the supjjosed Achromatism of the Eye, 163 



right in his argument, if his facts were correct. The refractions 

 are not all ^performed one way. The vitreous humour acts 

 as a concave lens, and the rays are refractedyrom the axis in 

 passing from the capsule of the crystalline into the vitreous 

 humour, and, as Mr. Powell justly observes, this case is pre- 

 cisely the same in 'principle as the construction for achroma- 

 tic microscopes which I have given in p. 4-08 of my Treatise 

 on New Philosophical Instruments, But in practice it is very 

 different. The refractive and dispersive powers of the cry- 

 stalline and vitreous humours are such that an achromatic 

 compensation is impossible. 



But there is another point of view in which I would beg to 

 submit this subject to Mr. PowelPs consideration. I have 

 elsewhere stated, (and Mr. Powell has quoted the passage 

 without pursuing the idea which it contains,)'* that no provi- 

 sion is made in the human eye for the correction of colour, 

 because the deviation of the differently coloured rays is too small 

 to produce indistinctness of vision." If the last of these two 

 propositions be true, the first will be instantly admitted ; for 

 it is inconceivable that the all-wise Author of nature, who never 

 works in vain, should have made the eye achromatic when it 

 was not required for the purposes of vision. 



The idea that the eye would answer the purposes of vision 

 more perfectly if it were achromatic, seems to be founded on 

 a hasty analogy. Because an achromatic telescope, or micro- 

 scope, or lens, is preferable to the same instruments when 

 they are not freed from colour, it is conceived that an achro- 

 matic eye should have the same superiority : the two cases, 

 however, are considerably different. In using the telescope, 

 &c., the eye views in succession every part of the image which 

 they form, in every part of the object within the field of view; 

 but there is no eye behind the retina to view in the same man- 

 ner the image which is formed upon that membrane. In point 

 of fact, the eye is incapable of seeing any object distinctly unless 

 it is situated in or near its axis, and hence it is of no importance 

 whatever to render the image distinct at a distance from the 

 axis. Whenever the eye wishes to examine an object, or a 

 part of an object, minutely, it instantly directs to it the axis 

 of its vision, and from the rapidity of its movements, and the 

 duration of the impressions of light, it thus obtains the most 

 perfect view of a given object, and can scrutinize in succession 

 its minutest parts. 



Now in order to obtain distinct, and a sensibly colourless 

 vision, near the axis of the eye, achromatic compensation is 

 not necessary. In order to prove this, look through a convex 

 lens, about an inch in focal length, at any sharp and well- 



Y2 



