REFRANGIBILITV 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 conftruction of this 

 kind, the correction of the fpherical aberration would be at- 

 tended with more difficulty. 



Having thus found an exception to the general refult of my 

 former experiments, which was, that thofe rays which in the 

 lead difperlive 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 pofTefTed of this property. It will appear from 

 what has been faid concerning attraction, 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 muft 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 attraction for the 

 green rays would be in an intermediate degree, and might be 

 rendered the fame, in proportion to the attraction for the red 

 and violet rays, which obtains in crown-glafs and other indif- 

 perfive mediums. 



It might be found a matter of no fmall difficulty to unite 

 the effential oils with the marine acid, fo as to form a colour- 



G 2. lefs 



