236 ON LIGHT. 



^^ scatteredy In the other mode the path of the ray, 

 subsequent to the point where it first encounters the 

 deflecting body, is wholly or partly witJi'ui it, and the 

 light is said to be " refracted," or "transmitted.'^ 



(19.) The first law observed in every case, whether of 

 direct or circuitous illumination, is gathered from ordi- 

 nary and universal experience. The illuminating and 

 illuminated points are mutually intercliangeable. By 

 whatever path, however circuitous, light is conveyed from 

 A to B, by tlie same it can be conveyed from b to a. 

 This condition alone suffices to determine the patli, and 

 to fix the situation of the point at which its flexure takes 

 place by reJiexio7i^ w^lien the light is " incident *' on any 

 polished surface, whether plane or curved. That point 

 (p) must be so situated on the surface, that the two lines 

 joining it and the illuminating and illuminated points (a, 

 b) shall there make equal angles with the surface, the 

 three points (a, b, p) all lying in one plane with a perpen- 

 dicular to the surface. For, ist, except the angles were 

 equal, the two directions (pb, pa) would not be similarly 

 related to the surface at the point of incidence ; so that 

 in reversing the path of the ray, the physical condition 

 which determined the obliquity of the incident ray to the 

 surface in proceeding from a to b, to be greater or less 

 than tliat of the reflected, would have to be reversed in 

 the passage of light from b to a. And similarly, if the 

 reflected ray lay in a plane to the right or left of that in 

 which the perpendicular and the incident one were con- 

 tained, the physical condition which determined it to 

 deviate to the one side or to the other of that plane, would 



