Abstract: An apparatus for performing measurements on the eye of a subject includes a headrest for supporting the head of the subject in a fixed position, and a frame holding a light source directing illumination at the eye. The frame is adapted for pivotal motion about a central axis of the eye. Imaging means provides an image of the first and fourth Purkinje images of the light, and means are provided for measuring the amount of rotation of the frame. The frame may be initially aligned so that a fixation target lies in the plane defined by the central axis and the light source, and then rotated until the first and fourth Purkinje images determine a straight line parallel to the central axis, whereby the plane is aligned with the optical axis of the eye. The net rotation of the frame then gives a direct reading of the angular difference between the line to the target and the optical axis of the eye. Preferably the apparatus includes an infrared light source.