424 



SPHERICAL ABERRATION. 



owing to the large quantity of minute blood-vessels which it 

 contains, it presents a bright red hue. The choroid coat, seen 

 through the pupil, has exactly the same aspect ; so that the 

 pupil is not readily distinguished. During the day the vision 

 of these Albinos is very indistinct, and the glare of light is 

 painful to them ; and it is only when twilight comes-on, that 

 they can see clearly and without discomfort. 



546. The eye is much more remarkable for its perfection 

 as an optical instrument, than we might be led to suppose 

 from the cursory view we have hitherto taken of its functions ; 

 for by the peculiarities of its construction certain faults and 

 defects are avoided, to which all ordinary optical instruments 

 are liable. One of these, termed spherical aberration, results 

 from the fact, that rays falling upon the central and the outer 

 portions of an ordinary convex lens, whose surfaces form part of 

 a sphere, are not brought to meet in one point, the focus of 

 the central portion being rather more distant than that of the 

 outer part. This is shown in fig. 211, where L L is the lens, 



Fig. 211. 



R L, R L, are rays falling upon its circumference, and R' L', 

 R' L', are rays falling near its centre. The former set of rays 

 meet in /; 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/ by the rays falling upon the outer 

 portion; and when the outer portion is covered, a distinct 

 image will be formed at F 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 /, 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 



