Page:Mind (Old Series) Volume 12.djvu/530

 THE PERCEPTION OF SPACE. (IV.) 517 but by actually exciting a new nerve-process, to which the modified feeling of colour immediately corresponds. The explanation is physiological, not psychological. The trans- formation of the original colour by the inducing colour is due to the disappearance of the physiological conditions under which the first colour was produced, and to the induction, under the new conditions, of a genuine new sensation, with which the " suggestions of experience " have naught to do. That processes in the visual apparatus propagate them- selves laterally, if one may so express it, is also shown by the phenomena of contrast which occur after looking upon motions of various kinds. Here are a few examples. If, over the rail of a moving vessel, we look at the water rushing along the side, and then transfer our gaze to the deck, a band of planks will appear to us, moving in the opposite direction to that in which, a moment previously, we had been seeing the water move, whilst on either side of this band another band of planks will move as the water did. Looking at a waterfall, or at the road from out of a car- window in a moving train, produces the same illusion, which may be easily verified in the laboratory by a simple piece of apparatus. A board with a window five or six inches wide and of any convenient length, is supported upright on two feet. On the back side of the board, above and below the window, are two rollers, one of which is provided with a crank. An endless band of any figured stuff is passed over these rollers (one of which can be so adjusted on its bearings as to keep the stuff always taut and not liable to slip), and the surface of the front board is also covered with stuff or paper of a nature to catch the eye. Turning the crank now sets the central band in continuous motion, whilst the margins of the field remain really at rest, but after a while appear moving in the contrary way. Stopping the crank results in an illusory appearance of motion in reverse directions all over the field. A disc with an Archimedean spiral drawn upon it, whirled round on an ordinary rotating machine, produces still more startling effects. " If the revolution is in the direction in which the spiral line approaches the centre of the disc the entire surface of the latter seems to expand during revolu- tion and to contract after it has ceased ; and 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 rotating disc towards any familiar object e.g., the face of a friend the latter seems to contract or recede in a