hich, if he had properly considered it,
might have led him to a contrary conclusion: from a beautiful
experiment he obtains data, which enable him with considerable
accuracy to determine the size of the insensible spot in his eye,
which he finds to be about 1/30 or 1/40 of an inch in diameter, and
consequently only about 1/5 or 1/6 of the diameter of the optic
nerve, that nerve being about 1/6 of an inch in diameter. I find that
in my eye likewise, the diameter of the insensible spot is about 1/40
of an inch, or something less. Whence it is evident that vision
exists where the choroid coat is not present, and consequently that
the choroid coat is not the organ of vision.

It is probably owing to the hardness and callosity of the retina
where the nerve enters, that we have this defect of sight, as it has
not yet acquired that softness and delicacy which is necessary for
receiving such slight impressions as those of the rays of light, and
this conjecture is rendered still more probable by an observation of
M. Pequet, who tells us, that a bright and luminous object, such as a
candle, does not absolutely disappear, but one may see its light,
though faint. This not only shows that the defect of sight is not
owing to a want of the choroides, but also that the retina is not
altogether insensible where the nerve enters. These circumstances, in
my opinion, render it certain, that the retina, and not the choroid
coat, is the organ of vision.

_Of our seeing Objects erect by inverted Images._

Another question concerning vision, which has very much perplexed
philosophers, is this; how comes it that we see objects erect, when
it is well known that their images or pictures on the retina are
inverted? The sagacious Kepler, who first made this discovery, was
the first that endeavoured to explain the cause of it.

The reason he gives for our seeing objects erect, is this, that as
the rays from different points of an object cross each other before
they fall on the retina, we conclude that the impulse we feel upon
the lower part of the retina comes from above; and that the impulse
we feel from the higher part, comes from below. Des Cartes afterwards
gave the same solution of this phenomenon, and illustrated it by the
judgment we form of the position of objects which we feel with our
arms crossed, or with two sticks that cross each other. But this
solution is by no means satisfactory: first, because it supposes our
seeing objects ere