REFRANGIBILITY of LIGHT, 27 
_., Wuen both fides of a lens are convex, and the proportion of 
their convexities is as one to fix, if the moft convex fide be ex- 
‘ pofed to parallel rays, the aberration will exceed the thicknefs 
of the lens one fourteenth, which is the {malleft poflible aber- 
ration of any convex lens. ‘aks 
Ir it is required to increafe the aberration, this may be done 
by grinding one fide of the lens convex, and the other fide con- 
cave, toa longer radius. Such a lens, with its concave fide 
_ turned towards parallel rays, will have more aberration than 
any plano-convex or double convex lens of the fame focal di- 
ftance. . : 
HuceEnNtuvs proceeds to fhew, that the fame aberration is pro- 
duced by concave lenfes as by fimilar convex ones. When a 
. plano-concave lens is expofed to parallel rays, with its plane fide 
outward, the external ray of the pencil, being produced back- 
ward after refraction, will interfect the axis of the lens nearer 
to it than its focus, by four times and a half the thicknefs of 
the lens. But if its concave fide be expofed to the parallel rays, 
the aberration will only exceed the thicknefs of the lens one 
fourteenth part. A double concave, whofe radii are as one to 
fix, with the moft concave fide turned outward, difperfes the 
rays with the leaft aberration; and a concave menifcus, with 
its convex fide outward, produces more aberration than any 
plano-concave or double concave lens, of an equal focal di- 
ftance.. é 
. Tuese are fufficient data for correcting the aberration from 
the fpherical figure, in cafes where both a convex and concave 
lens are required, in the conftruction of the compound. objeét- 
glafs. ; 
aathits : fifi : 
Fic. 10. Let AB reprefent a convex lens receiving a pencil of 
rays from the object S, and converging rays incident near the axis, 
as ST, to the point F; and external rays, as SB, to the point 
nod i arr reer D; 
