REFRANGIBILITY 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 
_ Jeaft refrangible, and the violet-making rays the moft refran- 
gible. 
Tue 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 refraction of the moft 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 refpeét to the mediums on which he made his 
experiments ; but there are many exceptions to it. 
For the experiments of Mr Dotionp 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. 
Tue 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 poflefled 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 pafles 
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 fhall be the moft refrangible, or 
whether there fhall be any difference in their refrangibility. 
t THE 
