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, 
Li 
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. Tut 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 
