344 On the Adjustment of the Eye, &c. 



inents, and no alteration whatever was made in the instru- 

 ment, except in the ancriure cf the lens. 



Whence it is evident, that a large pupil gives us a distinct 

 view of remote objects ; that a contracted pupil gives us a 

 clear view of near («l;jecis ; and that no part of the eye, ex- 

 cept the iris, suficrs any change in making these adjust- 

 ments. 



The same Property of Vision demonstrated by a more simple 

 Experimtnt. 



Let a remote object be ol^served through an aperture of 

 about J^ of an inch in diameter, n)ade m the manner al- 

 ready mentioned, and if the object be seen in a proper light 

 it will appear as distinct as to the naked eye. Ti.^n intro- 

 duce a pipj or any other sii;all object, in a line belwcin the 

 remote object and tht eye, at the distance of live or six 

 inches from it, and these two objects will appear as disti.ict 

 when seen together as when thev are viewed separately ; 

 but the moment the aperture is taken away from before the 

 eye, the near object vvill be seen very indistinctly ; and as 

 the aperture may be removed so instantaneously as to render 

 it impossible for any chanre to take place in the internal 

 parts of the eye, it is therefore evident, i!)at the iris is the 

 only organ by which the eye is adjusted to distinct vision. 



Hence it is as clearly proved as any proposition in optics, 

 that the ravs issuing from a neai object, atter having passed 

 through a small aj)erture, aiT conveyed to the same point 

 upon the retina as those from a remote one ; the rays from 

 both objects being nearly parallel. 



These experiments also prove, that an eye which has 

 lost the crystalline lens, if the iris has not received any in- 

 jurv, may still retain its pov.er (jf adjustment. 

 I am. sir, 



l.vnn, Your humble servant, 



6th Jan. 1808. Ez. WaLKER. 



L. On 



