REFRANGIBILITY of LIGHT. 51 
condary colour, this may be effected by a compound convex, 
which, inftead of lengthening, will fhorten the focal diftance 
of the compound object-glafs. But in a conftrudtion of this 
kind, the correction of the fpherical aberration would be at- 
tended with more difficulty. 
Havine thus found an exception to the general refult of my 
former experiments, which was, that thofe rays which in the 
leaft difperfive mediums conftitute the mean refrangible rays, 
are in more difperfive mediums found amongft the lefs refran- 
gible rays, it feemed not improbable that difperfive mediums 
might exift, which would feparate the differently coloured rays 
exactly in the fame proportion in which they are feparated by 
indifperfive mediums. . 
I HAD now indeed got hold of a pretty fure clue to lead me 
to mediums poffeffed of this property. It will appear from 
» what has been faid concerning attraétion, that when in a me- 
tallic folution or an effential oil, which feparate the red and 
violet rays in the fame degree in which they are feparated by 
the marine acid, the green rays are found amongft the lefs re- 
frangible rays in the former fluids, and amongft the more re- 
frangible in the latter fluid, the caufe of this difference mutt be, 
that the green light is more attracted by the marine acid than 
by effential oils or metallic folutions, when the attraction for 
the red and violet light is the fame in all thefe mediums. 
Hence it feemed reafonable to conclude, that in a medium 
compounded in a due proportion of the particles compofing 
thefe two kinds of difperfive mediums, the attra@tion for the 
green rays would be in an intermediate degree, and might be 
ndered the fame, in proportion to the attraction for the red 
and violet rays, which obtains in crown-glafs and other indif- 
perfive mediums. 
_ Ir might be found a matter of no {mall difficulty to unite 
the effential oils with the marine acid, fo as to form a colour- 
G 2. lefs 
