THE PERCEPTION OF SPACE. 258 



fields, helps us to see the primary lines double. The effect 

 is analogous to that shown in the cases which 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 psychophysiological laws. Thus, if in Fig. 70, I 

 and r be crossed by an horizontal line at the same level, 

 and viewed stereoscopically, they appear as a single pair of 

 lines, s, in space. But if the horizontal be at difi"erent 

 levels, as in V, r\ three lines appear, as in s' .* 



Let us then say no more about double images. All that 

 the facts prove is what Volkmann says,t that, although 

 there may be sets of retinal fibres so organized as to give 

 an impression of two separate spots, yet the excitement of 

 other retinal fibres may inhibit the effect of the first ex- 

 citement, and prevent us from actually making the dis- 

 crimination. Still farther retinal processes may, however, 

 bring the doubleness to the eye of attention ; and, once 

 there, it is as genuine a sensation as any that our life 

 a fiords.:}: 



(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 hut two other groups to puzzle us. The 

 first is that of the after-images distorted by projection on to 

 oblique planes ; the second relates to the instability of 

 our judgments of relative distance aud size by the eye, 

 and includes especially what are known as pseudoscopic 

 illusions. 



* See Archiv f. Ophtbalm., v. 2, 1 (1859), where many more examples 

 are given. 



f Uutersuchuugen, p. 250 ; see also p. 243. 



t 1 pass over certain difficulties aboyt double images, drawn from the 

 perceptions of a few squiuters (e.g. by Schweigger, Klin. Untersuch liber 

 das Schielen, Berlin, 1881 ; by Javal. Annales d'Oculistique, lxxxv. 

 p. 217), because the facts are exceptional at best and very difficult of inter- 

 pretation. In favor of the sensationalistic or nativistic view of one such 

 case, see the important paper by Von Kries, Archiv f. Ophthalm., xxiv. 

 4, p. 117. 



