One of the most curious facts relating to the science of vision is
the absolute insensibility of a certain portion of the retina to the
impression of light, so that the image of any object falling on that
point would be invisible. When we look with the right eye, this point
will be about fifteen degrees to the right of the object observed, or
to the right of the axis of the eye, or the point of most distinct
vision. When looking with the left eye, the point will be as far to the
left. The point in question is the basis of the optic nerve, and its
insensibility to light was first observed by the French philosopher,
Mariotte. This remarkable phenomenon may be experimentally proved in
the following manner:--
Place on a sheet of writing-paper, at the distance of about three
inches apart, two colored wafers; then, on looking at the left-hand
wafer with the right eye, at the distance of about a foot, keeping the
eye straight above the wafer, and both eyes parallel with the line
which forms the wafers, the left eye being closed, the right-hand wafer
will become invisible; and a similar effect will take place if we close
the right eye, and look with the left.