a6 On the UNEQUAL 
obfervations by which this was detected, it will be requifite to 
explain the method of removing the fpherical aberration, by a 
combination of convex and concave lenfes. For next to the in- 
diftinétnefs arifing from the unequal refrangibility of light, 
this aberration, occafioned by the fpherical figures of lenfes, is 
the great obftacle to the advancement of the powers of vifion. 
Of the aberration from the fpherical figure. 
Tus fubject has been treated of in all thecwartety of cafes 
which can occur in fingle glafs lenfes, by the great Hucenius, 
in his Dioptrics, a pofthumous work. He there demonftrates 
that the quantity of this aberration is very different in different 
lenfes of the fame focal diftance, according to the convexities 
or concavities. of their two fides, and the manner in which 
thefe are expofed to parallel rays. 
In convex lenfes, thofe rays which pafs at a diftance from the 
axis, are converged to a point nearer to the lens than its geome- 
trical focus. The diftance between the point at which the ex- 
ternal ray of a pencil incident on a lens, interfects its axis and 
the geometrical focus, is called the linear aberration of that 
lens. 
expofed to parallel rays, with its plane fide towards them, this 
aberration will amount to four times and a half the thicknefs 
of the glafs. By the thicknefs of a convex lens is meant its 
greateft thicknefs in the middle, after fubtracting its thicknefs, 
if it has any, at the outer edge; and by the thicknefs of a con- 
cave lens, is meant its thicknefs at the external edge, after de- 
ducting its thicknefs in the middle. 
On turning the convex fide of the lens towards the light, the 
linear aberration will only exceed the thicknefs of the lens by 
one fixth part. 
WHEN 
Hucentus demonftrates, that when a plano-convex lens is 
f 
