190 w. JAMES :

strength might be but half as great as that of a stationary shadow so faint as just to disappear. With a blurred shadow in indirect vision the difference in favour of motion was much greater namely, 13'3 : 40' 7. If we hold a finger between our closed eyelid and the sunshine we shall not notice its presence. The moment we move it to and fro, however, we discern it. Such visual perception as this reproduces the conditions of sight among the radiates. Enough has now been said to show that in the education of spatial discrimination the motions of impressions across sensory surfaces must have been the principal agent in breaking up our consciousness of the surfaces into a con- sciousness of their parts. Even to-day the principal function of the peripheral regions of our retina is that of sentinels, which, when beams of light move over them, cry ' Who goes there ? ' and call the fovea to the spot. Most parts of the skin do but perform the same office for the finger-tips. Of course finger-tips and fovea leave some power of direct per- ception to marginal retina and skin respectively. But it is worthy of note that such perception is best developed on the skin of the most movable parts (the labours of Vierordt and his pupils have well shown this) ; and that in the blind, whose skin is exceptionally discriminative, it seems to have become so through the inveterate habit they possess of twitching and moving it under whatever object may touch them, so as to become better acquainted with the conformity of the latter. Czermak was the first to notice this. It may be easily verified. Of course movement of surface under object is, for purposes of stimulation, equivalent to move- ment of object over surface. And the exquisite mobility of the eyeball is thus shown, apart from those measuring uses we have noticed already and shall notice again, to be of immense service in promoting discrimination pure and simple. (&) Their Comparison and Measurement. What precedes is all we can say in answer to the problem of discrimination. Turn now to that of measurement of the several spaces against each other, that being the first step in our constructing out of our diverse space-experiences the one space we believe in as that of the real world. If we were immovable and could only passively receive the pressure and motion of objects on our skin, without ever feeling one part of our skin with another, it is certain that we should have far vaguer perceptions of their extension and of our own form than we now possess. The differences of