VTSIOX. 321 



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 

 fronn the object to that focus, must be deflected, or bent from 

 their rectilineal course. This effect may be produced by 

 refraction, which takes place according to another optical 

 law; a law of which 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 recti- 

 lineal. 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 situ- 

 ated 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 

 obliquely 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 



perpendicularly to the surface of the medium, at the point s, 

 and within that medium. When the ray arrives at T, and 

 meets the posterior surface of the dense medium, passing 

 thence into one that is less dense, it is again refracted ac- 

 cording to the same law; that is, it inclines towards the per- 

 pendicular line T i, drawn from T, within the denser me- 

 dium, and describes the new course T u instead of T v. The 

 amount of the deflection corresponds to the degree of ob- 

 VOL. II. 41 



