Page:Mind (Old Series) Volume 12.djvu/544

 THE PEECEPTION OF SPACE. (IV.) 531 come to pass that instead of these parts eliminated by election a feeling of entirely different elements comes to consciousness elements not objectively contained in the stimulus. A group of sensations namely, for which a strong tendency to reproduction has become, by frequent repetition, ingrained in the nervous system will easily revive as a whole when not its whole retinal image, but only an essential part thereof, returns. In this case we get some sensations to which no adequate stimulus exists in the retinal image, and which owe their being solely to the reproductive power of the nervous apparatus. This is complementary (erganzendej reproduction. "Thus a few points and disconnected 'strokes are sufficient to make us f-'ee a human face, and without specially directed attention we fail to note that we see much that really is not drawn on the paper. Attention will show that the outlines were deficient in spots where we thought them com- plete. . . . The portions of the percept supplied by complementary reproduction depend however, just as much as its other portions, on the i eaction of the nervous apparatus upon the retinal image, indirect though this reaction may, in the case of the supplied portions, be. And so long as they are present, we have a perfect right to call them sensations, for they dift'er in no wise from such sensations as correspond to an actual stimulus in the retina. Often, however, they are not persistent ; many of them may be expelled by more close observation, but this is not proved to be the case with all. ... In vision with one eye. . . the distribution of parts within the third dimension is essentially the work of this com- plementary reproduction, i.e., of former experience. . . . When a certain way of localising a particular group of sensations has become with us a second nature, our better knowledge, our judgment, our logic, are of no avail. . . . Things actually diverse may give similar or almost identical retinal images ; e.g., an object extended in three dimensions, and its flat perspective picture. In such cases it often depends on small accidents, and especially on our will, whether the one or the other group of sensations shall be excited. . . We can see a relief hollow, as a mould, or vice versa; for a relief illuminated from the left can look just like its mould illuminated from the right. Reflecting upon this, one may infer from the direction of the shadows that one has a relief before one, and the idea of the relief will guide the nerve-processes into the right path, so that the feeling of the relief is suddenly aroused. . . . When- ever the retinal image is of such a nature that two diverse modes of reaction on the part of the nervous apparatus are, so to speak, equally, or nearly equally, imminent, it must depend on small accidents whether the one or the other reaction is realised. In these cases our previous knowledge often has a decisive effect, and helps the correct perception to victory. The bare idea of the right object is itself a feeble reproduction which with the help of the proper retinal picture develops into clear and lively sensation. But if there be not already in the nervous apparatus a disposition to the production of that percept which our judgment tells us is right, our knowledges strives in vain to conjure up the feeling of it ; we then know that we see something to which no reality corresponds, but we see it all the same " (Hermann's Handb. der Physiologic, iii. 1, pp. 565-71). Note that no object not probable, no object which we are not incessantly practised in reproducing, can acquire this vividness in imagination. Objective corners are ever chang- ing their angles to the eyes, spaces their apparent size, lines their distance. But by no transmutation of position in space does an objective straight line appear bent, and only in one