136 EXPLAINING MIRRORS AND LENSES. 



EXPLAINING MIRRORS AND LENSES. 



The author has met with the best success in explaining mir- 

 rors 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 object 

 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 decreased, 

 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 ap- 

 parent size of the object correspondingly diminished. The rays 

 of light are not brought to a focus, hence the second effect of a 

 concave mirror can not be seen. 



The same illustration can be used in explaining lenses, re- 

 membering 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. 



ASTRONOMY WITH AN OPERA-GLASS. 



In schools where there is no telescope, teachers may find 

 valuable suggestions for class observations in the articles en- 

 titled "Astronomy with an Opera-Glass," by Garrett P. Serviss, 

 in the Popular Science Monthly, April, June, August, and No- 

 vember, 1887, and February, 1888. 



