368 Mr. Walker on the Adjustment of the Eye to [May 



Article V. 



On the Adjustment of the Eye to Distinct Vision at Different 

 Distances. By John Walker, Esq. Assistant-Surgeon 

 to the Manchester Eye Institution. 



The adjustment of the eye to the distinct vision of objects 

 at various distances, is a question which is still considered 

 to remain undetermined. 



It is not my purpose to enter into a history of the various 

 explanations offered, but rather to point out a very simple 

 experiment, which, I think, elucidates the point in dispute. 

 I may, nevertheless, first observe that it is generally admitted 

 that when the eye, from contemplating an object at some 

 distance, suddenly fixes itself on some other object very near 

 to the organ, there is a contraction of the pupil always 

 accompanying this change. 



From this circumstance many have been led to believe 

 that the alteration in the size of the pupil is the chief, if not 

 the sole, change which occurs in the eye during this opera- 

 tion. Sir David Brewster has related experiments which 

 are easily verified, and which prove that the application of 

 a bright light to the eye, causing a contraction of the pupil, 

 enables the organ to adjust itself to distinct vision at a less 

 distance than before. Dr. Wells has also established the 

 fact that when the pupil is dilated by belladonna, the power 

 of adjustment is lost. It is also well known that when an 

 object is brought so close to the eye as to be invisible, by 

 looking at the same object, at the like distance, through a 

 a pin-hole perforated through a card, it becomes quite dis- 

 tinct. All these circumstances seem to favour the opinion 

 that it is merely the size of the aperture, through which the 

 rays of light are admitted into the eye, that regulates the 

 adjustment ; but still they are considered fallacious, and it 

 is thought that some alteration in the refracting media is 

 indispensable. 



It occurred to me, whilst debating these facts in my mind, 

 that, with an artificial eye-ball, which I possess, fitted up 

 with a cornea, iris, lens, and a retina of ground glass, each 

 bearing its proper relation to the others, by varying the 



