REFRACTION OF LIGHT. 399 



change of front causes a change in the direction of the ray 

 which is thus refracted toward a perpendicular. The wave 

 now moves forward in a straight line until the top of the 

 wave front strikes A C, the surface of the prism, as shown 

 at m. The upper end of the wave front emerging first 

 into the air gains upon the other end of the front which is 

 still moving more slowly in the glass. When the lower 

 end emerges from the glass, the wave has the position 

 shown at n. This second change of front involves another 

 change in the direction of the ray which is now refracted 

 from the perpendicular. 



619. Three Kinds of Refractors. When a ray 



of light passes through a refracting medium, three cases 

 may arise : 



(I.) When the refractor is bounded by planes, the re- 

 fracting surfaces being parallel. 



(2.) When the refractor is bounded by planes, the re- 

 fracting surfaces being not parallel. The refractor is then 

 called a prism. 



(3.) When the refractor is bounded by two surfaces of 

 which at least one is 

 curved. The refractor 

 is then called a lens. 



62O. Parallel 

 Plates. When a ray 

 passes through a me- 



dium bounded bv paral- 



111 j.u iL L- 

 lei planes the refractions 



at the two surfaces are equal and contrary in direction. 

 The direction of the ray after passing through the plate is 



