REFRANGIBILITY of LIGHT. 93 
ir appears from the experiments which have been made on 
indifperfive mediums, that the mean refrangible light is always 
the fame, and of a green colour. 
Now, in by far the largeft clafs of difperfive mediums, in- 
cluding flint-glafs, metallic folutions, effential oils, the green 
light is not the mean refrangible order, but forms one of the 
lefs refrangible orders of light, being found in the prifmatic 
fpectrum nearer to the deep red than the extreme violet. 
In another clafs of difperfive mediums, which includes the 
muriatic and nitrous acids, this fame green light becomes one 
of the more refrangible orders, being now found nearer to the 
extreme violet than the deep red. 
TueseE are the varieties in the refrangibility of light, when 
the refraction takes place in the confine of a vacuum; and the 
phenomena will fcarce differ fenfibly in refra@tions made in the 
confine of denfe mediums and air. 
But when light paffes from one denfé medium into another, 
the cafes of unequal refrangibility are more complicated. 
In refraétions made in the confine of mediums which differ 
only in ftrength, not in quality, as in the confine of water and 
crown-glafs, or in the confine of the different kinds of difper- 
five fluids more or lefs diluted, the difference of refrangibility 
‘will be the fame as above ftated in the confine of denfe medi- 
ums and air, only the whole refra&tion will be lefs. 
In the confine of an indifperfive medium, and a rarer me- 
dium belonging to either clafs of the difperfive, the red and 
violet rays may be rendered equally refrangible. If the dif- 
perfive power of the rare medium be then increafed, the violet 
rays will become the leaft refrangible, and the red rays the moft 
tefrangible. If the mean refractive denfity of the two medi- 
ums be rendered equal, the red and violet rays will be- refract- 
ed in oppofite directions, the one towards, the other from the 
perpendicular. 
* Vou. IIL. K Tuus 
