REFLECTION OF LIGHT. 



311 



from burning incense were caused to ascend at this point, an image would be 

 formed upon it, and appear suspended in the air in an apparently supernatural 

 manner. In this way, terrifying apparitions of skulls, daggers, etc., were 

 produced. 



669. A Convex Mirror may be considered 

 as any given portion of the exterior surface of 



Whit is a Con 

 vex JUrror ? 



a sphere. 



Where is the 

 principal focus 

 of a convex 

 mirror? 



The principal focus of a convex mirror lies 

 as far behind the reflecting surface as in con- 

 cave mirrors it lies before it. (See § 664.) 

 The focus in this case is called the virtual focus, because 

 it is only an imaginary point, toward which the rays of 

 reflection appear to be directed. 



Thus, let a b c d e, Fig. 2-48, be 

 parallel rays incident upon a convex 

 mirror, A B, whose center of curvature 

 is C. These rays are reflected diverg- 

 ent, in the directions a b' c d' e, as 

 though they proceeded from a point, 

 F, behind the mirror, corresponding 

 to the focus of a concave mirror. 



If the point C be the geometrical 

 center of the curve of the mirror, the 

 point F will be half way between C 

 and the surface of the mirror ; as this 

 focus is only apparent, it is called the virtual focus. 



Rays of light falling upon a convex mirror, 

 diverging, are rendered still more divergent by 

 reflection from its surface ; and convergent 

 rays are reflected, either parallel or less con- 



FiG. 249. 



670. The general efi'ect 



of convex mirrors is to 



produce an image smaller 



than the object itself. 



249, let D E be an object placed 

 before a convex mirror, A B ; the rays proceed- 

 ing from it will be reflected from the convex sur- 

 face to the eye at H K, as though they proceeded 

 from an object, d e, behind the mirror, thus pre- 

 senting an image smaller, erect, and much nearer 

 the mirror than the object. 



How are di- 

 Terging and 

 conver^ng 

 rays reflected 

 from a convex 

 mirror i 



vergent. 



What is the 

 n.iture of the 

 Images formed 

 by convex mir- 

 rors? 



Thus, in Fij 



