REFRANGIBILIfr of LIGHT. 65 



Dr Maskelyne has taken the pains to compute the quantity 

 of this aberration in the eye, and is of opinion that it is not 

 incompatible with diftinct vifion *. But as it has been juft af- 

 ferted as a matter of fact, that the aberration from difference 

 of refrangibility is not corrected in the human eye, it will be 

 expected that the proofs on which this afTertion is founded, 

 fhould be explained. Thefe are fo ample as to leave no caufe 

 of uncertainty ; nor are the neceflary experiments attended with 

 much trouble. For it happens that the humours are better 

 placed for the purpofe in the natural eye, than art could difpofe 

 them elfewhere. 



When I take the penknife which now lies before me, and 

 hold it between me and the fky, at the diftance to which the 

 eye is conformed for diftinct vifion, the blade appears diftinct, 

 and well defined. If the eye be now accommodated to a more 

 diflant object, the blade of the knife begins to be furrounded 

 with a penumbra ; and if this penumbra be carefully attended 

 to, it appears to be coloured, and the colour next to the knife is 

 red inclining to orange, which is the colour of the leaft refran- 

 gible rays. 



If the eye be again accommodated to the diftance necefTary 

 for feeing the knife distinctly, the bars of the window, which 

 is at a greater diftance than the knife, are furrounded with a 

 penumbra, and the colour of this penumbra is blue, which is 

 the prevailing colour of the mod refrangible rays. The fame 

 appearances will be obferved in all cafes where the confine of 

 a dark and luminous object is carefully examined, and will be 

 fo much the more confpicuous by how much the contrafl of 

 light and darknefs is ftronger. It requires, however, a ca- 

 pacity of viewing with attention an object to which the eye is 

 not conformed, which muft be acquired by habit. The fol- 

 lowing eafy experiment may be tried by any one. Shutting 

 one eye, obferve with the other the four well defined black pa- 

 Vol. III. I rallel 



* Fnilofophical Tranfaftions of London, Vol.lxxix. p. 256. 



