534 w. JAMES : them in perspective, as approximations, at least, to fore- shortened rectangular forms. 1 At the same time the genuine sensational form of the lines before us can, in all the cases of distortion by suggested perspective, be felt correctly by a mind able to abstract from the notion of perspective altogether. Individuals differ in this abstracting power. Artistic training improves it, so that after a little while errors in vertical bisection, in estimating height relatively to breadth, &c., become im- possible. In other words, we learn to take the optical sensation before us pure. 2 We may then sum up our study of illusions by saying that they in no wise undermine our view, that every spatial determination of things is originally given in the shape of a sensation of the eyes. They only show how very potent certain imagined sensations of the eyes may become. These sensations, so far as they bring definite forms to the mind, appear to be retinal exclusively. The movements of the eyeballs play a great part in educating our perception, it is true ; but they have nothing to do with constituting any one feeling of form. Their function is limited to exciting the various feelings of form, by tracing retinal streaks ; and to comparing them, and measuring them off against each other, by applying different parts of the retinal surface to the same objective thing. Helmholtz's analysis of the facts of our
- measurement of the field of view ' is, bating a lapse or two,
masterly, and seems to prove that the movements of the eye have had some part in bringing our sense of retinal 1 Hering would partly solve in this way the mystery of Figs. 7, 8, and 12. No doubt the explanation partly applies ; but the strange cessation of the illusion when we fix the gaze fails to be accounted for thereby. 2 Helmholtz has sought (Physiol. Optik, p. 715) to explain the divergence of the apparent vertical meridians of the two retinae, by the manner in which an identical line drawn on the ground before us in the median plane will throw its images on the two eyes respectively. The matter is too technical for description here ; the unlearned reader may be referred for it to J. Leconte's Sight in the Internat. Scient. Series, p. 198 ff. But for the benefit of those to whom verbum sat, I cannot help saying that it seems to me the exactness of the relation of the two meridians whether divergent or not, for their divergence differs in individuals and often in one individual at diverse times precludes its being due to the mere habitual falling-off of the image of one objective line on both. Leconte, e.g., measures their position down to a sixth of a degree, others to tenths. This indicates an organic identity in the sensations of the two retinae, which the experience of median perspective horizontals may roughly have agreed with, but hardly can have engendered. Wundt explains the divergence, as usual, by the Innervationsgefuhl (op. cit. ii. 99 ff).
