t this coat is the
proper organ of vision. A variety of other arguments in favour of the
choroides occurred to him, particularly he observed that the retina
is transparent, which he thought could only enable it to transmit the
rays further, and he could not persuade himself that any substance
could be considered as being the termination of the pencils, and the
proper seat of vision, at which the rays are not stopped in their
progress.

Mr. Pequet, in answer to Mariotte's observation, says, that the
retina is very imperfectly transparent, resembling oiled paper, or
horn: and, besides, that its whiteness demonstrates that it is
sufficiently opaque for stopping the rays of light as much as is
necessary for vision: whereas, if vision be performed by means of
those rays which are transmitted through such a substance as the
retina, it must be very indistinct.

Notwithstanding the plausibility of this opinion of M. Mariotte, and
the number of celebrated men who joined him in it, I must confess,
that none of their arguments, though very ingenious, have been able
to make me a convert to that opinion.

If we argue from the analogy of the other senses, in all of which the
nerves form the proper seat of sensation, we shall be induced to give
judgment in favour of the retina. And this argument from analogy is
much strengthened, by considering that the retina is a large nervous
apparatus, immediately exposed to the impressions of light; whereas
the choroides receives but a slender supply of nerves, and seems no
more fitted for the organ of vision than any other part of the body.
But facts are not wanting which make still more in favour of the
retina. It appears from observations made upon the sea calf and
porcupine, that these animals have their optic nerves inserted in the
axis of the eye, directly opposite the pupil, which renders it very
improbable that the defect in sight, where the optic nerves enter,
can be owing to the want of the choroides in that place; for were
this true, then in those creatures which have the optic nerves
inserted in the axis of the eye, and which by consequence do directly
receive on the extremity of the nerve the pictures of objects, all
objects would become invisible to which their eyes are turned,
because the choroides is wanting in that place where the image falls;
but this is contrary to experience.

M. Le Cat, though he strenuously supports Mariotte's opinion, takes
notice of a circumstance, w