THE PEECEPTION OF SPACE. (IV.)

523 primary lines double. The effect is analogous to that in the cases we despatched a moment ago, where given lines have their space-value changed by the addition of new lines, without our being able to say why, except that a certain mutual adhesion of the lines and modification of the resultant feeling takes place by psycho-physiological laws. Thus, if in Fig. 15 I and r be crossed by an horizontal at the Fig. 15. r same level, and viewed stereoscopically, they appear as a single pair of lines, s, in space. But if the horizontal be at different levels, as in /', /, three lines appear, as in s'. 1 Let us then say no more about double images. All that the facts prove is what Volkmarm says, 2 that, although there may be sets of retinal fibres so organised as to give an im- pression of two separate spots, yet the excitement of other retinal fibres may inhibit the effect of the first, and pre- vent us from actually making the discrimination. Still further retinal processes may, however, bring the double- ness to the eye of attention ; and, once there, it is as genuine a sensation as any that our life affords. 3 (c) These groups of illusions being eliminated, either as cases of defective discrimination, or as changes of one space- sensation into another when the total retinal process changes, there remain but two other groups to puzzle us. The first is that of the after-images distorted by projection 1 See Archiv. f. Opktlwzlm., v. 2, 1 (1859), where many more examples are given. 2 Untersuchungen, p. 250 ; see also p. 242. 3 I pass over certain difficulties about double images, drawn from the perceptions of a few squinters (e.g. t by Schweigger, Klin. Untersuch. liber das Schielen, Berlin, 1881 ; by Javal, Annales d' Oculistiqite, Ixxxv., p. 217), because the facts are exceptional at best and very difficult of interpretation. In favour of the sensationalistic or nativistic view of one such case, see the important paper by Von Kries, Archiv. f. Ophthalm., xxiv. 4, p. 117.