Cut a circular piece of white paper, about two inches in diameter, and
affix it to a dark wall. At the distance of two feet on each side, but
a little lower, make two marks; then place yourself directly opposite
the paper, and hold the end of your finger before your face, so that
when the right eye is open it shall conceal the mark on your left, and
when the left eye is open the mark on your right. If you then look with
both eyes at the end of your finger the paper disc will be invisible.