326 w. JAMES :

now cope with difficulties greater than any that have assailed us hitherto. A sensation is presumably the mental affection that follows most immediately upon the stimulation of the sense- tract. Its antecedent is directly physical, no psychic links, no acts of memory, inference or association intervening. Accordingly, if we suppose the nexus between neural process in the sense-organ on the one hand, and conscious affection on the other, to be by nature uniform, the same process ought always to give the same sensation ; and conversely, if what seems to be a sensation varies whilst the process in the sense-organ remains unchanged, the reason is presumably that it is really not a sensation but a higher mental product, whereof the variations depend on events occurring in other parts of the nervous system than the sense-organ in question, probably higher cerebral centres. Now the size of the field of view varies enormously in all three dimensions, without our being able to assign with any definiteness the process in the visual tract on which the variation depends. We just saw how impossible such assignment was in the case where turning down the head produces the enlargement. In general, the maximum feeling of depth or distance seems to take the lead in determining the apparent magnitude of the whole field, and the two other dimensions seem to follow. If, to use the former instance, I look close into a wash-basin, the lateral extent of the field shrinks proportionately to its nearness. If I look from a mountain, the things seen are vast in height and breadth, in proportion to the farness of the horizon. But when we ask what changes in the eye determine how great this maximum feeling of depth or distance (which is un- doubtedly felt as a unitary vastness) shall be, we find ourselves quite unable to point to any one of them as being its absolutely regular concomitant. Convergence, accom- modation, double and disparate images, differences in the parallactic displacement when we move our head, faintness of tint, dimness of outline, and smallness of the retinal image of objects named and known, are all processes that have something to do with the perception of ' far ' and of 1 near ' ; but the effect of each and any one of them in determining such a perception at one moment, may at another moment be reversed by the presence of some other sensible quality in the object, that makes us, evidently by reminding us of past experience, judge it to be at a different distance and of another shape. If we paint the inside of a pasteboard mask like the outside, and look at it with one