look at you with that which is usually
distorted, he will immediately turn the axis of it directly towards
you. If you bid him open the undistorted eye, and look at you with
both eyes, you will find the axis of this last pointed at you and the
other turned away, and drawn close to the nose, or perhaps to the
upper eye lid. From these facts, and some others mentioned by Dr.
Jurin, I think we may conclude that this defect is seldom, if ever,
occasioned by such a preternatural make of the eye, as M. de la Hire
supposed.

From the most accurate observations it will appear, that by far the
most common cause of squinting, is a defect in the distorted eye. Dr.
Reid examined above twenty people that squinted, and found in all of
them a defect in the sight of one eye; M. Buffon likewise, from a
great number of observations, has found that the true and original
cause of this blemish, is an inequality in the goodness, or in the
limits of distinct vision, in the two eyes. Dr. Porterfield says this
is generally the case with people who squint; and I have found it so
in all that I have had an opportunity of examining.

With regard to the nature of this defect, the distorted eye is
sometimes more convex, and sometimes more flat, than the sound one;
sometimes it does not depend upon the convexity, but upon a weakness,
or some other affection, of the retina, of the nature of which we are
ignorant.

It will be easy to conceive how this inequality of goodness in the
two eyes, when in a certain degree, must necessarily occasion
squinting, and that this blemish is not a bad habit, but a necessary
one, which the person is obliged to learn, in order to see with
advantage. When the eyes are equally good, an object will appear more
distinct and clear when viewed with both eyes than with only one; but
the difference is very little. The ingenious Dr. Jurin, who has made
some beautiful experiments to ascertain this point, has shown, that
when the eyes are equal in goodness, we see more distinctly with both
than with one, by about one thirteenth part only. But M. Buffon has
found that when the eyes are unequal, the case will be quite
different. A small degree of inequality will make the object, when
seen with the better eye alone, appear equally bright or clear, as
when seen with both eyes; a little greater inequality will make the
object appear less distinct when seen with both eyes, than when it is
seen with the stronger eye alone; and a sti