458 PHLLOSOPHICAL TRANSACTIONS. [anNO 1795. 



man's eye was adjusted to the same distance in the repeated trials that were 

 made with it. 



The perfect eye, with a glass of 64- inches focus, had distinct vision at 3 

 inches; the near limit was l^ inch, the distant one less than 7 inches. The 

 imperfect eye, with a glass of 2-^ inches focus, with an aperture ^Sg. of an inch, 

 had distinct vision at 2^ inches, the near limit 1-|- inch, the distant one 7 inches. 

 From the result of this experiment we find that the range of adjustment of the 

 imperfect eye, when the 2 eyes were made to see at nearly the same focal dis- 

 tance, exceeded that of the perfect eye. These experiments were made by Mr. 

 Ramsden, who took, particular care to avoid every thing that might be produc- 

 tive of error or deception ; and repeated them several times before any conclu- 

 sions were drawn from them. Several others were made on the same subject, all 

 tending to confirm those already mentioned. It may be proper to mention a 

 reason which suggested itself to Mr. Ramsden, why tlie point of distinct vision 

 of the imperfect eye appeared to the man himself nearer than it was in reality ; 

 it arose from his judging of distinctness by the legibility of the letters, which 

 were easier read when they subtended a greater angle, from the imperfection of 

 his eye, than at his real point of distinct vision. 



The result of these experiments convinced us that the internal power of the 

 eye, by which it is adjusted to see at different distances, does not reside in the 

 crystalline lens ; we were also satisfied by the facts and arguments adduced in 

 Mr. Hunter's letter on this subject, published in the last vol. of the Phil. Trans, 

 that it does not arise from a change in the general form of the globe of the eye ; 

 we therefore abandoned both of these theories. It suggested itself that any 

 change in the curve of the cornea, could it be produced, would vary the refrac- 

 tion of the rays, so as considerably to alter the focus of the eye ; and on consi- 

 dering this subject, Mr. Ramsdem made a rough calculation, from which it ap- 

 peared, that a very small alteration in that part would vary the adjustment of 

 the eye from parallel rays to its shortest distance of distinct vision. This opened 

 to us a new field of inquiry, and I endeavoured to ascertain how far the cornea 

 admitted of such a change, and if it did, how far that change operated in pro- 

 ducing this particular effect. 



For the first of these purposes I made the following experiments in the pre- 

 sence of Mr. Ramsden. A portion of the cornea -f of an inch broad, and 44- of 

 an inch long, was removed from the eye of a person 40 years of age, 2 days 

 after death, with a part of the sclerotic coat on each side attached to it. This 

 was laid on a piece of glass immersed in water, under which was a scale divided 

 into very minute parts, these divisions being very readily seen through the glass. 

 One end of the cornea was made fast by fixing the sclerotic coat, and a force 

 was applied to the other ; this power was found capable of elongating the cornea 



