THE PEECEPTION OF SPACE, (ill.) 333

These examples, to which I could easily add others if I had room, are perhaps sufficient to break down in the reader's mind the authority of a dictum which has been left so strangely unquestioned. So far from its being true, as Helmholtz says, that a genuine present sensation cannot have its character transformed by suggestions from past experience, it would seem as if the exact contrary were the rule, and as if, with Stumpf, 1 we might reverse Helmholtz's query, and ask : " What would become of our sense-perceptions in case experience were not able so to transform them?" Adding, "All wrong perceptions that depend on peculiarities in the organs are more or less per- fectly corrected by the influence of imagination following the guidance of experience ". If, therefore, among the facts of optical space-perception (which we must now proceed to consider in more detail) we find instances of an identical organic eye-process, giving us different perceptions at different times, in consequence of different collateral circumstances suggesting different objec- tive facts to our imagination, we must not hastily conclude, with the school of Helmholtz and Wundt, that the organic eye-process pure and simple, without the collateral circum- stances, is incapable of giving us any sensation of a spatial kind at all. We must rather seek to discover ly what means the circumstances can so have transformed a space-sensation, which, but for their presence, would probably have been felt in its natural purity. And I may as well say now in advance, that we shall find the means to be nothing more or less than association the suggestion to the mind of optical sensations not actually present, but more habitually associated with the " collateral circumstances " than the one which they now displace. But before this conclusion emerges, it will be necessary to have reviewed the most important facts of optical space-perception, in relation to the organic conditions on which they depend. Readers acquainted with German optics will excuse what is already familiar to them in the following section. (c) The Two Theories of Retinal Perception. Let us begin the long and rather tedious inquiry by the most important case. Physiologists have long sought for a simple law by which to connect the seen direction and distance of objects with the retinal impressions they pro- 1 Op. cit. p. 214.