen is remarkable for prolixity and scholastic subtilty. Only two of his works have been printed—his Treatise on Twilight, and his Thesaurus Opticoe. (See Casiri, Bibl. Arab. Hisp. Escur. )