522 w. JAMES :

Considering all the circumstances, I feel entirely justified in discarding this entire batch of illusions as irrelevant to our present inquiry. Whatever they may prove, they do not prove that our visual percepts of form and movement may not be sensations strictly so called. They much more pro- bably fall into line with the phenomena of irradiation and of colour-contrast, and with Vierordt's primitive illusions of movement. They show us, if anything, a realm of sensa- tions in which our habitual experience has not yet made traces, and which persist in spite of our better knowledge, tmsuggestive of those other space-sensations which we all the time know from extrinsic evidence to constitute the real space-determinations of the diagram. Very likely, if these sensations were as frequent and as practically important as they now are insignificant and rare, we should end by substituting their significates the real space-values of the diagrams for them. These latter we should then seem to see directly, and the illusions would disappear like that of the size of a tooth-socket when the tooth has been out a week. (&) Another batch of cases which we may discard is that of double images. A thoroughgoing anti-sensationalist ought to deny all native tendency to see double images when disparate retinal points are stimulated, because, he would say, most people never get them, but see all things single which experience has led them to believe to le single. * Can a doubleness, so easily neutralised by our knowledge, ever be a datum of sensation at all ? ' such an anti-sensation- alist might ask. To which the answer is that it is a datum of sensation, but a datum which, like many other data, must first be discriminated. As a rule, no sensible qualities are dis- criminated without a motive. And those that later we learn to discriminate were originally felt confused. As well pretend that a voice, or an odour, which we have learned to pick out, is no sensation now. One may easily acquire skill in discriminating double images, though, as Hering somewhere says, it is an art of which one cannot become master in one year or in two. For masters like Hering himself, or Leconte, the ordinary stereoscopic diagrams are of little use. Instead of com- bining into one solid appearance, they simply cross each other with their doubled lines. Volkmann has shown a great variety of ways in which the addition of secondary lines, differing in the two fields, helps us to see the