THE PERCEPTION OF SPACE. 247 



vice versa if the movement of revolution is in the opposite direction. If 

 in the former case the eyes of the observers are turned from the rotat- 

 ing disk towards any familiar object — e.g. the face of a friend — the latter 

 seems to contract or recede in a somewhat striking manner, and to 

 expand or approach after the opposite motion of the spiral." * 



An elementary form of these motor illusions seems to be 

 the one described by Helmlioltz 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 



Fig. 66. 



that line. Thus in Fig. 66: 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 compass-point passes from C to D, the line appears to 

 move downwards ; as it passes from D to E, the line appears 

 to move upwards ; at the same time the Avliole line seems 

 to incline itself in the direction FG during the first half 

 of the compass's movement ; and in the direction HI dur- 

 ing its last half ; the change from one inclination to an- 

 other 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 



* Bowditch and Hall, iu Journal of Physiology, vol. iii. p. 299. Helm- 

 holtz tries to explain this phenomenon by unconscious rotations of the eye- 

 ball. But movements of the eyeball can only explain such appearances 

 of movements 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 an- 

 other. 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 (Sitzuugsber. d. Wiener Akad., 

 Bd. Lxi.). See also Bowditch and Hall's paper as above, p. 300. 



