On the Mechaluftn of the Eye', 33 c 



mhiation of tlie refrafllve power of a variable medium, and its application to the conftitu- 

 tion of the cryftalUne lens» Secondly, the conftraftion of an inftrument for afcertaining, 

 upon infpedion, the exaft focal diftance of every eye, and the remedy for its iniperfedtions. 

 Thirdly, to fliew the accurate adjuftment of every part of the eye, for feeing with diflincl- 

 nefs the greatell poffible extent of objedls at the fame inftant. Fourthly, to meafure the 

 colleftive difperfion of coloured rays in the eye. Fifthly, by immerging the eye in water, 

 to demonllrate th.U its accommodation does not depend on any change in the curvature of 

 the cornea. Sixthly, by confining the eye at the extremities of its axis, to prove that nd 

 material alteration of its length can take place. Seventhly, to examine what inference can 

 be drawn from the experiments hitherto makle on perfons deprived of the lens ; to purfue 

 the inquiry, on the' principles fuggefted by Dr. Porterfield 5 and to confirm his opinion of 

 the utter inability of fuch perfons to change the refra£live ftate of the organ. Eighthly, i6 

 deduce, from the aberration of the lateral rays, a decifive argument in favour of a change 

 in the figure of the cryftalline; to afcertain, from the quantity of this aberration, the form 

 into which the lens appears to be thrown in my own eye, and the mode by which the change 

 mud be produced in that of every other perfon. And I flatter myfelf, that I fliall not be 

 deemed too precipitate, in denominating this feries of experiments' fatisfaftorily demon- 

 ftrative. 



EXPLANATION OF THE FIGURES. 



Plate XIII. Fig. I. Seepage 256. Prop. III. 



Fig. a. See page 256. Prop. 1 V. 



Fig. 3. See page 258. Prop. V. 



Fig. 4 — 6. Relating to the optometer. See page 259. 



Plate XIV. Fig. 7. The form of the ends of the optometer, when made of card. The 

 apertures in the fhoulders are for holding a lens : the fquare eftds turn under, and are' faf- 

 tened together. 



Fig. 8. The fcale of the optometer. The middle line is divided, from the lower end, 

 into inches. The next column fliews the number of a concave len* requifitc for a fhort'*' 

 fighted eye; by looking through the Aider and obferving the number oppofite to which the 

 interfe£lion appears when mod remotel By obferving the place of appafeht interfeftion 

 when neareft, the number requifite will be found in the other column, provided-that the 

 eye have the average power of accommodation. At the OthCf dnd, the ihiddle line is gi'a- 

 duated for extending the fcale of itiches by means of a lens four inches Irt' focus j ithe riegtt*' 

 tive numbers implying that fuch rays as proceed from them aTe made tO converge towardi a 

 point on the other fide of the lens; The other column (hews the fbcal length of con**e:^ 

 glafles required by thofe eyes teJ wHich the inferfedion appears, When neareft, oppcJli^e flW 

 refpediive places of the numbe'rSw - . . • 



Fig. g. A fide view of the optometer, half its fize. 



Fig. 10. The jippearance of the lines through the flidcV. ;'J 



Fig. 



