2S6 
NEAR AND DISTANT SIGHT. 
Varions cases 
of imperfec- 
tion in the 
power of focal 
arljiistmcnt in 
the eye. 
The ervf falline 
baniours ap- 
peai s iiuii-ipcn- 
Sii l)i<; to tlie, 
(li'itant vision 
at (liti'eient 
distances. 
Although old persons lose the power of distinguishing cor- 
rectly near objects, and require for this purpose the aid of con- 
vex glasses, they usually retain the sight of those that ate dis- 
tant, as w’ell as when they were young. Instances, however, 
are not wanting of persons advanced in life, who require the 
aid of convex glasses to enable them to sec- near as well as dis- 
tant objects. Ur. Wells is one of these. He informs us, in 
the paper to w'bich 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 proceeding from an object seven inches from the 
eye, but likewise a convex glass of thirty-six inches focus, 
to enable him to bring to a point parallel rays. There are 
also instances 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 consi- 
derable convexity is required to enable them to see distinctly, 
not only near objects, but also those that are distant 3 and it is 
remarkable, that the same glass will enable many such per- 
sons 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 s^uch 
persons differ from those who have had the crystalline humour 
removed by an operation } since the latter always require a 
gla.ss to enable them to discern distant objects, different from 
that which they use to see those that are near. This circum- 
staiice, in my apprehension, affords a convincing proof that the 
crystalline humour is indispensably necessary to enable the 
eye to see at different distances. It is also worthy of remark, 
sliow.that tlic faculty of scritig at different distances is produced by 
a i)o\ver in the crystalline humour, to beeome more or less convc.v, 
according as the object is more or less distant from the eye. 
without 
