CH. vii. ALHAZEN MAGNIFYING GLASSES. 



49 



and any object, because the two surfaces of the glass, be- 

 coming more and more oblique to each other as they 

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



FIG. 5. 



A 



.-.>----'*/?'. 



I 



-. 



B 



Arrow magnified by a convex lens. 



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 be- 

 cause it is so near, your eye cannot focus or collect together 

 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 take a magnifying glass, 

 c D, fig. 5, and hold it between your eye and the arrow. If 

 you hold it at the right distance you will now see the arrow 

 distinctly, because the greater part of the rays have been 

 bent or refracted by the rounded glass so as to come into 

 focus on your retina. But now comes another curious fact. 

 It is a law of sight, that when rays of light enter our eye we 

 follow them out in straight lines, however much they may 

 have been bent in coming to the eye. So your arrow will 

 not appear to you as if it were at a b, but, following out the 

 dotted lines, you will see a magnified arrow, A B, at the 



