48 



SCIENCE OF THE MIDDLE AGES. 



PT. II. 



He knew that the air round our globe grows denser as it 

 gets nearer the earth, so he argued that the slanting rays 

 from the sun, moon, and stars must become bent as they 

 approach the earth and pass through the denser air. This, 

 he said, causes us to see the sun after it has really sunk 

 below our horizon at night, and before it rises in the 

 morning ; for the rays are gradually curved by passing 

 through the denser air round our earth. Fig. 4 explains this. 



Fig. 4. 



Earth 



Bending of the Sun's rays by the atmosphere. 



s. Sun. s c and s d, Rays as they would travel if there were no atmosphere, 

 s B A, Ray bent so that the sun becomes visible at A. 



Supposing the sun to be at s, and a person at a, it is clear 

 that any straight ray from the sun, such as s d, could not 

 reach a, because part of the earth is in the way; neither 

 could a ray, s c, reach the earth, because it would pass 

 above it. But when the rays from s to c strike the at- 

 mosphere at B, they are bent out of their course, and are 

 gradually curved more and more by the denser air till they 

 are brought down to the earth at a, and so the sun becomes 

 visible. 



Alhazen was also the first to remark that a convex lens,  

 that is, a glass with rounded surfaces, such as our common 

 magnifying glasses and burning glasses, will make things 

 appear larger if held at a proper distance between the eve 



I 



