OH. vii. ALHAZEN MAGNIFYING GLASSES. 



OP THE 



UNIYERS 



^C/ f /me.* 1 



49 



same position, 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 



FIG. 5. 



Arrow magnified by a convex lens. 

 n t, Real arrow ; c D, Magnifying glass ; A B, Enlarged image of the arrow. 



as if it were at a b, but, following out the dotted lines, 

 you will see a magnified arrow, A B, at the distance at which 

 you usually see small objects distinctly. This observation 

 of Alhazen's about the bending inwards or converging of 

 rays through rounded glasses was the first step towards 

 spectacles. 



Besides the Arabians whom I have mentioned here, there 

 were many who were very celebrated, but we know very 

 little of their works. Among them was Avicenna A.D. 980, 

 whom you will often hear mentioned as a writer on minerals. 

 But the chief thing to be remembered, besides the clis- 



