ItEFLECTION OF LIGHT. 67 



as an elastic ball which is struck against a wall. A ray 

 of light striking perpendicularly upon a plane mirror, is re- 

 flected back in the same direction ; but those rays which 

 strike it obliquely, are reflected back in an opposite direction, 

 but with the same obliquity ; the angle of reflection, there- 

 fore, is exactly equal to the angle of incidence. If you stand 

 directly before a looking-glass, you see your image reflected 

 back to you. If you stand a little to the side, you cannot 

 see yourself; but a person who stands just as far on the other 

 side of it, can see your image in the glass, and you can see 

 his. If you place a candle a little to one side, you must go 

 as far on the other to see its image in the glass. This is the 

 same rule which takes place in the collision of elastic bodies 

 against any surface. If you strike an ivory ball or common 

 marble perpendicularly against the wainscot, it returns to 

 you ; but if you make it strike sideways, it goes off" at the 

 same angle with which it came to the wainscot. So it is 

 with rays of light ; the incident ray, or the ray which falls 

 upon a surface, makes an angle with a perpendicular line, 

 drawn from the point where it strikes, equal to that which 

 the reflected ray makes with it. 



With respect to a looking-glass, it is the silvering on the 

 glass which causes the reflection, otherwise the rays would 

 pass through it without being stopped, and if they were not 

 stopped they could not be reflected. No glass, however, is 

 so transparent but it reflects some rays : if you put your 

 hand near a window, you clearly see its image on the other 

 side, and the nearer the hand is to the glass, the more evi- 

 dent is the image. Whatever suffers the rays of light to 

 pass through it is called a medium, and the more transpa- 

 rent the body, the more perfect is the medium. But rays 

 of light do not pass through a transparent medium, (unless 

 they fall perpendicularly upon it) in precisely the same di- 

 rection in which they were moving before they entered it. 

 They are bent out of their former course, and this is called 

 refraction. When they pass out of a rarer into a denser 

 medium, as from air into water or glass, they are always re*- 

 fracted toward& a perpendicular to the surface, and the re* 

 fraction is, more or less, in proportion as the rays fall, more 

 or less, obliquely on the refracting surface. But when they 

 pass from a denser into' a rarer medium, as from glass or 

 water into air, they move in a direction farther from the 



