ON THE RAYS OF LIGHT. 



295 



standing that it is the favourite theory of the day, 

 difficulties appear still to encumber it. The the- 

 ory of undulations implies the advance and recoil 

 of the elastic medium, and that gives the idea of 

 retardation. The supposition of light being the 

 effect of the motion of an ether, does not fall in 

 w^ith our conceptions of the manner in which it 

 enters into the composition of bodies, or influences 

 chemical combinations, or affects the living pow- 

 ers of animals and vegetables. The merits of the 

 two theories, however, need not be discussed here. 

 It will be sufficient for our purpose to represent 

 a beam of light by a line drawn with the pen, and 

 to enter on the explanation of a few of the laws 

 which influence it in passing through transparent 

 media. 



When the ray of light, as a, passes perpendicu- 

 larly from a rarer into a denser medium, as from 

 air into water, it suflfers no change in its direction ; 

 but when it passes obliquely, as b, it takes a new 

 direction towards the perpendicular, making a 



