424 - SPHERICAL ABERRATION. 
the central portion being rather more distant than that of the - 
outer part, This is shown in fig. 211, where 1 x is the lens, 
Fig. 211. 
RL, RL, are rays falling upon its circumference, and R’ L/, © 
RL’, are rays falling near its centre. The former set of rays 
meet in f; whilst the latter pass-on to F, before they meet in 
a focus. This may be shown by covering the central and 
the outer portions of the lens, alternately, with some opaque 
substance, which shall stop all the rays of light proceeding 
through either. When the central portion is covered, a distinct 
image will be formed at f by the rays falling upon the outer 
portion ; and when the outer portion is covered, a distinct 
image will be formed at Fr by the rays that have passed 
through the central portion. But when the whole lens is 
employed, no distinct image is formed anywhere ; for if a 
screen be held at f, it will receive, not only the rays which 
are brought to a focus at that point, but also the rays which — 
are going-on to meet at F; whilst, on the other hand, if the 
