338 Observations relative to 
faculty of seeing those that are near ; since the lengthened 
axis of the eye leaves the power by which it is adjusted to 
see at different distances, precisely in the same state ir. 
which it was before the lengthening of the axis took 
place*. 
Although old persons lose the power of distinguishing 
correctly near objects, and require for this purpose the aid 
of convex glasses, they usually retain the sight of those 
that are distant as’ well as when they were young. Is- 
stances, however, are not wanting of persons advanced in 
life, who require the aid of convex glasses to enable them 
to see near as well as distant objects. Dr. Wells is one of 
these. He informs us, in the paper to which I have more 
than once adverted, that when twenty years younger, he 
was able, with his left eye, to bring to a focus on the retina, 
pencils of rays which flowed from every distance greater 
than seven inches from the cornea ; but at the age of fifty- 
five, he required not only a convex glass of six inches focus, 
to enable him to bring to a point on the retina rays pro- 
ceeding from an object seven inches from the eye, but like- 
wise a convex glass of thirty-six inches focus, to enable 
him to bring to a point parallel rays.—There are also in- 
stances of young persons, who have so disproportionate a 
convexity of the cornea or crystalline, or of both, to the 
distance of these parts from the retina, that a glass of con- 
siderable convexity is required to enable them to see di- 
stinctly, not only near objects, but also those that are di- 
stant ; and it is remarkable, that the same glass will enable 
many such persons to see both near and distant objects ; 
thus proving that the defect in their sight is occasioned 
solely by too small a convexity in one of the parts above 
mentioned, and that it does not influence the power by 
which their eyes are adapted to see at distances variously 
remote. In this respect such persons differ from those who 
have had the crystalline humour removed by an operation ; 
since the latter always require a glass to enable them to dis- 
cern distant objects, different from that which they use to 
see those that are near. This circumstance, in my appre- 
hension, affords a convincing proof that the crystalline hu- 
mour is indispensably necessary to enable the eye to see at 
different distances.—It is also worthy of remark, that per- 
*Dr. Young, inthe paper to which I alluded in page 335, has described 
a great number of ingenious experiments devised by him, to show that the 
facuity of seeing at different distances is produced by a power iu the crystal- 
line humour, to become more or Jess convex, according as the object is more 
or less distant from the eye. 
sons 
