426 CHROMATIC ABERRATION :-ACHROMATISM OF THE EYE. 
l l (fig. 211) has been corrected for spherical aberration; and 
that r l, r l, are violet rays falling upon it, whilst r' l', r' l', 
are red rays. The former are capable of being refracted in a 
much higher degree than the latter ; so that they are brought 
to a focns at/, whilst the others do not meet until f. Hence 
if a screen be placed to receive the image at / the picture 
will be formed by the violet rays only, and will be surrounded 
by red fringes occasioned by the red rays which are passing 
on to their focus at f ; whilst, on the other hand, if the screen 
be placed at f, the picture will be chiefly formed by the red 
rays, and will be surrounded by violet fringes produced by 
the violet rays, which, having met in / have crossed and 
passed-on to g and h. How as from each point of almost 
every object proceed rays in which the different colours are 
blended, the refraction of an ordinary lens produces a sepa¬ 
ration of these, and a consequent indistinctness and false 
colouring in the picture. This is particularly the case with 
regard to the rays that pass through the outer portion of the 
lens; for, as these are subject to greater change in their 
direction than are those which pass through its centre, the 
separation of the differently-coloured rays of which they are 
composed is more considerable. 
549. In practice, this error is got over, like the preceding, 
by very much contracting the aperture of the lens; so that 
only the central rays, in which the colours are but little 
separated, are allowed to pass. But it may be perfectly cor¬ 
rected by combining lenses formed out of different materials, 
which possess a different refracting power; the errors of 
these being made to counterbalance one another. Such 
lenses, which are termed achromatic , are now employed in 
all superior Telescopes and Microscopes; but no artificial 
combination can surpass that which exists in the Eye, the 
different density of whose humors is adjusted in such a 
manner as completely to answer this purpose. The contrac¬ 
tion of the pupil which takes place when we look at a very 
near object, prevents the only imperfection which could occur; 
and thus the picture on the retina, in a healthy eye, is always 
rendered free from false colours. It is said that the first idea 
of uniting glasses of different kinds, so as to produce an 
achromatic lens, was taken from the Eye ; and this is not at 
all improbable. In this, as in many other instances, Nature 
