becomes the two-dimensional plane through the two straight horopter lines. In detail, the identification of the theoretical/geometrical horopter with the Vieth-Müller circle is only an approximation. It was pointed out in Gulick and Lawson (1976) that Müller's anatomical approximation that the nodal point and eye rotation center are coincident should be refined. Unfortunately, their attempt to correct this assumption was flawed, as demonstrated in Turski (2016). This analysis shows that, for a given fixation point, one has a slightly different horopter circle for each different choice of the nodal point’s location. Moreover, if one changes the fixation point along a given Vieth-Müller circle such that the vergence value remains constant, one obtains an infinite family of such horopters, to the extent that the nodal point deviates from the eye’s rotation center. These statements follow from the Central Angle Theorem and the fact that three non-collinear points give a unique circle. It can also be shown that, for fixations along a given Vieth-Müller circle, all the corresponding horopter circles intersect at the point of symmetric convergence. This result implies that each member of the infinite family of horopters is also composed of a circle in the fixation plane and a perpendicular straight line passing through the point of symmetric convergence (located on the circle) so long as the eyes are in primary or secondary position. When the eyes are in tertiary position away from the two basic horopter lines, the vertical disparities due to the differential magnification of the distance above or below the Vieth-Müller circle have to be taken into account, as was calculated by Helmholtz. In this case the horopter becomes a single-loop spiral passing through the fixation point and converging toward the vertical horopter at the top and bottom extremities and passing through the nodal point of the two eyes. This form was predicted by Helmholtz and subsequently confirmed by Solomons. In the general case that includes the fact that the eyes cyclorotate when viewing above or below the primary horopter circle, the theoretical horopter components of the circle and straight line rotate vertically around the axis of the nodal points of the eyes. Empirical Binocular Horopter As Wheatstone (1838) observed, the empirical horopter, defined by singleness of vision, is much larger than the theoretical horopter. This was studied by Peter Ludvig Panum in 1858. He proposed that any point in one retina might yield singleness of vision