ON LIGHT. 



241 



to the surface. As it is quite essential to the under- 

 standing of what follows that this, " the law of ordinary 

 refraction," should be clearly apprehended, we will illus- 

 trate it by a figure. Let A c B be a section of the surface 



by the plane in which the ray D c, incident at c, and 

 p c Q the line perpendicular to the surface at c, both lie, 

 and c E the refracted ray. Taking c for a centre, with 

 any radius, c M, describe a circle cutting the incident 

 and refracted rays in M and N, from which points draw 

 M R, N s perpendicular to P c Q. Then will these two 

 lines be to each other, in one and the same invariable 

 proportion, whatever be the inclination of the original 

 ray D c to the surface, or to the perpendicular c P. 

 This latter inclination is what is understood by the 

 "angle of incidence" and the corresponding inclination 

 (to the perpendicular Q c P) of the refracted ray, by " the 

 angle of refraction' 1 



Q 



