46 Oa 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 notwithftanding 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. r 
An objeét-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 neceffary merely to corre¢t 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 refradct- 
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 obje¢t-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 
