46 SCIENCE OF THE MIDDLE AGES. rr. n. 



that a Greek writer, Diophantus, made use of this method 

 some time in the fifth century. This name ' Algebra ' is an 

 Arabian word, and the Arabs were very clever at this way 

 of making calculations. Ben Musa is the first writer we 

 know of who used the Indian numerals i, 2, 3, 4, 5, 6, 7, 

 8, 9, o, instead of the clumsy Roman numerals I. II. III. 

 IV. ; etc If you try to do a sum with the Roman numerals 

 you will see what a troublesome business it is, and what a 

 great gain the Indian numerals are. The Arabs learned these 

 figures from the Hindoos, and always used them after the 

 time of Ben Musa, so that they are now generally called 

 the Arabic numerals. About the year 1000, a Frenchman 

 named Gerbert, Archbishop of Rheims, and afterwards Pope 

 Sylvester the Second, who had been educated at the famous 

 Arabian University of Cordova in Spain, introduced them 

 into Europe. The word cipher, which we use for o, comes 

 from an Arabic word, ciphra, meaning empty or void. 



Alhazen's discoveries in Optics, 100O. Another 

 Arabian whom we must specially mention, was an astro- 

 nomer and mathematician named Alhazen, who was born at 

 Bassora, in Asiatic Turkey, about A.D. 1000, but who spent 

 most of his life in Spain. He made discoveries chiefly in 

 optics^ or the science of light and vision. He was the first 

 to teach that we see things because rays of light from the 

 objects around us strike upon the retina or thin membrane 

 of our eye, and t*he impression is carried to our brain by a 

 nerve. When the object is itself a light, like the flame of 

 a candle, it gives out the rays which reach our eye ; but 

 when, like a book or a chair, it is not luminous, then the 

 rays of the sun or any other light-giving body are reflected 

 from it to our eye and make a picture there. Alhazen also 

 explained why we do not see two pictures of one object, 

 although we look at it with two eyes ; he pointed out that, 



