CH. vii. ALHAZEN ON REFRACTION. 47 



we see things because rays of light from the objects around 

 us strike upon the retina or thin membrane of our eye, and 

 the 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, as the reflection of any 

 given point of the object is formed on the same part of the 

 one eye as of the other, only one united picture reaches the 

 brain. This is the best explanation which has ever been 

 given of why we only see one image, but even to this day we 

 are not quite certain that it is satisfactory. 



Alhazen discovered another wonderful thing about light. 



If you take a straight stick and hold 



i . A- . u FlG ' 3 ' 



it in a slanting direction in a basin 



of water so that half of it is under 

 water, the stick will appear to bend 

 at the point A, where it touches 

 the surface of the water, and in- 

 stead of going along the dotted 

 line to B, will look as if it went 



to the point c. This is because rays of light are bent in a 

 slanting or oblique direction when they pass through sub- 

 stances of different density. Water is more dense than air, 

 and therefore the rays of light reflected from the stick are 

 bent as they pass out of the water into the air on their way 

 to your eye. This is called refraction, or the breaking-back 

 of a ray, and the discovery of it led Alhazen to find the 

 explanation of a very curious natural fact. 



