I. Combination of Similar Stereoscopic Images. § II. Ideational Change in the Combination of Different Images. § III. Reflection and Lustre; Theory of Lustre. § IV. Phenomena of Suppression in Binocular Vision.

THE fusion of the two retinal images to a single idea is only a particular instance of a general law of the formation of ideas. In the visual idea which comes from the two eyes we do not discover any trace of the perception of one eye as distinct from that of the other; but we blend them at once into a single and indissoluble perception. In this sense it is true that the two eyes constitute only a single organ of vision. That they are really like two independent observers, regarding things from two different points of view, and that we become acquainted with the properties of objects only by combining the result of these observations, are facts which we do not remark; there is given in consciousness simply the result of this combination. That is to say, it is not until the two perceptions of binocular vision have fused that we have an idea; or, in other words, the fact that the two eyes unite to perform a common function is a necessary result of the nature of the mental processes of association.