Alhazin, who wrote about A. D. 1080, made the great discovery of atmospheric refraction--that a ray of light when it touches the atmosphere is bent from a straight line; and {38} consequently we see the sun before it rises and after it sets, in the same manner that an object lying at the bottom of a bucket filled with water appears in quite a different position from that in which it really is. He was the first to give that beautiful and scientific explanation of twilight, viz., the refraction of light, which is still regarded by modern scientists as the true one. He even attempted to ascertain the height of the atmosphere, which he estimated to be about fifty-eight and a half miles. This philosopher also wrote a treatise on weights and measures, and introduced that excellent system of weighing by means of a small, movable weight attached to the longer arm of a lever, as in our modern scales or steelyards. The Arabian astronomer, Ebn-Junis, was the first who made use of the pendulum in the machinery of clocks for the accurate measurement of the hours. In the golden age of the Saracen empire, there were colleges in every part of its vast dominions. So numerous were these institutions, that more than six thousand students received instructions in them annually. In the far east were the college and astronomical observatory of Samercand; while in the western province of Andalusia were the famous school and observatory of Giralda. The first medical college established in Europe was that founded by the Saracens at Salerno, in Italy; the first famous school of mathematics and astronomy was that established by them at Seville, in Spain. Among them, learning was not confined to the rich, but every class received its benefits. The teachers of their colleges were paid liberal salaries for their services, and an allowance was made for indigent scholars, so that the son of the mechanic could graduate from the same class as the heirs of the Caliphs. At first glance it seems remarkable that the wild ferocity of the Arabs should so suddenly change into a passion for intellectual pursuits; yet it should not be forgotten that this ferocity was to a great extent caused by religious enthusiasm. Thus, when the General Akbah had conquered his way from Egypt to the Atlantic ocean, opposite the Canary Islands, he rode his horse into the sea and drew his sword, exclaiming, {39} "Great Allah! if my course were not stopped by this sea, I would still go on to the unknown regions of the West, preaching the unity of thy holy name, and putting to the sword the rebellious nations who worship any other gods than thee."