RELATIVE DIFFERENCES IN THE SIZE AND SHAPE OF OCULAR IMAGES Leaving aside pathologic conditions, one usually considers but two factors in determining whether the eyes of a person are functioning properly : first, the refraction and, second, the muscle balance. If there are emmetropia and orthophoria for both distance and near, it is assumed that there is nothing the matter with the eyes as far as their functioning is concerned. There is a third factor, however, which appears to be of importance. This is the relative size and shape of the ocular images of the two eyes. The importance of this factor becomes evident when one considers the use of the eyes in binocular vision and the physiologic part played by the brain when the ocular images of the two eyes are combined. For the purpose of elucidation, figure 1, which represents a mechanical model of the physiologic processes of