VISION. 335 



Dr. Young have rendered it probable that some change takes 

 place in the figure of the lens, whereby its convexity, and 

 perhaps, also, its distance from the retina, are increased. He 

 has shown, by a very decisive experiment, that any change 

 which may take place in the convexity of the cornea has 

 but little share in the production of the effect; for the eye 

 retains its power of adaptation when immersed in water, in 

 which the form of the cornea can in no respect influence the 

 refraction. 



But the rays of light are of different kinds; some exciting 

 the sensation of red, others of yellow, and others again of 

 blue; and these different species of light are refracted, under 

 similar circumstances, in different degrees. Hence, the 

 more refrangible rays, namely, the violet and the blue, are 

 brought to a nearer focus, than those which are less refran- 

 gible, which are the orange and-the red rays: and this want 

 of coincidence in the points of convergence of these different 

 rays, (all of which enter into the composition of white light,) 

 necessarily impairs the distinctness of all the images pro- 

 duced by refraction, shading off their outlines with various 

 colours, even when the object itself is colourless. This de- 

 fect, which is incident to the power of a simple lens, and 

 which is termed the Chromatic Aberration, is remedied 

 almost perfectly in the eye, by the nice adjustment of the 

 powers of the different refracting media, which the rays of 

 light have to traverse before they arrive at the retina, pro- 

 ducing what is called an achromatic combination;^' and it is 

 found that the eye, though not an absolutely achromatic in- 

 strument, as was asserted by Euler,t is yet sufficiently so 

 for all the ordinary practical purposes of life. 



The object, then, of the whole apparatus appended to the 

 optic nerve, is to form inverted images of external objects 

 on the retina, which, as we have seen, is the expanded ex- 



* For the exposition of the principles on which these achromatic combi- 

 nations of lenses correct this source of aberration, I must refer to works 

 which treat professedly on Optics. 



f For the rectification of this error we are indebted to Ur. Young. 



