Abstract: An apparatus for measuring torsional rotation of an eyeball about the visual axis by means of an afterimage. The apparatus includes a frame mountable to a patient so as to be fixed with respect to the patient's skull. Mounted on the frame are means for creating a transitory bright image to imprint the retina of the patient's eye with an afterimage and means for creating a target image alignable with said afterimage. Measuring means is included in the apparatus so as to indicate the angular displacement of the target image relative to a preset reference position corresponding to a different stimulus condition. In the preferred embodiment, the apparatus is a tubular assembly supported by a bite bar.