AbstractWhen equally spaced, vertical, parallel straight lines are rotated about a horizontal axis perpendicular to the plane of the grid, the horizontal separation between the lines increases with the angle of rotation. With the grid presented binocularly, the increase in horizontal separation between the lines can be used to measure the vergence limits of subjects. Results obtained by this method were compared with those obtained by Risley prisms and by a haploscope. The new method has advantages over the traditional methods: it is not affected by the aberrations and distortions generated by prisms and it presents a more natural stimulus situation than the tubular haploscope.