Refrangibillty of Light. 151 



rays proceeding out of the denser medium (the 

 water) into the rarer (the air), they are bent out 

 of their course more obliquely towards the eye 

 of the spectator. Have you ever seen a skilful 

 marksman shoot a fish in the water with a bullet? 

 If you have, the sportsman could tell you that 

 he took his aim considerably (perhaps a foot) 

 below the fish as it appeared, because it seemed 

 much nearer the top of the water than it really 

 was. The distortion of objects through a wrinkled 

 or crooked pane of glass, arises also from the 

 unequal refraction of the rays that pass through 

 it. When light passes out of pure space into 

 air, it is also refracted ; and therefore the sun is 

 visible, by means of the refraction of our atmo- 

 sphere, some minutes before he rises above the 

 horizon in the morning, and some minutes after 

 he sets below it in the evening. It has been cal- 

 culated that, in looking through the common 

 glass of a window, objects appear about one-thir- 

 tieth part of an inch out of their real place by 

 means of the refraction. 



But the most excellent use to which this prin- 

 ciple has been applied is the construction of op- 

 tical glasses ; for, by grinding the glass thinner at 

 the edges than in the middle, those rays of light, 

 which would strike upon it in a straight line, or 

 perpendicularly if it were plain, strike upon it ob- 

 liquely, and consequently suffer a refraction, and 

 are made to converge ; and ? on the contrary, by 

 making the glass thinner in the middle than at 



