Apparent displacement of position was examined with a small light spot moving vertically downward. The apparent positions of the moving spot were measured by means of the comparison spot, presented to the right side of tile motion-track at each of four time intervals, 80, 200, 360, and 720 msec, after (1) the start of, (2) the end of, or (3) 200 or 720 msec after the marker spot was flashed while the moving spot was in motion. Results from five Ss revealed the following tendencies. Any point on the motion-track always displaced its apparent position in the direction of the motion. The starting point showed the greatest displacement at the time interval of 720 msee. The end point displaced its apparent position considerably even at the time interval of 80 msec, indicating no significant increase of the displacement at longer intervals. Intermediate points showed greater displacement than the starting and the end points. Even a moving spot which was flashed for only a second and whose motion was hardly noticeable, also displaced its apparent position downward, and increased the displacement with time after the spot was turned off. These findings were considered in relation to the shifts of reference system in the perception of motion.
In describing a physical motion we usually choose an appropriate origin of the coordinates according to the physical situation. But, in our daily life, some objects are mainly chosen as the origin, e.g., the surface of the earth in describing the speed of a motor car. In this example, we can say the surface of the earth serves as the reference system for the motor ear (Johansson, 1966). Many studies of motion have shown how perceived motions are seen differently from physical motions. When these experimental results are discussed in relation to the reference system, they will be regarded as shifts of the reference system in seen motion, that is, shifts from the surface of the earth to something else. The following three examples are the typical cases of such kinds of shift of the reference system.
Duneker (1929) demonstrated that two enclosing patterns of the same form presented stroboscopically can be seen as stationary, while the point presented at the same physical position in the area enclosed by * The author is indebted to Professor T. Oyama, Chiba University, for his valuable suggestions and criticism in preparing the manuscript.