REFRANGIBILlTr of LIGHT. 69 



controverted, that when the refraction is made in the confine 

 of any medium whatever, and a vacuum, the rays of different 

 colours are unequally refracted, the red-making rays being the 

 lead refrangible, and the violet- making rays the moft refran- 

 gible. 



The difcovery of what has been called a different difperfive 

 power in different refractive mediums, proves thofe theorems of 

 Sir Isaac Newton not to be univerfal, in which he concludes 

 that the difference of refraclion of the mod and leaft refrangi- 

 ble rays, is always in a given proportion to the refraction of the 

 mean refrangible ray. There can be no doubt that this pofi- 

 tion is true with refpect to the mediums on which he made his 

 experiments ; but there are many exceptions to it. 



For the experiments of Mr Dollond prove, that the diffe- 

 rence of refraction between the red and violet rays, in propor- 

 tion to the refraction of the whole pencil, is greater in fome 

 kinds of glafs than in water, and greater in flint-glafs than in 

 crown-glafs. 



The firft fet of experiments above recited, prove, that the 

 quality of difperfing the rays in a greater degree than crown- 

 glafs, is not confined to a few mediums, but is poffeffed by a 

 great variety of fluids, and by fome of thefe in a moft extraor* 

 dinary degree. Solutions of metals, effential oils, and mineral 

 acids, with the exception of the vitriolic, are moft remarkable 

 in this refpect. 



Some confequences of the combinations of mediums of dif- 

 ferent difperfive powers, which have not been fufficiently at- 

 tended to, are then explained. Although the greater refrangi- 

 bility of the violet rays than of the red rays, when light paffes 

 from any medium whatever into a vacuum, may be confidered 

 as a law of nature ; yet in the paffage of light from one medi- 

 um into another, it depends entirely on the qualities of the 

 mediums, which of thefe rays fliall be the moft refrangible, or 

 whether there fhall be any difference in their refrangibility. 



The 



