On tSe Mechanifm of the Eye, 



zCi 



in breadth, divided by fpaces fomewhat broader; fo that each obfcrver may cboofe that 

 which befl: fuits the aperture of his pupil. In order to adapt the inftrument to the ufe of 

 prefbyopic eyes, the other end muft be furnirtied with a lens of four inches focal length; 

 and a fcale muft be made near the line on each fide of it, divided from one end into inches, 

 and from the other according to the table here calculated from Cor. 7. Prop. IV, by means 

 of which, not only diverging, but alfo parallel and converging rays from the lens are referred 

 to their virtual focus. The inftrument is eafily applicable to the purpofe of afcertainlng the 

 focal length of fpedtacles required for myopic or prefbyopic eyes. Mr. Cary has been fo 

 good as to furnifli me with the numbers and focal lengths of the glaffes commonly made ;. 

 and I have calculated the diftances at which thofe numbers muft be placed on the fcale of the 

 optometer, fo that a prefbyopic eye may be enabled to fee at eight inches diftance, by 

 ufing the glaffes of the focal length placed oppofite to the neareft crofling of the lines ; and a 

 myopic eye with parallel rays, by ufing the glaffes indicated by the number that ftand op- 

 pofite their furtheft croffing. To facilitate the obfervation, I have alfo placed thefe numbers 

 oppofite that point which will be the neareft croffmg to myopic eyes; but this, upon the 

 arbitrary fuppofition of an equal capability of change of focus in every eye, which I muft 

 confefs is often far from the truth. It cannot be expe£ted, that every perfon, on the firft 

 trial, will fix precifely upon that power which beft fuits the defe£l: of his fight. Few can 

 bring their eyes at pleafure to the ftate of full a£lion, or of perfeft relaxation ; and a power 

 two or three degrees lower than that which is thus afcertained, will be found fufficient for 

 ordinary purpofes. I have alfo added to the fecond table, fuch numbers as will point out the 

 fpe£tacles neceffary for a prefbyopic eye, to fee at twelve and at eighteen inches refpeftively : 

 the middle feries will perhaps be the moft proper for placing the numbers on the fcale 

 The optometer fhould'be applied to each eye; and, at the time of obferving, the oppofite. 

 eye fhould not be fliut, but the inftrument fhould be fcreened from its view. The place of 

 interfeiHrion may be accurately afcertained, by means of an index Aiding along the fcale. 



The optometer is reprefented in Plate XIV. Fig. 8 and 9 ; and the manner in which the. 

 lines appear, in Fig. 10. 



TalU- 



