Fig. 10 For convergence at any distance other than infinity, all points that do not lie on the Vieth-Müller circle or the vertical horopter line project to the retina with either a vertical disparity or both a vertical and horizontal disparity. Dashed lines show geometric horopter for symmetric fixation (dotted lines from foveae). Full lines represent light rays from a selected point in space. The vertical disparity arises from the differential magnification occurring when the point is closer to one eye than the other, as must occur with all points off the vertical axis. The three-dimensional point horopter is therefore not a surface but two lines in space.