used and
indistinct; whence, in order to see things distinctly, people whose
eyes are so formed are obliged to bring the object very near their
eyes; by which means the rays fall upon the eye in a more diverging
state, so that a distinct picture will be formed on the retina, by
which the object will be distinctly seen: from the circumstance of
such persons being obliged to hold objects near their eyes, in order
to see them distinctly, they are called short sighted.

If a short sighted person look at an object through a small hole made
in a card, he will be able to see even remote objects, with tolerable
distinctness, for this lessens the circles of dissipation on the
retina, and thus lessens the confusion in the picture. For the same
purpose, we commonly observe short sighted people, when they wish to
see distant objects more distinctly, almost shut their eye lids: and
it is from this, says Dr. Porterfield, that short sighted persons
were anciently called myopes.

The sight of myopes is remedied by a concave lens of proper
concavity, which, by increasing the divergency of the rays, causes
them to be united into a focus on the retina: and they do not require
different glasses for different distances, for, if they have a lens
which will make them see distinctly at the distance most commonly
used by other persons, for example, at the distance at which persons
whose eyes are good generally read, they will, by the help of the
same glass, be able to see distinctly at all the distances at which
good sighted people can see distinctly: for the cause of
shortsightedness, is not a want of power to vary the conformation of
the eye, but is owing to the whole quantity of refraction being too
great for the distance of the retina from the cornea.

The other defect to be mentioned, is of an opposite nature, and
persons labouring under it are called long sighted, or presbytae: it
is caused by the cornea and crystalline, or either of them, being too
flat in proportion to the distance between the crystalline and
retina: whence it follows, that the rays which come from an object at
an ordinary distance, will not be sufficiently refracted, and,
consequently, will not meet at the retina, but beyond it, which will
render the picture on the retina confused, and vision indistinct.
Whence, in order to read, such persons are obliged to remove the book
to a great distance, which lessens the divergency of the rays falling
on the eye, and makes t