21 



the image will be found to enlarge again. Return 

 until the most intense point is reached again. By 

 holding sufficiently long, especially if the paper be 

 dark, it will be found to burn, due to the concen- 

 tration of heat rays and hence is called the burn- 

 ing point or focal point. Or, in a room opposite a 

 window or lamp, hold a rule against a white wall 

 and placing the edge of a lens upon it, move it 

 slowly toward the wall until a greatly redticed, but 

 bright image of either object appears and read off 

 the distance between the wall and edge of lens, 

 which is its focus. 



\ 



A 



/ 'N 



A 

 / I \ 



/i 



Fig. S. 



Magnifying Power. Magnifying power of a 

 convex lens depends also upon its convergence. 

 In Fig. 8 a b represents the object, c d the 

 lens, e the pupil of the eye. B}' following the 



