38 On the UNEQUAL 



For when the whole refraction of the lead difperfive me- 

 dium AB is fuch as juft to unite the red and violet rays, the 

 green ray, which is more refracted by this medium A B, in 

 proportion to the whole refraction of the red and violet rays in 

 the medium A B, than it is refracted by the difperfive medium 

 C D, in proportion to the whole refraction of the red and violet 

 rays in the medium C D, will, when the red and violet rays 

 are united by contrary refractions through thefe two mediums, 

 be refracted too much ', the balance of refraction being always, 

 in this cafe, in favour of the leaft difperfive medium j and 

 therefore the green light will emerge from this compound re- 

 fraction more refracted than the united red and violet light, 

 and the inclination of the emergent green light to the emergent 

 united red and violet light, will be greater or lefs according as 

 the ratio in which the red, green and violet light are feparated 

 by the refraction of the two mediums, approaches more or 

 lefs to equality. What this inclination amounts to, in any par- 

 ticular inftance, muft be determined by experiment. 



Hence if the cafe of unproportional difperfion, above fta- 

 ted, fhould be found to hold true in fact, we fhall arrive at 

 this new truth in optics, That though in the refraction of a pen- 

 cil of folar light, made in the confine of any medium, and a 

 vacuum, the deep red rays are always the leaft refrangible, and 

 the violet rays are always the molt refrangible ; yet it depends 

 entirely on the fpecific qualities of the medium, which fhall be 

 the mean refrangible ray ; the very fame ray, which in the 

 refraction through one medium is the mean refrangible ray, 

 being found in others among the lefs refrangible rays. For it 

 is manifeft that the ray which bifects the angle formed by the 

 moft and leaft refrangible rays, and falls in the middle of the 

 coloured fpectrum, is to be accounted the mean refrangible 

 rayr 



Thus, in Fig. 14. the green ray G^ is the mean refrangi- 

 ble. But in Fig. 15. the green ray Ry is found among the 



lefs 



