74 On the UNEQUAL 



Thus it happens to the red and violet rays, whichfoever 

 clafs of difperfive mediums be employed. But the refrangibi- 

 lity of the intermediate orders of rays, and efpecially of the 

 green rays, will be different when the clafs of difperfive medi- 

 ums is changed. 



Thus in the fir ft cafe, where the red and violet rays are ren- 

 dered equally refrangible, the green rays will emerge mofl re- 

 frangible, if the firft clafs of difperfive mediums is ufed, and 

 lead refrangible if the fecond clafs is ufed. And in the other 

 two cafes, where the violet become leaft refrangible, and the 

 red mod refrangible, and where thefe two kinds of rays are re- 

 fracted in oppofite directions, the green rays will join the red, 

 if the firft clafs of difperfive mediums be employed, and will 

 arrange themfelves with the violet, if the fecond clafs be made 

 life of. 



Only one cafe more of unequal refrangibility remains to be 

 ftated ; and that is, when light is refracted in the confine of 

 mediums belonging to the two different claffes of difperfive 

 fluids. In its tranfition, for example, from an effential oil, or 

 a metallic folution, into the muriatic acid, the refractive denfity 

 of thefe fluids may be fo adjufted, that the red and violet rays 

 fhall fufFer no refraction in pafling from the one into the other, 

 how oblique foever their incidence be. But the green rays will 

 then fufFer a confiderable refraction, and this refraction will be 

 from the perpendicular, when light paffes from the muriatic 

 acid into the effential oil, and towards the perpendicular, when 

 it palTes from the effential oil into the muriatic acid. The other 

 orders of rays will fufFer fimilar refractions, which will be 

 greateft in thofe adjoining the green, and will diminifh as they 

 approach the deep red on the one hand, and the extreme violet 

 on the other, where the refraction ceafes entirely. 



The manner of the production of thefe effects, by the at- 

 traction of the feveral mediums, may be thus explained. 



We 



