§ I. The Separation of Visual Ideas; Influence of Boundary Lines. § II. Ideas of Depth. § III. Relations between Apparent Magnitude and Distance of Objects.

WE have now shown at some length how the mind comes to arrange visual impressions in spatial order upon a plane surface. But our knowledge of the formation of the field of vision has given us no idea either of the nature of external objects, or of the visible parts of our own body. The impressions, though spatially disposed, have not as yet been brought into those relations in virtue of which they are arranged as separate ideas, each apprehended as a whole of definite spatial form. How does this separation come about? How do we pass from the spatial perception which leaves its objects side by side without distinction or difference to the idea of objects which are spatially separate?