VISION. 453 



to collect into one point a great number of rays 

 proceeding from the corresponding point of the 

 object to be represented. Such a collection of 

 rays proceeding from any point, is termed, in 

 the language of optics, a pencil of rays ; and the 

 point into which they are collected is called a 

 focus. For the purpose of collecting a pencil of 

 rays into a focus, it is evident that all of them, 

 except the one which proceeds in a straight line 

 from the object to that focus, must be deflected, 

 or bent from their rectilineal course. This effect 

 may be produced by lefraction, which takes 

 place according to another optical law ; a law 

 of wl ich the following is the exposition. 



It is only when the medium which the rays 

 are traversing is of uniform density that their 

 course is constantly rectilineal. If the density 

 change, or if the rays pass obliquely from one 

 medium into another of a different density, they 

 are refracted ; each ray being deflected towards 

 a line situated in the medium of greatest density, 

 and drawn from the point where the ray meets 

 the new medium, perpendicular to the refracting 

 surface. Thus the ray r. Fig. 408, striking ob- 

 liquely on the surface of a denser medium, at the 

 point s, instead of pursuing its original course 

 along the line s o, is refracted, or turned in the 

 direction s t, which is a line situated between s o, 

 and s p ; this latter line being drawn perpen- 

 dicularly to the surface of the medium, at the 



