Achrematic Lens h afmgle RefraBien. 7 



of the lefs refrangible orders of light, being found in the prifmatic fpeflrum nearer to the 

 deep red than to the violet ; and that in another clafs of difperfive mediums, which includes 

 tlie muriatic and nitrous acids, this fame green light becomes one of the more refrangible 

 orders, being now found nearer to the extreme violet than to the deep red. Whenever, 

 therefore, the light pafles out of one of thefe three clafTes of mediums into another of a 

 different clafs, the difperfive powers will not accurately counteraft each other, even though 

 they maybe adopted to caufe the extreme red and violet rays to become parallel ; but the 

 rays v/hich would occupy the interior pans of the fpeftrum will be difperfed, and that in 

 a greaterdegree the more remote they are from the extremes. 



Thefe feveral cafes of refradion were likewife tried with compound objeft-glafles, which 

 (hew the efFecl: better than prifms. Thus, if a plano-convex lens have its plane fide turned 

 toward a diftant obje£t, the rays will enter it, as to fenfe, perpendicularly, and will therefore 

 fuffer no refraiSion. If the convex furface of this lens be brought in contaft with a fluid 

 of lefs mean refraftive denfity than the glafs, but exceeding it in difperfive power in that 

 de'gree which occafions an equal refraftion of all the rays, all thefe rays will then be con- 

 verged to the faiie point, which are incident at the fainc diftance from the axis of the lens. 

 The focal diilance of this compound lens will be greater or lefs, in proportion to its radius 

 of convexity, and to the difference of refraiSlion between it and the fluid made ufe of. 

 While the fluid is confined on one fide by the plano-convex lens, let the lens which is 

 brought in contail with it on the oppofite fide have one of its fides ground convex, and 

 the other concave ; the radii of their fphericlties being equal to the focal diftance at which the 

 rays are made to converge by the refraflion which takes place when light palTes from the 

 plano-convex lens into the fluid. It is manifeft that the light will now both enter into this 

 compound lens, and emerge from it, perpendicularly, and will therefore futTer no refrac- 

 tion, except in the confine of the convex fide of the plano-convex, and the difperfive fluid 

 where all the rays are equally refrangible. A compound lens of this kind is reprefented in 

 the fecond figure, PI. I. which requires no farther explanation ; excepting only, that, iiiflead 

 of being fpherical, it is reprefented with that curvature which converges homogeneal rays 

 incident at all diftances from the axis to the fame point. If the required curvature could be 

 given to lenfes with fufficient accuracy, this figure feems to reprefeiitas perfeft a conftruc- 

 tion of the obje£t-glafs of a telefcope as can be defired. But there is reafon to think 

 that a fpherical figure may be communicated, not only much eafier, but with greater accu- 

 racy than a fpheroidal or hyperboloidal, which would then be required ; and even if this 

 difficulty could be got over, there would ftill remain a fundamental fault In the theory. 

 Before relating the obfervations by which this was detedted. Dr. Blair explains the method 

 of removing the fpherical aberration by a combination of convex and concave lenfes. 

 For, next to the indiflinilnefs arifing from the unequal refrangibility of light, this aberra- 

 tion occafioned by the fpherical figures of lenfes is the great obftacle to the advancement 

 of the powers of vifion. The aberration from the fpherical figure has been treated of, 

 in all the variety of cafes which can occur in fingle glafs lenfes by the great Hugenius in his 

 Dioptrics, a pofthumous work. He there demonfl;rates, that the quantity of this aberration 

 is very diflTercnt in different lenfes of the fame focal diftance, according to the convexities 

 or concavities of their two fides, and the manner in which they arc expofad to parallel rays. 



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