ON THE LIMITS OF SINGLE VISION. ] 35 



luinous raj's should be applied to that organ as they come 

 from the Inininoiis body, so that they may not undergo any 

 decomposition before they reach it, and prove that the de- 

 composition has been effected in their passa«^e through the 

 humours of this organ. Now this is the case in neither of 

 the ejiperiments qijoted : in the former the decomposition is 

 produced by the attractive power of the edges by which the 

 rays pass ; in the latter the heterogeneous rays, isolated by 

 the prism separately applied, may seem to be unequally re- 

 frangible, because the spectrum ceases to be linear through- 

 out its whole extent. But on account of this extent they 

 arrive at the transparent cornea at different angles, in pro- 

 portion to these they are uneqiyd, and they reach parts of a 

 refracting medium, the curvature and density of which are 

 unequal : can we then expect them to be equally refracted? 

 The eye is intended to preserve the natural mixture of the 

 elementary rays applied to it : to pretend, that it should re- 

 compose those, the composition of which has been altered 

 by foreign causes, would be to require of this organ what 

 it cannot accomplish. 



The perfection of the achromatic system of the eye ap- The eye a 

 pears to me therefore demonstrated ; and this perfection J^'ad^c^'msuu-" 

 can result only from an accurate proportion between the ment, 

 curvatures of the solid or fluid lenses and menisci that com- 

 pose it, and the i-efractive power of these subbtances, Ihere 

 can be no doubt, that a more accurate imitation of the 

 structure of their organ would give us still more perfect in- 

 struments. The labours of Mr. Rochon, and those of Dr. 

 Robert Blair, who by an ingenious combination of glass 

 menisci and fluids enclosed in them produced instruments 

 perfectly achromatic, afford us the moft promising hopes of 

 success in the application of these principles to the con- 

 struction of large instruments. Mr. Chenevix has made 

 known to us the chemical nature of the humours of the 

 eye. Let a new Petit and another Zinn unite their efforts ^ptj should 

 with a rival of Jiuler, tg determine with the greatest accu- '^ad to thfi 

 racy the structure, figure, density, and refractive and dis- 0^,^^^^^^^ ^^ 

 persive powers of each of the humours of the eye, and by telescopes. 

 their assistance our modern Dollonds will still farther im- 

 prove the achromatic telescope, 



