Page:Mind (Old Series) Volume 12.djvu/531

 518 w. JAMES : somewhat striking manner, and to expand or approach after the opposite motion of the spiral." 1 An elementary form of these motor illusions seems to be the one described by Helmholtz on pp. 568-571 of his Optik. The motion of anything in the field of vision along an acute angle towards a straight line, sensibly distorts that line. Fig. 11. Thus in Fig. 11 : Let AB be a line drawn on paper, CDE the tracing made over this line by the point of a compass steadily followed by the eye, as it moves. As the com- pass-point passes from C to D, the line appears to move downwards ; as it passes from I) to E, the line appears to move upwards ; at the same time the whole line seems to incline itself in the direction FG during the first half of the compass's movement ; and in the direction HI during its last half; the change from one inclination to another being quite distinct as the compass-point passes over D. Any line across which we draw a pencil-point appears to be animated by a rapid movement of its own towards the pencil-point. This apparent movement of both of two things in relative motion to each other, even when one of them is absolutely still, reminds us of the instances quoted from Vierordt on page 188, and seems to take us back to a primi- tive stage of perception, in which the discriminations we now make when we feel a movement have not yet been made. If we draw the point of a pencil through ' Zollner's pattern ' (Fig. 7, p. 343), and follow it with the eye, the whole figure becomes the scene of the most singular apparent unrest, of which Helmholtz has very carefully noted the conditions. The illusion of Zollner's figure vanishes entirely, or almost 1 Bowditch and Hall, in Journal of Physiology, vol. in., p. 299. Helmholtz tries to explain this phenomenon by unconscious rotations of the eyeball. But movements of the eyeball can only explain such appearances of move- ment as are the same over the whole field. In the windowed board one part of the field seems to move in one way, another part in another. The same is true when we turn from the spiral to look at the wall the centre of the field alone swells out or contracts, the margin does the reverse or remains at rest. Mach and Dvorak have beautifully proved the impossibility of eye-rotations in this case (Sitzungsber. d. Wiener Akad. Bd. Ixi.). See also Bowditch and Hall's paper as above, p. 300.