Supplement to the Fourth, Fifth, and Sixth Editions of the Encyclopædia Britannica/Alhazen

ALHAZEN, an Arabian author of the eleventh century, who is better entitled to the appellation of philosopher, than most of those of his countrymen by whom it has been obtained. The place of his birth was Bassorah; the year, uncertain; but his death took place at Cairo in 1038. There was another author of the same name, who translated the Almagest of Ptolemy; but that writer lived during the reign of the Caliph Almamon. In some accounts of Alhazen, we find it said, that he lived in Spain; but it appears from Casiri, (Bibl. Arabico-Hispana Escurialensis), that, after he left his native city, Egypt was his place of residence. It also appears, that he was invited to that country by one of the Fatemite Caliphs; and that the reason was, some boasts which he had made of being able to obviate the evils attendant upon the overflowing, as well as the decrease of the waters of the Nile. He surveyed the country with a view to this project, to aid which, every thing that he asked was liberally furnished by the Caliph; but finding that his imagination had seduced him into a wild and impracticable scheme, he feigned madness, thereby to avoid the punishment which he dreaded; and he continued to play this humiliating part till the Caliph’s death relieved him from his apprehensions.

But whatever figure he may have made as a projector, there can be no doubt that he was a skilful geometrician; and that his name deserves a conspicuous place among the improvers of the science of optics. He was not a mere compiler from the ancients, or commentator upon their works; he followed the bent of his own genius; and, striking into the right path of experiment and observation, his inquiries were productive of a real accession to the stock of knowledge, in regard to some of the most interesting phenomena of nature. He refuted the error of the ancient philosophers, that vision was produced by rays emitted from the eye. He gave the first sensible explanation of the cause of the apparent increase of the sun and moon when seen near the horizon; showing that this is occasioned by their being then supposed, owing to the number of intermediate objects, to be at a greater distance from the spectator. He was the first who applied the laws of refraction, to show how the heavenly bodies are sometimes seen as if above the horizon when still below it; and who, in the same way, explained the cause of the morning and evening twilight;—of that beneficent provision of nature, by which the glories of day are made gradually to approach, and gradually to withdraw. These dioptrical discoveries of the Arabian philosopher have furnished M. Bailly with one of the many fine passages which embellish his celebrated work on the History of Astronomy. (Astron. Moderne, L. vi. sect. 20.)

As a writer, Alhazen is censurable for unmeaning prolixity, and scholastic subtlety. It appears, from Casiri, that his works were numerous; but only two of them have been published, namely, his Treatise on Optics, and that on the Twilight. They were both published in Latin, in 1572, by Frederic Risner, under the title of Opticæ Thesaurus.