THE ILLUSION OF SUBTRACTION.
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.