REFRANGIBILirr of LIGHT. 75 



it 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, efTential 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. 



These 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 refractions made in the 

 confine of denfe mediums and air. 



But when light pafTes from one denfe medium into another* 

 the cafes of unequal refrangibility are more complicated. 



In refractions 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 refraction 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 lead refrangible, and the red rays the moft 

 refrangible. If the mean refractive den'fity 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. 



Vol. III. K Thus 



