As Wheatstone (1838) observed, the empirical horopter, defined by singleness of vision, is much larger than the theoretical horopter. This was studied by P. L. Panum in 1858. He proposed that any point in one retina might yield singleness of vision with a circular region centred on the corresponding point in the other retina. This has become known as Panum's fusional area, or just Panum's area, although recently that has been taken to mean the area in the horizontal plane, around the Vieth-Müller circle, where any point appears single.