16 



APPLICATION OF THE ARGUMENT. 



billing t05;ether lenses composed of different substances. 

 i. e of substances which possessed different refracting 

 powers. Our artist borrowed from thence his hint ; and 

 produced a correction of the defect by imitating, in glasses 

 inadc from different materials, the effects of the different 

 humours through which the rays of light pass before they 

 reac'i the bottom of the eye. Could this be in the eye 

 without purpose, which suggested to the optician the only 

 effectual 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, which being found in the laws 

 that regi^iate both, may furnish topics of fair and just com- 

 parison. Two things were wanted, to the eye, which 

 were not 'vanted, at least in the same degree, to the teles- 

 cope ; and these were the adaptation of the organ, first, 

 ^o different degrees of light ; and, secondly, to the vast 

 diversity of distance at which objects are viewed by the 

 naked eye, viz. from a few inches to as many miles. These 

 difBcuities present not themselves to the maker of the 

 telescope. He wants all the light he can get ; and he 

 nsver directs his instrument to objects near at hand. In 



'•'The hint taken from the eye, and applied by Mr. Dollond in the 

 construction of achromatic telescopes, has been since carried still far- 

 ther, and illustrates more strongly, if possible, the point so admi- 

 rably inp".sted upon by Dr. Paley, namely, the resemblance between 

 the eye and our optical instruments. In the best achromatic tele- 

 scopes, composed of the different kinds of glass, according to the dis- 

 covery of Mr. Dollond, white or luminous objects are not shown per- 

 fectly fiee from colour, their edges being tinged on one side with a 

 claret coloured, and on the other with .i greenish fringe. This remain- 

 ing imperfection has been got rid of by" the combination of solid and 

 fluid lenses in the object and eye-glasses of telescopes. For this beau- 

 tiful discovery science is indebted to Dr. Blair of Edinburgh, who 

 found that by placing a concave lens of muriatic acid with a metallic 

 solution, between two convex lenses of glass, a combined lens was 

 formed which refracted rays with perfect regularity and equality. A 

 lens like this has been used with great advantage. The most impor- 

 tant point is, however, to consider this improvement in its application 

 to the argument, and it will be seen how" much nearer this construc- 

 tion brings the telescope to the eye. In Dollond's telescope there is a 

 combination of solid lenses of different substances. — In Blair's, a com- 

 bination ol fluid and solid ; which is exactly the case in the human 

 eye. The only difference is, that in the eye there is a solid lens be- 

 tween tvvo fluid ones; and in the telescope a fluid between two solid. 

 The combination is closely similar, and the final cause in both proba- 

 bly the same, namely, to correct the unequal refraction of light." — See 

 Ediiiburs^h Journal of Science, No. viii, p. 212 : and Library of 

 Useful Knowledge, No. 1 & 12. [Ed, 



