322 w. JAMES :

ambiguities, as not directly relevant to the space-problem ; it is certain that the same retinal image makes us see quite differently-sized and differently-shaped objects at different times, and it is equally certain that the same ocular move- ment varies in its perceptive import. It ought to be possible, were the act of perception completely and simply intelligible, to assign for every distinct judgment of size, shape and position, a distinct optical modification of some kind as its occasion. And the connexion between the two ought to be so constant that, given the same modification, we should always have the same judgment. But if we study the facts closely we soon find no such constant con- nexion between either judgment and retinal modification, or judgment and muscular modification, to exist. The judgment seems to result from the combination of retinal, muscular and intellectual factors with each other; and any one of them may occasionally overpower the rest in a way which seems to leave the matter subject to no simple law. The scientific study of the subject, if we omit Descartes, began with Berkeley, and the particular perception he analysed in his New Theory of Vision was that of distance or depth. Starting with the physical assumption that a difference in the distance of a point can make no difference in the nature of its retinal image, since " distance being a line directed endwise to the eye, it projects only one point in the fund of the eye which point remains invariably the same, whether the distance be longer or shorter," he con- cluded that distance could not possibly be a visual sensation, but must be an intellectual " suggestion " from " custom" of some non-visual experience. According to Berkeley this experience was tactile. His whole treatment of the subject was excessively vague no shame to him, as a breaker of fresh ground but, as it has been adopted and enthusiastically hugged in all its vagueness by nearly the whole line^of British psychologists who have succeeded him, it will be well for us to begin our study of vision by refuting his notion that depth cannot possibly be perceived in terms of purely visual feeling. (a) The Third Dimension. Berkeley ans unanimously assume that no retinal sensa- tion can primitively be voluminous ; if it be extended at all (which they are barely disposed to admit), it can be extended only in the first two dimensions, not in the third. At start- ing we have denied this, and adduced facts to show that all sensations are voluminous in three dimensions. It is