46 On the UNEQUAL 



which difperfes the rays in a much greater degree. The 

 convexity of this compound lens is fuch, as to unite, at 

 a convenient diftance, rays diverging as from the virtual 

 focus of the compound concave. The whole refraction 

 through the convex is confequently much greater than through 

 the concave. But notwithstanding this, the angle formed by 

 the green ray with the united red and violet rays, is reprefented 

 equal in the two lenfes. For as the effect of the mediums of 

 which the compound concave is formed, is to feparate the united 

 red and violet rays from the green rays, much more than thofe 

 of which the compound convex is formed, when the refraction 

 of the pencil is equal, it becomes neceflary, in order to render 

 this feparation equal in both lenfes, to diminifh the refraction 

 through the concave. 



An object-glafs formed of fuch a compound concave and 

 compound convex, appears more complicated than it is in rea- 

 lity. It may. be rendered complete without employing more 

 than two fluid mediums and three glafs lenfes, which were 

 found neceflary merely to correct the aberration from the fphe- 

 rical figure. Thus, in the nineteenth figure, the two compound 

 lenfes are reprefented in contact ; and it is manifeft, that the 

 pieces of plain glafs with parallel fides, which were neceflary to 

 confine the fluid when the lenfes were feparate, are now ufelefs ; 

 for it is the very fame fluid which is on both fides of thefe 

 plain pieces of glafs ; and as they produce no effect in refract- 

 ing the light, they are better removed, as reprefented in this 

 figure. 



Parallel rays incident on the concave lens, are here re- 

 prefented converged to a focus, without any aberration what- 

 ever. This is a neceflary confequence of what hath been re- 

 lated concerning the properties of the refracting mediums, of 

 which this compound object-glafs is formed. 



In both the concave and convex, the red and violet rays are 

 united, and form the leaft refrangible rays, and in both, the 



green 



