Against the wall of a room fix three small pieces of paper, about the
size of a sixpence; let them be about half a foot asunder, and the
height of the eye; stand about a yard distant, and, keeping both eyes
steadfastly fixed on the centre piece, all three pieces are visible. Now
shut the _right_ eye (keeping the left still on the centre), and the
piece which is opposite to the _left_ eye disappears; or close the
_left_ eye, and the _right_ piece cannot be seen: so that if either eye
be shut, the paper opposite its fellow becomes invisible; plainly
proving, that some objects opposite the left eye are viewed by the
right, and _vice versâ_, with the left eye closed, and the right piece
consequently invisible; remove the right eye from the centre, and carry
it to the piece on the left; the right piece now becomes visible, but
the centre disappears; and so on alternately, the three pieces not being
visible at the same time, as when both eyes are open, showing one of the
uses of having two eyes.