20 



site; instead of converging the light toward the 

 axis, it diverges the light from the axis as shown 

 in Fig. 7. The imaginary extension of the 



diverging rays should meet at c and the distance 

 e d indicates the virtual focus ^ in contradistinction 

 to the real focus in the convex lens. 



To Determine the Focus of a Lens.- -The 



focus of a lens may be quite accurately determined 

 by the following methods: Taking the sun as an 

 object, hold the lens toward it and bringing a piece 

 of paper under it, move it 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 minute 

 and as the paper is brought still closer to the lens, 



