If all the rays from one point of the object, a, are to be concentrated on a corresponding point of the retina, a which would then become the exclusive representative of a, we must have one or more refracting surfaces interposed, to gather the rays together. The presence of the lens, with its various coatings, has made representation of point by point possible for the eye. The absence of such an instrument makes the same sort of representation impossible to other senses, such as the nose, which does not smell in one place the effluvia of one part of the environment and in another place the effluvia of another, but smells indiscriminately the combination of all. Eyes without lenses like those possessed by some animals, undoubtedly give only a consciousness of diffused light, without the possibility of boundaries or divisions in the field of view. The abstraction of colour from form is therefore by no means an artificial one, since, by a simplification of the organ of sense, one may be perceived without the other. But even if the lens enables the eye to receive a distributed image of the object, the manifold which consciousness would perceive would not be necessarily a manifold of parts juxtaposed in space. Bach point of the retina might send to the brain a detached impression; these might be comparable, but not necessarily in their spatial position. The ear sends to the brain such a manifold of impressions (since the ear also has an apparatus by which various external differences in rapidity of vibrations are distributed into different parts of the organ). But this discriminated manifold is a manifold of pitches, not of positions. How does it happen that the manifold conveyed by the optic nerve appears in consciousness as spatial, and that the relation between its elements is seen as a relation of position?