330 w. JAMES :

Faces, colours, shapes, change in the twilight, according as we imagine them to represent this or that object. Motion- less things appear to move under the same circumstances. The colour of the marginal field of view is seen like that of the central in the absence of any reason why we should judge it different (as in looking at the blue sky or a white wall), though a small marginal patch seen alone would be quite different. Colour is surely a sensation ! But leave the optical realm, where everything has been made doubtful. Touch is a sensation ; yet who has not felt (pp. 96-8), gives a theory which is to me so obscure that I only refer the reader to its place, adding that it seems to make of distance a fixed function of retinal sensation as modified by focal adjustment. Besides these three authors I am ignorant of any, except Panum, who may have attempted to define distance as in any degree an immediate sensation. And with them the direct sensational share is reduced to a very small proportional part, in our completed distance-judgments. Professor Lipps, in his singularly acute Psychologische Studien (pp. 69 ff.), argues, as Ferrier, in his review of Berkeley (Philosophical Remains, ii. 330 ff.), had argued before him, that it is logically impossible we should perceive the distance of anything from the eye by sight ; for a seen distance can only be between seen termini ; and one of the termini, in the case of distance from the eye, is the eye itself, which is not seen. Similarly of the distance of two points behind each other : the near one hides the far one, no space is seen between them. For the space between two objects to be seen, both must appear beside each other, then the space in question will be visible. On no other condition is its visibility possible. The conclusion is that things can properly be seen only in what Lipps calls a surface, and that our knowledge of the third dimension must needs be conceptual, not sensational or visually intuitive. But no arguments in the world can prove a feeling which actually exists to be impossible. The feeling of depth or distance, of farness or awayness, does actually exist as a fact of our visual sensibility. All that Professor Lipps's reasonings prove concerning it is that it is not linear in its character, or in its immediacy fully homogeneous and consubstautial with the feeling of lateral distance between two seen termini ; in short, that there are two sorts of optical sensation, each inexplicably due to a peculiar neural process. The neural process is easily discovered, in the case of lateral extension or spread-outness, to be the number of retinal nerve-ends affected by the light ; in the case of protension or mere farness, it is more complicated and, as we have found, is still to seek. The two sensations unite in the primitive visual bigness. The measurement of their various amounts against each other obeys the general laws of all such measurements. We discover their equivalencies by means of objects, apply the same units to both, and translate them into each other so habitually that at last they get to seem to us even quite similar in kind. This final appearance of homo- geneity is doubtless much facilitated by the fact that in binocular vision two points situated on the prologation of the optical axis of one of the eyes, so that the near one hides the far one, are by the other eye seen laterally apart. Each eye has in fact a foreshortened lateral view of the other's line of sight. In The Times for Feb. 8, 1884, is an interesting letter by J. D. DotLgal. who tries to explain by this reason why two-eyed rifle shooting has such advantages over shooting with one eye closed.