REFRANGIBILITY of LIGHT. 37 
if the difference of the angles of refra@tion of the green ray 
and deep red ray, in the medium CD, be alfo double of the 
difference of the angles of refraction of thefe rays in the me- 
dium AB, that is to fay, the angle yRe double the angle gGr; 
I fhould fay that the two mediums difperfed thefe three kinds 
of rays, namely the red, green and violet rays proportionally. 
But if, when the difference between the angles of refraction of 
the green ray, and extreme violet ray, in the medium CD, is 
double of what it is in the medium AB; the difference of the 
angles of refraction of the green ray, and deep red ray in the 
medium CD, fhall be found to exceed the difference between 
the angles of refraction of thefe rays, in the medium AB, only 
one half, for example ; then I would fay that the two mediums 
do not difperfe thefe differently refrangible rays proportion- 
ally. 
_For in this cafe the medium CD difperfes or feparates the 
green ray, and extreme, violet ray, twice as much as the me- 
dium A B does ; whereas the feparation of the green ray, and. 
deep red ray, in this fame medium CD, exceeds only by one 
half their feparation in the medium AB. 
Ir is farther manifeft, that the red, green and violet rays 
cannot be rendered parallel by any combination of the refrac- 
tions of the two mediums, upon the laft mentioned fuppofition. 
The whole refraction, through a prifm compofed of the me- 
_ dium CD, may be fuch as to give exaétly the fame inclination 
of the red and violet rays, which a prifm compofed of the me- 
dium AB does, when both rays fuffer a greater refra@ion 
‘through the latter; and therefore both thefe rays may be 
equally refraéted and converged to the fame point by means of 
a convex lens. of the leaft difperfive medium AB, and a concave 
lens duly proportioned to it, formed of the moft seed pe me- 
dium CD. 
Bur if we now add to thefe the green ray, jit is evident that 
at too cannot be refraéted parallel with the red’ and violet rays.. 
For 
