SPHERICAL AND CHROMATIC ABERRATION. 495 
screen be held at F, it will receive, not only the rays which 
are brought to a focus there, but also those which, having 
met at f, have crossed and passed-on to G and H. 
547. Now this indistinctness is ordinarily got over in 
practice, by employing only the central portion of the lens ; 
so that only those rays which correspond to R’ L’, R’ L’, shall 
pass. through it. This we observe in ordinary Microscopes 
and Telescopes ;—a stop or perforated partition being inter- 
posed behind the lenses, so as to allow the light to pass 
only a small aperture in their centre. By this plan 
a great deal of light is cut off, so that the image is rendered 
dark. The spherical aberration may be completely corrected, 
_ however, by a certain adaptation of two or more lenses whose 
surfaces have different curvatures ; the effect of which is, to 
_ bring all the rays that have passed through every part of this 
compound lens to the same focus. Now this very adjustment 
is made in the eye, by the arrangement of the curvatures of 
the cornea and of the two surfaces of the crystalline lens; 
and in the well-formed eye it is so perfect as to produce 
complete distinctness in the image or picture thrown upon 
_ the retina. The only case in which this would not occur, is 
when an object is brought very near the eye; for the rays 
then diverge from each other at so wide an angle, that those 
which fall upon different parts of the lens would not be all 
brought to the same focus. This error is corrected by the 
contraction of the pupil, which takes place involuntarily 
when an object is brought very near the eye, and thus 
euts-off the rays that would otherwise render the picture 
indistinct. 
548. But there is another imperfection to which ordinary 
optical instruments are liable, that is completely corrected in 
the eye. If we look through a common Microscope, especially 
when a high power is employed, by the light iy a lamp or 
‘candle, we see that the edges of the image are bordered by 
coloured fringes, which very much impair its distinctness, 
and prevent it from being seen in its true aspect. This is 
‘the result of what is termed chromatic aberration; and it 
‘results from the fact, that the rays of different colours, which 
are all blended in ordinary colourless light, are refracted by 
same lens in different degrees, so as to be brought toa 
at different points. Thus we will auppose that the lens 
