Fix a similar disc of paper, two inches in diameter, at the height of
your eye on a dark wall; a little lower than this, at the distance
of two feet on the right hand, fix another of about three inches in
diameter; now place yourself opposite the first sheet of paper, and,
shutting the left eye, keep the right eye still fixed on the first
object, and when at the distance of about ten feet, the second piece of
paper will be invisible.