The difference in the relative sizes of the ocular images of the two eyes that is found in marked anisometropia and in cases of aphakia is easily demonstrated and its physiologic importance has long been recognized. Such a difference may be considerable, varying from conditions in which one ocular image is 5 per cent larger in overall size than the other, in the anisometropic cases, to differences of 30 per cent in patients operated on for aphakia. A difference of less than 5 per cent in the relative size of the ocular images, however, has been generally assumed to be too small to have any physiologic importance and hence could be neglected. There would appear to be two grounds for this assumption. First, the ocular resolving power decreases so rapidly in peripheral vision that small differences in the size of the ocular images are below the threshold of discrimination.