48 SCIENCE OF THE MIDDLE AGES. PT. IT. 



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 atmo- 

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

 gradually curved more and mo^e by the denser air till they 

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

 visible. 



FIG. 4. 



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 from A. 



Alhazen was also the first to explain why a convex lens, 

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

 magnifying glasses and burning glasses, will make things ap- 

 pear larger if held at a proper distance between the eye and 

 any object ; namely, because the two surfaces of the glass, 

 becoming more and more oblique to each other as they 

 approach the sides, bend the rays inwards, so that they 

 come to a focus in the eye. To understand this, draw a 

 line of any kind, say a little arrow, on a sheet of paper, and 

 bring your eye near to it. Your arrow being so close 

 would look very large if you could see it distinctly, but just 

 because it is so near, your eye cannot focus or collect to- 

 gether the rays coming from it so as to make a picture on 

 the retina at the back of the eye ; therefore you see nothing 

 but an indistinct blur. But now, keeping your eye in the 



