LIGHT. 27 



the earth, while those that fall obliquely undergo Refrac- 

 tion, or are bent as it were downwards out of their direct 

 course. It is this refractive power of the atmosphere 

 that causes twilight, and so gradually introduces in the 

 morning the light of the sun, which, were it not for this, 

 would blaze suddenly out from the thickest gloom; and 

 at night, instead of quitting us so gradually and gently, 

 would leave us all at once enveloped in darkness. 

 Through the same cause the heavenly bodies appear 

 higher in the heavens than they really are, and that in 

 proportion to their proximity to the horizon ; and when 

 near the edge of the horizon, they appear above it, when 

 they are in reality below it ; so that when the lower 

 limb of the sun or moon apparently just touches the 

 horizon, although the whole body appears above it, it is 

 in reality below it, as the upper limb is at that instant 

 just beginning to touch the horizon. Rays of light be- 

 come refracted in passing obliquely through water, glass, 

 or any other diaphanous or transparent body, as well as 

 through the atmosphere ; and in passing from a rare to a 

 denser medium, as from air into water, they are refracted 

 towards a perpendicular to the surface ; while on the con- 

 trary, in passing from a dense into a rarer medium, they 

 are refracted farther/ro/n a perpendicular. If a stone be 

 thrown obliquely into the water, when it strikes the water 

 it will begin to fall more perpendicularly towards the bot- 

 tom ; just so is the direction of light in passing into water. 

 The refractive nature of water may be seen by putting a 

 stick partly into it, and observing the apparent change of 

 the line of direction of the stick, both in and out of it : 

 at the surface of the water it will appear broken. It is 

 on the principle of the refractive power of light that the 

 science of Dioptrics depends ; which is one of the most 

 useful and pleasing sciences, for by its means the most 

 remote objects are brought fully into our view, and the 

 most minute are so magnified as to show every part of 

 them distinctly. 



When rays of light fall on an opaque body, through 

 which they cannot pass, they either become absorbed by 

 the body, or are reflected back again. It is by means of 

 Reflection that bodies are rendered visible. Every body 



c 2 



