the near and distant Sight of different Persons. 337 
lens not unfrequently retains ils transparency for some time 
after an opacity has commenced in the part that. surrounds 
its centre. 
It is evident, that near-sightedness has no dependence on 
the greater or smaller degree of convexity possessed by the 
cornea, when this circumstance is considered alone; since 
the length of the axis of the eye from the cornea to the re- 
tina, and the greater or smaller degree of convexity in the 
erystalline humour, must be also regarded, before the di- 
stance of accurate vision can be determined. 
It is no less evident, that near-sightedness is not neces- 
sarily occasioned by a morbid protrusion of the whole eye ; 
since some persons are born with eyes of this description, 
and others acquire the peculiarity, when further advanced 
in life, in consequence of a morbid accumulation of adeps 
at the bottom of the orbit, without either of them being 
more near-sighted than those who are free from this im- 
perfection. 
I have seen many instances in which old persons, who have 
been long accustomed to use convex glasses of considerable 
power, have recovered their former sight at the advanced 
age of eighty or ninety years, and have then had no further 
need of them. Dr. Porterfield was of opinion that in such 
cases the amendment is occasioned by a decay of adeps at 
the bottom of the orbit ; in consequence of which the eyes 
from a want of the usual support behind, is brought, by 
the pressure of the muscles on its sides, into a kind of oval 
_ figure, in which state the retina is removed to its due focal 
distance from the flattened cornea. But if a morbid ab- 
sorption of adeps at the bottom of the orbit were sufficient 
to restore the presbyopic to a good sight, it might be ex- 
pected that a morbid accumulation of adeps in this part 
would produce a presbyopic or distant sight. This, how- 
ever, has not happened in any of the cases that have come 
under my notice. On the contrary, in some such persons 
a degree of near-sightedness has been induced by the ac-. 
cumulation ; and in others the sight, with regard to di- 
stance, has not been affected by it. It appears to me more 
probable, that this remarkable revolution in the sight of 
old persons is occasioned by an absorption of part of the vi- 
treous humour; in consequence of which, the sides of the 
sclerotica are pressed inward, and the axis of the eye, by 
this lateral pressure, is proportionably lengthened. An al- 
teration of this kind is also sufficient to explain the reason, 
why such aged persons retain the power of distinguishing 
objects at a distance, at the same time that they recover the 
Vol. 42. No, 187. Nov. 1813, © Y faculty 
