and Experiments on Vision. 381 
ments, were nearly the same as they had been in my youth, 
and that the convex glasses which I used did very little more, 
than supply, with respect to near objects, the place of a living 
power which 1 had lost, without compensating, except in a 
very small degree, for any alteration in the external shape of 
the eye, or any change in the configuration of its interior 
parts. I ascertained, for instance, that to give my left eye the 
refractive power which it formerly possessed while in its most 
relaxed state, that by which it was enabled to bring a pencil 
of parallel rays to a point on the retina, a glass of thirty-six 
inches focus was fully sufficient ; whereas to produce an equal 
effect upon rays proceeding from a point at the distance of 
seven inches from my eye, the other extremity of my ancient 
range of perfect vision, I was now obliged to employ a glass 
having a focus of only six inches. I regret much, that I had 
not made such experiments frequently before, as .1 think it 
very probable, that I should have found a period in the pro- 
gress of my vision to its present state, in which my capacity 
of seeing distant objects was the same as in my youth, and 
when therefore the whole of my imperfect vision of near ob- 
jects would have been owing to a loss of the muscular powers 
cf my eye. 
As there can be no good reason for supposing, that the 
changes which have occurred in my eyes are different from 
those, which the eyes of by far the greater number of per- 
sons, who are not short-sighted, undergo at the approach of 
old age, it is evident, that the experiments of Dr. Young* on 
the eye of Hanson, whom the learned author considered as a 
very fair subject for such trials, furnish no proof, that the 
* Phil. Trans. 1801, p. 66 . 
