232 



FERGUSON'S LECTURES. 



LECT. earth's surface, D E F the atmosphere that covers it, 

 ^ IT< and E B G H the sensible horizon of an observer at B. 

 As every point of the sun's surface sends out rays of 

 light in all manner of directions, some of his rays will 

 constantly fall upon, and enlighten, some half of the at- 

 mosphere; and therefore, when the sun is at /, below 

 the horizon /f, those rays which go on in the free space 

 I k K preserve a rectilineal course until they fall upon 

 the top of the atmosphere ; and those which fall so 

 about K, are refracted at their entrance into the atmo- 

 sphere and bent down in the line K m B, to the observer's 

 place at B : and therefore to him, the sun will appear 

 at L, in the direction of the ray B m K, above the ho- 

 rizon B G H, when he is really below it at 1. 



The angle contained between a ray of light, and a 



perpendicular to the refracting surface, is called the 



angle of incidence / and the angle contained between the 



incidence, same perpendicular, and the same ray after refraction, 



refraction. 



Thus, let L B M be 

 - 



is called the angle of refraction. 

 ^ ie refracting surface of a 

 medium (suppose water), and 

 A B C a perpendicular to 

 that surface ; let D B be a 

 ray of light, going out of air 

 into water at B, and therein 

 refracted in the line BH; 

 the angle A B D is the angle 

 of incidence, of which DF 

 is the sine ; and the angle 

 K B His the angle of refrac- 

 tion, whose sine is K L 



When (he refracting medium is water, the sine of the 

 angle of incidence is to the sine of the angle of refrac- 

 tion, as 4 to 3 : which is confirmed by the following 

 experiment, taken from Doctor SMITH'S Optics. 



Describe the circle DA E C on a plane square board, 

 and cross it at right angles with the straight lines ABC, 



