REFRANGIBILItr of LIGHT. 37 



if the difference of the angles of refraction of the green ray 

 and deep red ray, in the medium C D, 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 yR^ double the angle gOr \ 

 I mould 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 C D, 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 C D, fhall be found to exceed the difference between 

 the angles of refraction of thefe rays, in the medium A B, 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 C D 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 C D, exceeds only by one 

 half their feparation in the medium A B. 



It 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 lafl mentioned fuppofition. 

 The whole refraction, through a prifm compofed of the me- 

 dium C D, may be fuch as to give exactly the fame inclination 

 of the red and violet rays, which a prifm compofed of the me- 

 dium A B does, when both rays fuffer a greater refraction 

 through the latter ; and therefore both thefe rays may be 

 equally refracted and converged to the fame point by means of 

 a convex lens of the lead difperfive medium AB, and a concave 

 lens duly proportioned to it, formed of the mod difperfive me- 

 dium C D. 



But if we now add to thefe the green ray, it is evident that 

 it too cannot be refracled parallel with the red and violet rays.. 



For 



