THE ARGUMENT APPLIED. 23 



are separated into diilerent colors, thereby tinging the object, 

 especially the edges of it, as if it were viewed through a 

 prism. To correct this inconvenience had been long a desid- 

 eratum in the art. At last it came into the mind of a saga- 

 cious optician, to inquire how this matter was managed in 

 the eye, in which there was exactly the same difficulty to 

 contend with as in the telescope. His observation taught 

 him that in the eye the evil was cured by combining lenses 

 composed of difierent substances, that is, of substances which 

 possessed difierent refracting powers. Our artist borrowed 

 thence his hint, and produced a correction of the defect by 

 imitating, in glasses made from different materials, the effects 

 of the difierent humors through which the rays of light pass 

 before they reach the bottom of the eye. Could this be in 

 the eye without purpose, which suggested to the optician the 

 only efiectual means of attaining that purpose ? 



But further, there are other points, not so much perhaps 

 of strict resemblance between the two, as of superiority of 

 the eye over the telescope, yet of a superiority which, being 

 founded in the laws that regulate both, may furnish topics 

 of fair and just comparison. Two things were wanted to 

 the eye, which were not wanted, at least in the same degree, 

 to the telescope ; and these were the adaptation of the organ, 

 first, to different degrees of light, and secondly, to the vast 

 diversity of distance at which objects are viewed by the na- 

 ked eye, namely, from a few inches to as many miles. These 

 difficulties present not themselves to the maker of the tele- 

 scope. He wants all the light he can get ; and he never 

 directs his instrument to objects near at hand. In the eye. 

 both these cases were to be provided for ; and for the purpose 

 of providing for them, a subtile and appropriate mechanism 

 IS introduced. 



I. In order to exclude excess of light when it is exces- 

 eive, and to render objects visible under obscurer degrees of 

 it when no more can be had, the hole or aperture m the eye 

 through which the light enters is so formed as to contract 



