1. Apparatus for generating from arriving television signals, video signals for displaying simultaneously a succession of images of an object moving in an otherwise substantially static scene, said apparatus comprising: storage means for storing a reference field of video representing said scene at an operator controllable start time; means for comparing picture elements of selectable time-separated subsequently arriving television fields with corresponding picture elements of said reference field; means operative in response to detection of differences between corresponding picture elements of said subsequently arriving fields and said reference field for tagging and storing those picture elements of said subsequently arriving fields that caused the detected difference; means for successively mixing in synchronism with the arrival of selected subsequently arriving television fields previously tagged and stored picture elements that caused detected differences with corresponding picture elements of each of the said selectable subsequently arriving fields and producing a resultant signal containing a controllable fractional amplitude portion of the previously stored picture elements and a controllable fractional amplitude portion of corresponding picture elements of each of the selectable subsequently arriving fields and storing the resultant signal with said previously tagged and stored picture elements; and means connected to receive said tagged and stored picture elements and said subsequently arriving video fields for continuously providing for display, following the said start time, output video signals representing said scene with the substitution therein of those picture elements representing a controllable number of images of said moving object preceding the current image thereof, and in which said images progressively decrease in intensity in the direction toward the earlier image positions of the moving object.