Spherical Aberration. In considering the refrac- 

 tion of light by a lens, we have up to this point 

 purposely avoided mentioning another quality which is 

 incident to it. In magnifying an object with a single 

 lens it will be noticed that the virtual image as seen 

 through its central portion is quite clear, while that 

 near the margin or edge is quite indistinct. This is 

 due to spherical aberration and the extent of this 

 aberration increases with the power of the lens. 



It is due to the difference in refraction of those 

 rays passing near the margin and those passing through 

 the central portion, so that the rays, instead of combin- 

 ing at the focal point, come together at different inter- 

 vals along the central line or principal axis. 



By reference to Fig. 10 it will be seen that the 

 outer or marginal rays are refracted at e and f so 



that they will combine at o, and the inner or central 

 rays are refracted at g and k so that they will meet 



