REFRACTION OF LIGHT. 



315 



^ , . ,^ 674. The application of the laws of refraction of light ac- 



Explain the .... . , 



phenomena of count for many curious deceptive appearances in the at- 



Mir;ige. mosphere, which are included under the general name of 



Mirage. In these phenomena, the images of objects far remote are seen at an 

 elevation in the atmosphere, either erect or inverted. Thus travelers upon a 

 desert, where the surface of the earth is highly heated by the sun, are often 

 deceived by the appearance of water in the distance, surrounded by trees and. 

 villages. In the same manner at sea, the images of vessels at a great distance 

 and below the horizon, will at times appear floating in the atmosphere. Such 

 appearances are frequently seen with great distinctness upon the great Amer- 

 ican lakes. These phenomena appear to be due to a change in the density of 

 the strata of air which are immediately in contact with the surface of the earth. 

 Thus it often happens that strata resting upon the land may be rendered much 

 hotter, and those resting upon the water much cooler, by contact with the 

 surface, than other strata occupying more elevated positions. Rays, there- 

 fore, on proceeding from a distant object and traversing these strata, will be 

 unequally reflected, and caused to proceed in a cun-ilinear direction ; and in 

 tiiis way an object situated behind a hill, or below the horizon, may be 



brought into view and appear suspend- 

 ed in the air. This may be readily 

 understood by reference to Fig. 253. 

 Suppose the rays of light from tlis 

 ship, S, below the horizon to reach 

 the eye, after assuming a curvilinear 

 direction by passing through strata of 

 air of varj-ing density; then, as an 

 .<! object always appears in the direction 

 in which the last rays proceeding from 

 it enter the eye, two images will be seen in the direction of the dotted 

 lines, one of them being inverted. 



These phenomena may be sometimes imitated. Thus, if we look along a 

 red hot bar of iron, or a mass of heated charcoal at some image, a short dis- 

 tance from it, an inverted reflection of it will be seen. In the same manner, 

 if we place in a glass vessel liquids of different densities, so that they float 

 one above another, and look through them at some object, it will be seen 

 distorted and removed from its true place, by reason of the unequal refractive 

 and reflective powers of the liquids employed. 



675. The angle of refraction of light is not, 

 like the angle of reflection, equal to the angio 

 of incidence ; but it is nevertheless subject 

 to a definite law, which is called the law of 



Is the an^le 



o!' rofraction 

 equal to the 

 an^Ie of inci- 

 dence? 



Sines. 



What is a sine f 



A sine is a right line drawn from any point in one of th« 

 lines inclosing an angle, perpendicular to the other line. 



