426 CHROMATIC ABERRATION I ACHROMATISM OP 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 focus at/, whilst the others do not meet until F. Hence 

 if a screen be placed to receive the image at f y 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 P ; 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. Now 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 



