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.