CM. vii. ALHAZEN ON REFRACTION. 47 



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 explana- 

 tion 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 

 it in a slanting direction in a basin 

 of water so that half of it is under 

 water and look at it from above, the 

 stick will appear to bend at the 

 point A, where it touches the surface 

 of the water, and instead 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 

 substances 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. 



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 morn- 

 ing ; for the rays are gradually curved by passing through 

 the denser air round our earth. Fig. 4 explains this. Sup- 

 posing 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 



