Figure 2.

The complete set of correspondences and differences between sequential images of a line moving uniformly is defined by four factors. (A) Identity. A line point or segment at time t that occupies any position on the visible line at time t + 1 defines identical elements in the two images (in this and subsequent figures, identity is indicated by a black line segment). (B) Appearance. The elements of the line indicated in blue at time t + 1 do not correspond to any of the elements visible at time t and have thus appeared in the interval between the generation of the two images. (C) Disappearance. The elements of the line indicated in red at time t do not correspond to any visible elements at time t + 1 and have thus disappeared in the interval. (D) Deformation. The projected line images can also differ as a result of movement of the source toward (or away) from the observer, by rotation, or by physical deformation (here indicated by a uniform extension of the line segment during the interval; deformation in subsequent figures is also indicated in green). Because the contributions of identity, appearance, disappearance, and deformation of the line elements are conflated in any two sequential images, the physical displacement underlying the stimulus generated by a moving line is profoundly ambiguous. Any account of the possible sources of the stimulus must therefore take into account the contributions of these four factors.