26 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- 

 diftincT:nefs arifing from the unequal refrangibility of light, 

 this aberration, occasioned 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. 



This fubjed has been treated of in all the variety of cafes 

 which can occur in fingle glafs lenfes, by the great Hu genius, 

 in his Dioptrics, a poflhumous work. He there demonftrates 

 that the quantity of this aberration is very different in different 

 lenfes of the fame focal diflance, 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 diflance from the 

 axis, are converged to a point nearer to the lens than its geome- 

 trical focus. The diflance between the point at which the ex- 

 ternal ray of a pencil incident on a lens, interfecls its axis and 

 the geometrical focus, is called the linear aberration of that 

 lens. 



Hu genius demonftrates, that when a plano-convex lens is 

 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 fubtracling its thicknefs, 

 if it has any r at the outer edge ; and by the thicknefs of a con- 

 cave lens v is meant its thicknefs at the external edge, after de- 

 dueling 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 



