Fig. 7. 



In the convex lenses, parallel or nearly parallel 

 rays are converged to one point as shown in Fig. 7. 

 This point c is the 

 principal focus, or 

 focal point, and the 

 distance from the 

 point b, called the 

 principal point, to 

 ris the focal length. 

 With the same lens, 

 if a flame be placed 

 at c, all the rays 

 which strike the lens will emerge in a parallel direction 

 on the opposite side. The straight line d b c which 

 passes through the middle of the lens is called the 

 principal axis and the distance a c, which for the sake 

 of simplicity has been taken from the focal point as 

 center, the radius. 



The radius of curvature in combination with the 

 refracting power of the glass determines the converg- 

 ing quality of a lens and consequently its focal length, 

 and as the radius is lengthened the focus becomes 

 longer. In the ordinary lenses the glass used is, with 

 very slight variation, of the same refracting power, so 

 that the difference in focus is dependent upon the curva- 

 ture of the surfaces. If in a double convex lens the 

 radius of each surface is one inch, the lens has a focus 

 of one inch, and if in a plane convex lens the convex 



