240 ON LIGHT. 



example. And here we may pause for a moment to 

 observe that at this point we already find ourselves in- 

 troduced to an assemblage of relations between light and 

 material objects, which divide the whole universe of such 

 objects, infinite as they are in variety, into classes, char- 

 acterized by their habitudes with respect to light in its 

 reflexion from and passage through them. The import- 

 ance of this remark will grow upon us as we advance 

 further into the subject, and come to perceive that the 

 classification of bodies according to their " optical pro- 

 perties" stands in direct connexion with their most 

 intimate peculiarities of mechanical structure and chemi- 

 cal constitution ; and brings us, so to speak, into con- 

 tact with all those more recondite properties and reac- 

 tions of the ultimate particles of bodies which constitute 

 the domain of molecular physics. 



(24.) Confining ourselves now to the case where the 

 refraction is single, the rule which determines the course 

 of the refracted ray is as follows. Suppose at the " point 

 of incidence" {i.e., where the ray first enters the medium) 

 a line be draw^n perpendicular to the surface. Then, 

 first, the refracted ray will lie in the same plane which 

 contains both the incident ray and this perpendicular ; 

 and, secondly, the ray will be so bent at that point that 

 the exterior and interior portions shall make with the 

 perpendicular, not equal angles as would be the case 

 were there no flexure, but angles so related that their 

 sines (not the angles) shall bear to each other a certain 

 invariable proportion, whatever be the angles them- 

 selves, or whatever be the obliquity of the incident ray 



