O'S THE MECHANISM OF THE EYE. 



SB'S 



12 riiclies, nnd as near as five: for nearer 

 oljects she added another of equal focus, and 

 could then read at 7 inches, and at 2^. 



4. Hanson, a carpenter, ageil 63, iiad a 

 cataract extracted a few years since from one 

 eye : the pupil was clear and large, and he 

 saw well to work with a lens of 2^- inches 

 focus; and coi;!d read at 8 and at J 5 inches, 

 but most conveniently at 1 1. With the same 

 glass, the lines of tlie optometer appeared 

 always to meet at 11 inches; but he could 

 not perceive that thev crossed, the line be- 

 ing too strong, and the intersection too distant. 



The experiment was afterwards repeated 

 with the small optometer : he read the letters 

 from 2 to 3 inches; but the intersection was 

 always at 24- inches. He now fully under- 

 stood the circumstances that were to be no- 

 ticed, and saw the crossing with perfect dis- 

 tinctness : at one time, he said it was a tenth 

 of an inch nearer ; but! observed that he had 

 removed his eye two or three tenths from the 

 glass, a circumstance which accounted for 

 this small difference. 



5. Notwithstanding Hanson's age, I consi- 

 der him as a very fair subject for the experi- 

 ment. But a still more unexceptionable eye 

 was that of Mrs. Maberly. She is about 30, 

 and iiad the crystalline of both eyes extracted 

 a few years since, but sees best with her 

 right. She walks without glasses ; and, 

 with the assistance of a lens of about four 

 inches focus, can read and work with ease. 

 She could distinguish the letters of the 

 small optometer from an inch to2f inches; 

 but the intersection was invariably at the same 

 point, about ly tenths of an inch distant 

 A portion of the capsule is stretched across 

 the pupil, and causes her to see remote ob- 

 jects double, when without her glasses nor 



VOL. II, 



can she, by any exertion, bring the two- 

 images nearer together, although the exer- 

 tion makes them more distinct, no doubt bv 

 contracting the pupil. The experiment 

 with the optometer was conducted, in the 

 presence of Mr. Ware, with patience and 

 perseverance ; nor was any opinion given to 

 make her report partial. 



Considering the difficulty of finding an 

 eye perfectly suitable for the experiments, 

 these proofs may be deemed tolerably satis- 

 factory. But, since one positive argument 

 will counterbalance many negative ones, 

 provicjed that it be equally grounded on fact, 

 it becomes necessary to inquire into the com- 

 petency of the evidence employed to ascer- 

 tain the power of accommodation, attributed, 

 in the Croonian Lecture for 1794, to the 

 eye of Benjamin Clerk. And it appears, that 

 the distinction long since very properlj- made 

 by Dr. Jurin, between distinct vision and 

 perfect vision, will readily explain away the 

 whole of that evidence. 



It is obvious that vision miiy be made dis- 

 tinct to any given extent, by means of an 

 aperture sufficiently small, provided, at the 

 same time, that a sufficient quantity of light 

 be left, while the refractive powers of the 

 eye remain unchanged. And it is relnark- 

 able, that in those experiments, when the 

 comparison with the perfect eye was made, 

 the aperture of the imperfect eye only was 

 very considerably reduced. Benjamin Clerk, 

 with an aperture of -^\ of an inch, could read 

 with the same glass at If inch,' and at 7 

 inches*. With an equal aperture, I can 

 read at ly inch and at SO inches : and I can 

 retain the state of perfect relaxation, and 

 read with the same aperture at 2|r inches, 

 without any real change of refractive power,. 

 • Phil.Trau». 1795. 0. 

 4 o 



