When the Roman Empire was destroyed by the barbarians from the north, it was the Arabs, strange to say, who took up the cultivation of science where it had been left by the Greeks and Romans. The Khalifs who built Bagdad, Cairo, and Cordova in Spain, loved to surround themselves with learned men from all parts of the world. An Arabian writer says, “To Cordova came from all parts of the world students eager to cultivate poetry, to study the sciences, or to be instructed in divinity or law, so that it became the meeting-place of the eminent in all matters, the abode of the learned, and the place of resort for the studious. Its interior was always filled with the eminent and the noble of all countries, its literary men and scholars were continually vying with each other to gain renown, and its precincts never ceased to be the arena of the distinguished, the recourse of readers, the halting place of the noble, and the repository of the true and virtuous. Cordova was to Spain what the head is to the body, or what the breast is to the loin.” The sciences generally cultivated among the Arabs were Chemistry and Optics. In chemistry they discovered a number of new metals and acids. They knew how to distil a liquid and crystallise a solid, and how to mingle various metals to form new alloys with variable properties. They were excellent makers of sword-blades, and their scimitars or curved swords were dreaded by all the soldiers of Christendom. [Illustration: Fig. 14.--Cordova after the Expulsion of the Moors.] The science of Optics was originally cultivated by a native of Mesopotamia named Alhazen, who died in the year 1038 A.D. Alhazen was called to Egypt by one of the Khalifs who had heard that he had thought out plans for regulating the flow of the Nile in such a manner that each year there should be plenty of water for inundations. Alhazen went to Egypt and inspected the land, but found that his plan was not suitable. The Khalif was angry, and would have punished Alhazen, but the latter pretended to be mad, and managed to hide himself until the Khalif died. He then reappeared and became famous as a scholar and instructor of youth, copying old manuscripts and writing books on Astronomy, Mathematics, and Optics. Alhazen’s greatest work was done on the subject of the laws which govern the production of light. He proved, first of all, that light travels along straight lines. “If the light of the sun,” he says, “or the light of the moon, or the light of fire, enters a dark room through a narrow slit, and dust is in the room, or dust is made to fly in it, the light entering through the slit is made clearly visible in the air mixed with dust; it is also visible on the floor or on the opposite wall of the room. And it is found that the light travels through the slit to the floor or to the opposite wall along straight lines. And when a straight rod is held along that visible light, it is found that the light travels along the straight rod.