Framework-Relative Paths of Motion

                                                                                 

With three dots moving on the screen. A very different event is perceived, especially if only the dots are visible (that is, dots in limbo). The center dot is seen in vertical motion between the two end dots. If the surroundings are not completely featureless (that is, if the edges of the projection screen ate dimly discernible, or (here is a stationary dot), the entire set of dots may be detected as moving horizontally