1. In apparatus for deriving video signals from arriving television signals for displaying simultaneously a succession of selected separated images of an object moving over a selectable time period in an otherwise substantially static scene including means including storage means adapted to store a reference field of video, switching means normally operative to couple arriving video signals to an apparatus output terminal and operative in response to initiation of the start of a selectable time period to couple to said storage means an arriving video signal representing said scene at the start of said selectable time period, means for comparing picture elements of said stored video signal with corresponding picture elements of selected television frames subsequently arriving during said selectable time period and for controlling said switching means in response to detection of a difference between corresponding picture elements of the stored and subsequently arriving frames and to identify and store the picture element of the subsequently arriving frame that caused the difference, and means for controlling said switching means to derive at said apparatus output terminal an output video signal containing at least the said subsequently identified and stored picture elements representing a series of positions of said moving object, the apparatus further comprising: enhancing means responsive to said identifying means for modifying the brightness value of the output video signal elements representing the moving object to increase the difference, in the output video signal, between the brightness of the moving object and the brightness of the static scene.