67 
on the Mechanism of the Eye. 
glasses ; nor can she, by any exertion, bring the two images 
nearer together, although the exertion makes them more dis- 
tinct, no doubt by 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 
satisfactory. But, since one positive argument will counter- 
balance many negative ones, provided it be equally grounded 
on fact, it becomes necessary to inquire into the competency of 
the evidence employed to ascertain the power of accommoda- 
tion attributed, in the Croonian Lecture for 1794, to the eye of 
Benjamin Clerk. And it appears, that the distinction long 
since very properly made by Dr. Jurin, between distinct vision 
and perfect vision, will readily explain away the whole of that 
evidence. 
It is obvious that vision may be made distinct 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 
remarkable, 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 1 f- inch, and at 7 inches. * With an equal aperture, I 
can read at 1 i inch and at 30 inches : and I can retain the state 
of perfect relaxation, and read with the same aperture at 2 £ 
inches ; and this is as great a difference as was observed in 
* Phil. Trans, for 1795. p. 9. 
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