38 On th UNEQUAL 
For when the whole refraétion of the leaft 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 AB, 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 CD, will, when the red and violet rays 
are united by contrary refrations through thefe two mediums, 
be refracted too much ; the balance of refraction being always, 
in this cafe, in favour of the leaft difperfive medium; 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 faét, 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 moft 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 bifeéts 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 
ray- 
Tuus, in Fig. 14. the green ray Gg is the mean refrangi- 
ble. an in Fig. 15. the green ray Ry is found among the 
lefs 
