EXPLAINING MIRRORS AND LENSES. 



The author has met with the best success in explaining 

 mirrors and lenses to his pupils, by using the following 

 method. 



A Concave Mirror. Holding up before his eye the fore- 

 finger of each hand, he represents to the pupil how the rays 

 of light enter his eye converging ; how he then sees the ob- 

 ject on diverging rays : thus the ^visual angle being increased, 

 the apparent size of the object is correspondingly increased. 

 By crossing his two forefingers before his eye he represents 

 the focus, and shows how diverging rays then enter the eye ; 

 the object is seen on converging rays, the visual angle is de- 

 creased, and the apparent size of the object correspondingly 

 decreased. 



A Convex Mirror. Using the fingers in the same way, he 

 illustrates how diverging rays enter the eye, the object is seen 

 on converging rays, the visual angle is diminished, and the 

 apparent size of the object correspondingly diminished. The 

 rays of light are not brought to a focus, hence the second ef- 

 fect of a concave mirror cannot be seen. 



The same illustration can be used in explaining lenses, 

 remembering that the effect of a convex lens is like that of a 

 concave mirror, and of a concave lens that of a convex mirror. 



At the close of the explanation and illustration with the 

 fingers, the following formula is put on the blackboard, and 

 the pupil applies it to each class of mirrors and lenses : 



CONVERGING (diverging) RAYS ENTER THE EYE, THE 



OBJECT IS SEEN ON DIVERGING (converging) RAYS ; HENCE 

 THE VISUAL ANGLE IS INCREASED (decreased), AND THE 

 IMAGE IS LARGER (smaller) THAN LIFE. 



