OP OPTICS. 



235 



and, in leaving the glass, it will be refracted in the line LECT. 

 CD. J^ 



A ray of light 

 C D, falling oblique- 

 ly on the middle of a 

 convex glass, will go 

 forward in the same 

 direction D E, as if 

 it had fallen with the 

 same degree of ob- 



iqnity on a plane 

 glass ; and will go out of the glass in the same direction 

 with which it entered : for it will be equally refracted at 



he points D and JE, as if it had passed through a plane 



urface. But the rays C G and C I will be so refracted, 

 as to meet again at the point F. Therefore, all the rays 

 which flow from the point C, so as to go through the 



;lass, will meet again at F; and if they go farther on- 

 ward, as to L, they cross at F, and go forward on the 

 opposite sides of the middle ray C D E F, to what they 

 were in approaching it in the directions H F and K F. 



When the parallel rays, as The ^ 



A B C, fall directly upon a piano- /^ ^Xo A perties of 

 convex glass D E, and pass 



hrough it, they will be so re- 



racted, as to unite in a point f 



>ehind it ; and this point is called 



he principal focus ; the distance 



of which, from the middle of the glass, is called the 



focal distance ; which is equal to twice the radius of the 



sphere of the glass's convexity. And, 

 When parallel rays, as 



ABC, fall directly upon a 



glass D E, which is equally 



convex on both sides, and 



pass through it, they will be 



so refracted, as to meet in a 



different 

 lenses. 



