they converge to a point called the focus of the lens, and 

 there form a distinct image. The distance of the lens from 

 the focus or the point where the most distinct image is 

 formed is called the focal length. 



II. Place a candle on one side of a convex lens and 

 a sheet of paper on the 



other. Adjust the candle 

 and the paper until a distinct 

 image larger than the object 

 appears (Fig. 44). Meas- 

 ure the distance between 

 the candle and the center 

 of the lens and call the 

 distance n. Measure the distance between the image and 

 the center of the lens and call this distance v. 



III. Change the position of the candle and the paper 

 and adjust them until a distinct image smaller than the 



object appears on the paper. 

 Again determine the dis- 

 tances n and v. 



IV. Change the positions 

 of the candle and paper 

 again and adjust both until a 



FIG. 45. The lens is placed in such a ... 



position that the image is about the distinct image exactly the 



FlG. 44. By means of a lens an en- 

 larged image is obtained. 



same size as the object. 



size 



(See Fig. 45.) 



of the object appears 

 Determine u and v as 



upon the paper, 

 before. 



In I, II, III, and IV different values were determined 

 of u and v, and it is found that as u changes, v also 

 changes, although the focal length of the lens remains 

 the same. There must be some definite relation existing 

 between u, v, and /. Experiment shows that there is 



