serve for both; and, moreover, that our hearing single
with both ears is not the effect of custom. No person will doubt that
things which are produced by custom, may be undone by disuse, or by a
contrary custom. On the other hand, it is a strong argument, that an
effect is not owing to custom, but to the constitution of nature,
when a contrary custom, long continued, is found neither to change
nor weaken it. Now it appears, that from the time we are able to
observe the phenomena of single and double vision, custom makes no
change in them, every thing which at first appeared double, appearing
so still in the same circumstances. Dr. Smith has adduced some facts
in favour of his opinion, which, though curious, seem by no means
decisive. But in the famous case of the young man couched by Mr.
Cheselden, after having had cataracts in both his eyes till his
thirteenth year, it appears that he saw objects single from the time
he began to see with both eyes. And the three young gentlemen
mentioned by Dr. Reid, who had squinted, as far as he could learn,
from infancy, as soon as they learned to direct both eyes to an
object, saw it single.

In these cases it is evident that the centres of the retina
corresponded originally, for Mr. Cheselden's young man had never seen
at all before he was couched, and the other three had never been
accustomed to direct the axes of both eyes to the same point. These
facts render it probable, that this correspondence is not the effect
of custom, but of fixed and immutable laws of nature.

With regard to the cause of this correspondence, many theories have
been proposed, but as none of them can be looked on in any other
light than as probable conjectures, I think it would be to little
purpose to notice them. That of the illustrious Newton is the most
ingenious of any, and though it has more the appearance of truth than
any other, that great man has proposed it under the modest form of a
query.

Having given a short account of the principal phenomena of vision, I
proceed next to treat of some of the diseases to which this sense is
subject, I shall first take notice of the deformity called squinting.

_Of Squinting._

Though this is a subject which well deserves our particular
attention, yet having spoken of such a variety of subjects in the
preceding part of this lecture, I have not time for many observations
on this. I shall just mention the principal opinions, concerning the
cause of this de