REFRANGIBILirr of LIGHT. %% 



experiments have proved, that this fuppofed general law of re- 

 fraction does not hold in nature. 



In the inftance before us, if we fuppofe the force with which 

 glafs attracts the red, green and violet rays to be reprefented by 

 the numbers five, fix and feven \ then may the force with which 

 the difperfive fluid attracts thefe rays, be reprefented by the 

 numbers four, five and fix. For the rcafon why all the rays are 

 equally refracted in their tranfition from one of thefe mediums 

 into the other, is becaufe the rare medium has the property of 

 refracting the violet rays more, and the red rays lefs, than the 

 denfe medium, when the obliquity of incidence is fo propor- 

 tioned to their denfity, that the mean refrangible ray fliall fuffer 

 the fame refraction in both. 



Now, in the cafe above ftated, the attraction of the rare 

 medium for the feveral rays, is fo proportioned to the attraction 

 of the denfe medium for thefe fame rays, that the difference of 

 thefe attractions is every where equal, and confequently the re- 

 fraction arifing from thefe differences of attraction is alfb equaL 

 Thus the green ray is attracted by the denfe medium with the 

 force fix, and by the rare medium with.the force five, the dif- 

 ference of which is one ; and there is the fame difference be- 

 tween the attracting forces acting on the red and violet rays in 

 the two mediums, being in one cafe the difference between five 

 and four, and in the other between fix and feven ; fo that the 

 difference of attracting force, and confequently the refraction 

 fuppofed to arife from it, is the fame in all the rays, being al- 

 ways that which will be produced by an attracting force, repre- 

 fented by unity. 



If the difperfive power of the rare medium, of which the 

 prifm B C D is formed, be flill farther increafed, the red rays 

 will become the mofl refrangible, and the violet rays the leafl 

 refrangible ', a law of refraction, which, as has been already 

 explained, obtains when light is refracted in the confine of 

 crown-glafs and oil of turpentine, and of many other fluids. 



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