t*liYSICAL AND LITERARY. -4^ 



^afs previoufly thro' any number of tranfpa- 

 tent bodies any how difpofed^^". 



It is inipoffible therefore to know, whe- 

 ther an alteration of the fwiftnefs, with 

 which a homogeneal ray ftrikes the retina^ 

 tvould alter its colour ; I mettn the fenfation 

 pf colour produced by it in the mind : fince 

 it is impoffibie to alter, at pleafure, thd 

 denfity of that fluid which determines its 

 final velocity. 



One may diftingulfli t\vo different effeds 

 of the refradive power on the rays of lights 

 viz. the change of direction and change of 

 velocity. Sir Jfaac Newton has proved with 

 refpedt to the firft, That it is different iii the 

 differently-coloured rays, and of a determined 

 degree in each : he has further proved, That 

 refradion^ confidered in its firft effed, does 

 not change the colour of any fimple ray. 

 But it appears, from what we have now faid. 

 That none of his experiments prove thd 

 Vol. II. G immutability 



* Here it is proper to obferve, that the hyvotkefit which 

 fuppofes the intervals of the fits to be determined by the ve- 

 locity of the ray, agrees well with a remarkable obfervatiort- 

 of Sir Ifaac Nenjoton (Optics, "b. ii. part. I. obfcrv. 21.): 

 'viz. That thefe intervals in any medium, at a given angle of 

 incidence, are of a given magnitude, v^ithout regard to the 

 denfity of the furronnding medium. 



