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have the fame momentum; it is replied, ift, That it has not yet 

 been proved, that the fenfation of different colours depends on 

 the different momentum of the rays. 2dly, The rays may have 

 different momentums, and yet be equally refrangible ; for fince 

 refraftion is fuppofed to depend on the attradive force of the 

 denfer medium, we muft fuppofe it analogous to the attradlive 

 force of gravity, which is proportional to the quantity of matter ; 

 and therefore the greater or lefs quantity of matter in a particle 

 of light would produce no alteration in its refradlion. Neither 

 can the different refrangibility depend on the different velocity 

 of the rays; becaufe the difference of refrangibility of the red 

 ^nd violet rays is much greater in flint glafs than in crown 

 glafs ; and this would require a proportlonably greater difference 

 in the original velocities, which cannot be. And this fame argu- 

 ment holds equally againft the former hypothefis, that the diffe- 

 rence of refrangibility depends on the different magnitude or 

 denfity of the particles of light, jdly, Refradion feems to arife 

 from a fpecies of eledive attradlion, fince different meditims 

 which adl on the mean rays equally, adl on the extreme rays 

 unequally : hence rays of the fame quantity of matter and ve- 

 locity, and therefore of the fame momentnm, may be diverfely 

 refradted ; and rays of different momentums equally refracfled. 



Nor is it to be wondered at that the rays of light fhould be 

 differently refrangible, independent of any regard to their mo- 

 mentum, when we confider, that the different coloured rays ap- 

 pear 



