On the Mechamfm ef the Eyi. 253 



lime z&s upon thr colourltig oxidfcs or ochres ; and, laftly, to render the procefs gene* 

 rally.ufeftt!. By this union of effort the prefent valuable branch of the arts would be fitn- 

 piified, individuals would obtain confiderable favings, our habitations would become more 

 neat and cleanly, and particularly more healthy, as there is nothing which alters the purity 

 of the air more than oil paint. 



IV. 



On the Mechanifm of the Eye. By Thomjs Youwo, M.D. F.R.S.* 



I. XN the year 1793, I had the honour of laying before the Royal Society, fome obferva- 

 tions on the faculty by which the eye accommodates itfelf to the perception of objedls at 

 different diftances. f The opinion which I then entertained, although it had never been 

 placed exaflly in the fame light, was neither fo new, nor fo much forgotten, as was fup- 

 pofed by myfclf, and by moft of thofe with whom I had any intercourfc on the fubje£l. 

 Mr. Hunter, who had long before formed a fimilar opinion, was ftill Icfs aware of having 

 been anticipated in it, and was engaged, at the time of his death, in an inveftigation of the 

 fafts relative to it ; | an inveftigation for which, as far as phyfiology was concerned, he 

 was undoubtedly well qualified. Mr. Home, with the affiftance of Mr. Ramfden, whofc 

 recent lofs this Society cannot but lament, continued the inquiry which Mr. Hunter hicf 

 begun; and the refults of his experiments appeared very fatisfaftorily to confute the 

 hypothecs of the mufcularity of the cryftallinc lens. § I therefore thought it incumbent 

 on me, to take the carlicft opportunity of tcRifying my perfuafion of the juftice of 

 Mr. Home's conclufions, which I accordingly mentioned in a differtation publifhed at Got-- 

 tingen in 1796, H and alfo in an effay prefcnted lafl: year to this Society. ** About three 

 months ago, I was induced to refnme the fubjeft, by perufing Dr. Porterfield's paper on 

 tlie internal motions of the eye ; ft and I have very unexpedlcdly made fome obfervations, 

 which I think I may ventute to fay, appear to be finally conclufive in favour of my former' 

 opiiiion, as far as that opinion attributed to the lens a power of changing its figure. At the 

 fame time, I mufl remark, that every perfon who has been engaged in experiments of this 

 nature, will be avrare of the extreme delicacy and precaution rcquifite, both in condudlifrg; 

 them, and in drawing inferences from them ;. and will alfo readily allow, that no apology is 

 neceffary for the fallacies which have mifled. many others, as well as myfelf, in the applica-- 

 tion of thofe experitnents to optical and phyfrological determinations. 



* Philof. TranT. 18W. t Phil. Tranf. for 1793, p. 169,. 



J Phil. Tranr. for 1794, p. 21. § Phil. Tranf. for 1785, p. 2i.. 



II De Corporis hiimani Viribus confervatricibus, p- 68. 



♦• flul. Tranf, for 1800, p. 146. f\ E(Unb. Med. Eflays, Vol. IV. p. 124.. 



