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40 On the UNEQUAL 
the correction of colour was rendered the moft perfect poffible, 
a fringe of. purple within the focus, and a fringe of green be- 
yond the focus; and thefe coloured. fringes appeared, whether 
the concave confifted of flint-glafs, or of an effential oil. There- 
fore, in this kind of compound refraction, the rays of light, 
when their union is rendered the moft perfect poffible, emerge 
differently refrangible ; and the rays which emerge moft re- 
frangible, have the property of exciting in us the idea of a 
green colour; and the rays which emerge leaft refrangible, 
have the property of exciting in us the idea of a purple co- 
lour. 
Wuen, for the fake of brevity, I fpeak here, or elfewhere, 
of the union of the red and violet rays, as if it were performed 
by a fingle refraction, whereas, in general, the moft that can 
be effected is to render them parallel by oppofite refractions, 
I would be ufderftood to refer to the moft fimple and perfect 
cafe of achromatic refraction, in which the extreme red and 
violet rays are really equally refrafted, and confequently united, 
by a fingle refraction, as already explained in the references to 
the fifth and ninth figures. 
’ Tue fringe of purple light is formed in part by an union of 
the red and violet rays, which in fimple refraction differ moft in 
refrangibility, but which are here equally refrangible; and 
partly of the united orange and indigo light, whiclr are alfo 
united, and form the fecond order of coloured light in this fe- 
condary fpectrum. 
Tue green fringe is compofed in pa of the homogeneal 
green rays, which, in common refraction, are the mean re- 
frangible, or nearly fo, but are now the moft refrangible of . 
all. The remainder of this green fringe is formed by an union. 
of the yellow and blue rays, pak ass what may be termed an. 
heterogeneal green. 
Ir will appear from the foregoing ftatements, in what man- 
ner this difpofition of the rays is a neceflary confequence of 
the: 
