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XLTX. On that Power of the Eije, hy which It is adjusted 

 to sec Objects dhtlnctlij at different Distances. By 

 Ez. Walkkr, Esq. 



To Mr. Ti/loch. 



P ''""^^ 



JThilosophers have varied much in their opinions, for 



near two hundred years, respecting that power of the eye 



by wliich it is adjusted to see objects distinctly at different 



distances. 



Kepler, TNI. le Cat, and son)e other philosophers sup- 

 posed, that when we view distant. objects the eye contracts 

 itself and Ijecouics flatted, by which means the distance be- 

 tween the crvstalhne lens and the retina is decreased; but 

 when near objects are seen, the axis of the eye is lengtliened, 

 and this distance is therefore greater, Des Cartes, Dr. Pem- 

 berton, and Dr. Young, have adopted the opinion that the 

 form of the crystalline lens alters with the distance of the 

 object from the eye. Dr. Ponerlicld thinks, that the ad- 

 justment of the eye is produced by an alteration in the cur- 

 vature oi' the cornea and a n-ioiion of the crystalline lens. 

 De la Hire imagined, that the eye undergoes no change, 

 except the contraction and dilatation of the pupil. Mr. 

 Kamsdea and Mr. Elome conclude from their experiments, 

 that the adjustment is produced by three changes in the 

 eye; a motion in tlie crystalline lens, an elongation of the 

 axis of vision, and anincrea-'e of curvatiuT; in the cornea, 

 llaller, Smilli, .Turin, Monro, and many others, have also 

 written upon this subject. But as none of the hypotheses 

 hitherto advanced have been demonstrated, either by deci- 

 sive experiments or from the known laws of dioptrics, 

 this property of vision seems still to remain in much ob- 

 scurity. 



On entering t^pon an in([uiry of this nature, it will be 

 necessary to reject all suppositions that are not conform- 

 able to those beautiful mechanical laws which arc seen in 

 every other part of the animal oeconomy. I cannot there- 

 i'ore suppose that the whole globe of the eye alters its form 



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