SPHERICAL AND CHROMATIC ABERRATION. 425 



screen be held jat" r, it will receive, not only the rays which 

 are brought to a focus there, but also those which, having 

 met at/, 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' I/, R' i/, 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 

 through 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 

 cuts-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 of 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 

 the same lens in different degrees, so as to be brought to a 

 focus at different points. Thus we will suppose that the lens 



