Abstract: A device for measuring a subject's ocular parameters, in particular the interpupillary distance, comprises a front plate against which the face of the subject is placed, comprising two windows substantially corresponding to the lenses of a pair of eyeglasses. A lens is movable along its optical axis, which is perpendicular to the front plate, from an origin position near the front plate. A viewing eyepiece is disposed at the focus of the lens when in the origin position. A semi-reflecting mirror disposed at 45.degree. renders the eyepiece and a point source coincident, as far as the subject is concerned. Parallel to the semi-reflecting mirror, another mirror reflects an image of a cathode ray tube screen to the eyepiece through another lens. Adjustable calibrated voltage sources are used to produce coincidence of the reflections of the source on the corneas of the eyes of the subject and the luminous traces on the screen of the cathode ray tube, as seen by an operator looking through the eyepiece.