33^ On the Mechanifm if the Eye. 



Fig. 1 1. Method of meafuTiiig the magnitude of an image ou the retina. See page 293. 

 ■ Fig. 12. Diagonal fcale drawn on a looking-glafs. 



Fig. 13. The method of applying a lens with water to the cornea. 



Fig. 14. The appearance of a fpeclrum oGcafioned by preflure ; and tiie infle£Hon of 

 Ilraight lines feen within the limits of the fpeflrum. 



Fig. 15. An illuftration of the enlargement of the image, which would be the confe- 

 cj&ence of an elongation of the eye : the images of the candles which, in one inftance, fall 

 on the infertion of the nerve, falling, in the other inftance, beyond it- 

 Plate XV. Fig. 16. The fucceffive forms of the image of a large diftant object, as it 

 would be delineated by each refractive furface in the eye; to {hew how tliat form at laft 

 coincides with the retina. E G is the diftance between the foci of horizontal and vertical 

 lays in my eye. 



Plate XV. Fig. 17. Vertical fe£tioa of my right eye, feen from without; twice the na^- 

 tural fize. 



Fig. 1 8. Horizontal fe£tion, feen from above. 



Fig. 19. Front view of my left eye when the pupil is contra£ted j of the natural fize^ 



Fig. 20. The fame view when the pupil is dilated. 



Fig, 21. Outline of the eye and its ftraight mufcles when at reft. 



Fig. 42. Change of figure which would be the confequence of the zQilon of thofe muf- 

 dcs upon the eye, and upon the adipofe fubftance behind it. 



Fig. 23. Scale of the fmall optometer. 



Fig. 24. Appearance of four images of a line feen by my eye when its focus Is fliorteft. 



Fig. 25. Outline of the lens when relaxed ; from a comparifon of ,M. Petit's meafures 

 with the phenomena of my own eye, and on the fuppoGtion that it is- found in a relaxed 

 ftate after death. 



Fig. 26. Outline of the lens fufficiently changed to produce the fliortefl focal dlftance.^ 



Fig. 27. Apparatus for afcertalning the focal length of the lens in water. 

 ■' Plate XV. Fig. 28. Various forms of the image depifted by a cylindrical pencil of rays 

 obliquely refracted by a fpherical furface, when received on planes at diftances progreffively 

 greater. 



Fig. 29. Image of a minute lucid obje£l held very near to my eye. 



Fig. 30. The fame appearance when the eye has been rubbed. 



Fig. 21—37. Different forms of the image of a lucid point at greater and greater dif- 

 tances; the moft perfeft focus being like Fig. 3.3, but much fmaller. 



Fig. 38. Image of a very remote point feen by my right eye» 



Fig. 39. Image of a remote point feen by my left eye ^ being more obtufe at one end» 

 probably from a lefs obliquity of the pofterior furface of the cryftalline lens. 



Fig. 40. Combination of two figures fimilar to the fifth variety of Fig. 28 ; to imitate 

 Fig. 38. 



Fig. 41. Appearance of a diftant lucid point when the eye Is adapted to a very near objed. 



Fig. 



