MISCELLANEOUS PAPERS. 



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I. OBSERVATIONS ON VISION. BY THOMAS YOUNG. 



COMMUNICATED BY RICHARD BROCKLESBY, M.D, F. R.S. 



FROM THE PHILOSOPHICAL TRANSACTIONS. 



Head before Ihe Ro.yal Society, Mai/ 30, 1793. 



It is well kaowp, that the eye, when not 

 acted upon by any exertion of the mind, con- 

 veys a distinct impression of those objects 

 only which are situated at a certain distance 

 from itself; that this distance is different in 

 different persons, and that the eye can, by 

 the volition of the mind, be accommodated 

 to view other objects at a much less distance : 

 but how this accommodation is effected, has 

 long been a matter of dispute, and has not 

 yet been satisfactorily explained. It is 

 equally true, and it has indeed already been 

 observed by Dr. Porterfield, that no exer- 

 tion of the mind can accommodate the eye 

 to view objects at a distance greater than that 

 of indolent vision, a circumstance which 

 may easily be experienced by any person to 

 whom this distance of indolent vision is less 

 than infinite. 



The principal parts of the eye, and of its 

 appertenances, have been described by 

 various authors. VVinslovv is generally very 



accurate ; but Albinus, in Musscheubroek's 

 Introductio, has represented several particu- 

 lars more correctly. I shall suppose their 

 account complete, except where I mention or 

 delineate the contrary. 



The first theory that I find of the accom- 

 modation of the eye is Kepler's.' He sup- 

 poses the ciliary processes to contract. the dia- 

 meter of the eye, and lengthen its axis, by 

 a muscular power. But the ciliary processes 

 neither appear (o contain any muscular fibres, 

 nor have they any attachment by which they 

 can be capable of performing this action. 



Descartes imagined the same contrac- 

 tion and elongation to be effected by a mus- 

 cularity of the crystalline, of which he sup- 

 posed the ciliary processes to be the tendons. 

 He did not attempt to demonstrate this mus- 

 cularity, nor did he enough consider the 

 connexion with the ciliary processes. He 

 says, thai the lens in the mean time beconie* 

 more convex, but does not appear to attri- 



