/ Hold the lens toward the sun with one hand. 

 With the other hold a piece of paper under it. 

 Move the paper slowly toward the lens. A large bright 

 spot will appear, which, as the paper is brought nearer, 

 will decrease in size but increase in intensity until 

 it becomes quite small, and as the paper is brought 

 still closer to the lens the image will be found to 

 enlarge again. Return again until the spot decreases 

 to smallest she. The distance between lens and paper 

 is the focal length. By holding sufficiently long, 

 the paper, especially if it be dark, will be found to 

 burn, due to the concentration of heat rays and hence 

 this point is called the burning point or focal point. 



II In a room opposite a window or at a con- 

 siderable distance from a lamp, hold the end of a 

 ruler against a white wall and place the edge of 

 the lens against it, so that the axis of the lens will 

 be parallel with the ruler. Move the lens sfowly 

 toward or away from the wall until a greatly 

 reduced but bright image of either object appears 

 sharply defined. Read off the distance between the 

 wall and edge of lens. This is the focal length. 



In a lens of considerable thickness measure from 

 the centre of the lens. 



Magnifying Power. Magnifying power of a 

 convex lens depends upon its converging power. 



In Fig. 9, a b represents the object, c d the lens, 

 <? the pupil of the eye. iy following the coirse of 



