NATURAL THEOLOGY. 23 



The resemblance between the two cases is still 

 more accurate, and obtains in more points than 

 we have yet represented, or than we are, on the 

 first view of the subject, aware of. In dioptric te- 

 lescopes there is an imperfection of this nature. 

 Pencils of light, in passing through glass lenses, 

 are separated into ditferent colours, tlTereby ting- 

 ing the object, especially the edges of it, as if it 

 were viewed through a prism. To correct this in- 

 convenience had been long a desideratum in the art. 

 At last it came into the mind of a sagacious opti- 

 cian, to inquire how this matter was managed in 

 the eye : in which there was exactly the same dif- 

 ficulty to contend with as in the telescope. His 

 observation taught him, that, in the eye, the evil 

 was cured by combining lenses composed of dif- 

 ferent substances, i. e. of substances which pos- 

 sessed different refracting powers. Our artist 

 borrowed thence his hint ; and produced a correc- 

 tion of the defect by imitating, in glasses made 

 from different materials, the effects of the different 

 humours through wiiich the rays of light pass be- 

 fore they reach the bottom of the eye. Could this 

 be in the eye without purpose, which suggested 

 to the optician the only effectual means of attain- 

 ing that purpose ?* 



* This is an interesting part of the inquir)', which will be found 

 more fully explained in the Appendix. 



It is not, accurately speaking, " glasses of different refracting 

 powers" which are required. Refraction is the new direction 

 which the ray takes in passing from one transparent body into 



