REFRANGIBILITY 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 diftiné vifion *. But as it has been juft-af- 
 ferted 4s 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 affertion 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. HH) 
Wuen 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 diftine vifion; the blade appears diftindt, 
and well defined. If the eye be now accommodated to a more 
diftant 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. 
Ir the eye be again accommodated to the diftance neceflary 
' for feeing the knife diftinétly, 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 moft 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 contraft of 
light and darknefs is. flronger. It requires, however, a ca- 
pacity of viewing with attention an obje@ to which the eye is 
hot 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- 
Vou. III. I rallg] 
* Philofophical Tranfaétions of London, Vol. lxxix. p- 256. 
