bjects single with two Eyes._

That we should have two pictures of an object, and yet see it single,
has long been looked upon as a curious circumstance by philosophers:
and of consequence, many attempts have been made to account for it,
few of which, however, are satisfactory.

As it would take up too much time to give a view of all the opinions
on this subject, I shall pass over the opinions of Galen, Gassendus,
Baptista Porta, Rohault, and others, which do not deserve a serious
refutation; and shall content myself with making a few observations
on the hypothesis of Bishop Berkeley.

But it seems the most proper way of proceeding, first of all to
consider the phenomena of single and double vision, in order, if
possible, to discover some general principle to which they lead, and
of which they are necessary consequences; and, for the sake of
perspicuity, we shall premise the following definition.

When a small object is seen single with both eyes, those points on
the two retinas on which the pictures of the object fall, may be
called corresponding points: and when the object is seen double, we
shall call such points, non-corresponding points.

Now we find that in sound and perfect eyes, when the axes of both are
directed to one point, an object placed in that point is seen single;
and in this case, the two pictures which show the object single, are
painted on the centres of the retinas. Hence, the centres of the two
retinas correspond.

Other objects at the same distance from the eyes, as that to which
their axes are directed, do also appear single: and in this case, it
is evident to those who understand the principles of optics, that the
pictures of an object to which the eyes are not directed, but which
is at the same distance as that to which they are directed, fall both
on the same side of the centre, that is, both to the right, or both
to the left, and both at the same distance from the centre. Hence it
is plain, that points in the retina, which are similarly situated
with respect to the centres, are corresponding points.

An object which is much nearer, or much more distant from the eyes,
than that to which their axes are directed, appears double. In this
case, it will easily appear, that the pictures of the object which is
seen double, do not fall upon points which are similarly situated.
From these facts, we are led to the following conclusion, viz. that
the points of the two retinas, which are similarly