OF THE SENSE OF SIGHT. 



121 



perpendicularly on the surface of a transparent body, they 

 traverse it without any change in their direction ; but falling 

 obliquely, they are always affected more or less in their direc- 

 tion. If they are passing from 



a rarer into a denser medium, a 



as from air into water, they are 

 refracted towards the perpen- 

 dicular ; the opposite happens 

 in passing from a denser into 

 a rarer medium ; they are then 

 refracted from the perpendi- 

 cular. A straight rod, for 

 example, plunged into water, 

 appears bent at the point of 

 immersion; and by placing a 

 coin in an empty basin (Fig. 



77 a), so that it shall be invisible to the eye of the observer, 

 it will become visible by merely filling the basin with water 

 (e), for then the rays of light from the ewer which formerly 

 took the direction of <?, and did not reach the eye of the 

 observer, will now pass in the direction of d b, and so the 

 coin will be seen. 



The form of the surface of the body on which the light 

 falls modifies greatly the further direction of the ray. Thus, 

 let us suppose that three rays start from the same point, a 

 (Pig. 78), traverse the air, and fall on the surface of a convex 

 lens. The ray a c striking the surface of the lens perpendi- 

 cularly will pass directly through it, experiencing no deviation ; 



Fig. 77. 



- 9 



Fig. 78. 



