This Demonstration plots the point horopter, which is the locus of points that project images to identical retinal positions in the two eyes. The sliders allow the user to specify the point of fixation with respect to the head and the horopter is then plotted. The dashed black lines show the lines of fixation. The calculations are based on the derivation of the horopter given by Helmholtz in his Treatise on Physiological Optics, but have been simplified by the use of vector and matrix algebra concepts which were not current in the nineteenth century.