Page:Mind (Old Series) Volume 12.djvu/338

 THE PEECEPTION OF SPACE, (ill.) 325 the mind will presumably react upon it after its usual fashion (which is that of unifying all data which it is in any way possible to unify), and prefer to consider it the move- ment of a constant object rather than the transformation of a fluctuating one. Now, the sensation of depth it receives is awakened more by the far than by the near end of the object. But how much depth ? What shall measure its amount ? Why, at the moment the far end is ready to be eclipsed, the difference of its distance from the near end's distance must be judged equal to the stick's whole length ; but that length has already been judged equal to a certain optical sensation of breadth. Thus amounts of the visual depth-feeling become signs of fixed amounts of the visual breadth-feeling. The measurement of distance is, as Berke- ley truly said, a result of suggestion and experience. But visual experience alone is adequate to produce it, and this he erroneously denied. Suppose a colonel in front of his regiment at dress-parade, and suppose he walks at right angles towards the midmost man of the line ; the line will visibly shrink as he advances, and at the same time the colonel will perceive his distance from the extreme man at each end of the line to increase relatively to his distance from the midmost man whom he approaches. When he finally touches this midmost man, his distance from the ends is felt by him to be at its maxi- mum, although the line as a whole subtends hardly any retinal angle. What distance shall he judge it to be ? Why, half the length of the regiment as it was originally seen, of course ; but this length was a moment ago a retinal object spread out laterally before his sight. He has merely equated a retinal depth-feeling with a retinal breadth-feeling. If the regiment moved, and the colonel stood still, the result would be the same. In such ways as these, a creature endowed with eyes alone could hardly fail of measuring out all three dimensions of the space he inhabited. And we ourselves, I think, although we may often ' realise ' distance in locomotor terms (as Berkeley says we must always do), yet do so no less often in terms of our retinal map, and always in this way the more spontaneously. Were this not so, the three dimensions could not possibly feel to us as homo- geneous as they do, nor as commensurable inter se. Let us, then, admit distance to be at least as genuinely optical a content of consciousness as either height or breadth. The question immediately returns, Can any of them be said in any strictness to be optical sensations ? W"e have contended all along for the affirmative reply to this question, but must