4 o On the UNEQUAL 



the correction of colour was rendered the mod 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 confided 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 moil 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. 



When, 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 uhderftood to refer to the moft fimple and perfect 

 cafe of achromatic refraction, in which the extreme red and 

 violet rays are really equally refracted, and confequently united, 

 by a fingle refraction, as already explained in the references to 

 the fifth and ninth figures. 



The 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, which are alfo 

 united, and form the fecond order of coloured light in this fe- 

 cdndary fpectrum. 



The green fringe is compofed in part 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, compofing what may be termed an 

 heterogeneal green. 



It will appear from the foregoing ftatements, in what man- 

 ner this difpofition of the rays is a neceflary confequence of 



the 



