as a
power of accommodating itself to view objects distinctly enough at
several different distances.

_Concerning the Seat of Vision._

No subject has been more canvassed than that concerning what is
improperly called the seat of vision. In early times, the crystalline
lens was thought to be best qualified for this office; but this
substance, though situated in the middle of the eye, which Baptista
Porta thought to be the proper centre of observation, had universally
given place to the better founded pretensions of the retina: and,
from the time of Kepler, few ventured to dispute its claim to that
office, till M. Mariotte was led, from some curious circumstances, to
think that vision was not performed by the retina, but by the choroid
coat. Having often observed in the dissections of men, as well as of
brutes, that the optic nerve is not inserted exactly opposite to the
pupil, that is, in the place where the picture of the objects upon
which we look directly, is made: and that in man it is somewhat
higher, and on the side towards the nose, he had the curiosity to
examine the reason of this structure, by throwing the image of an
object on this part of the eye. In order to do this, he fastened on a
dark wall, about the height of his eyes, a small round paper, to
serve for a fixed point of sight; and he fastened such another paper
on the right hand, at the distance of about two feet, but rather
lower than the former, so that light issuing from it, might strike
the optic nerve of his right eye, while the left was kept shut. He
then placed himself over against the former paper, and drew back by
degrees, keeping his right eye fixed, and very steady upon it, and
when he had retired about ten feet, he found that the second paper
entirely disappeared. This, he says, could not be imputed to the
oblique position of the second paper, with respect to his eye,
because he could see more remote objects on the same side. This
experiment he repeated by varying the distances of the paper and his
eye. He also made it with his left eye, while the right eye was kept
shut, the second paper being fastened on the left side of the point
of sight; so that by the situation of the parts of the eye, it could
not be doubted that this defect of vision is in the place where the
optic nerve enters, where only the choroides isdeficient.

From this he concludes, that the defect of vision is owing to the
want of the choroid coat, and, consequently, tha