1. Apparatus for use with equipment producing a first television image, originating from a first television camera capable of being pointed in a variable direction so as to view differing parts of a foreground scene, said apparatus having the capability of producing a second television image formed by combination of parts of a pair of further images originating from a pair of further television cameras pointing in fixed directions so as to view different parts of a background scene, with horizontally displaced overlapping fields of view, said second television image moving coincident with, and corresponding to, horizontal motion of said first television image arising from change in the direction of viewing of said first television camera, in such manner that said first television image and said second television image may subsequently be combined, by the method of chroma-key, being known art, to form a combined image, said combined image then having the appearance of having been viewed by said first television camera, the position of objects portrayed in said combined image undergoing displacement in accordance with the horizontal motions of said first television camera, said displacement applying equally to that part of said combined image deriving from said first television camera capable of being pointed in a variable direction, and to that part of said combined image deriving from said second television image itself being derived from said further television cameras pointing in fixed directions, comprising: a. means for sensing the direction of viewing of said first television camera, comprising a backing surface placed behind those parts of said foreground scene viewed by said first television camera, said backing surface having areas of a first color and an area of a second color different from said first color such as to cause said first television camera to generate two distinct and different values of its electrical output, the boundaries between said areas of said first color and said area of said second color being vertical lines, said boundaries performing the function of markers, the positions of said markers in the image formed by said first television camera being dependent on the direction of viewing of said first television camera, b. marker detector means for detecting transitions between said distinct and different values of said electrical output generated by said first television camera when viewing said first color and said second color, thereby detecting said markers, c. means for determining the positions within the image originating from said first television camera at which said transitions corresponding to said markers occur, and thereby producing indication of the direction of viewing of said first television camera, d. displacement generator means for generating displacements of the positions of said pair of further images originating from said pair of further television cameras, consisting of means for accepting a first reference signal having synchronizing pulses in accordance with standard television practice; means for generating a second reference signal likewise having synchronizing pulses in accordance with standard television practice; and means for effecting a controllable variable time delay between the synchronizing pulses of said second reference signal and the synchronizing pulses of said first reference signal, e. means for generating a pair of further reference signals also having synchronizing pulses in accordance with standard television practice, f. means for effecting individually changeable time delays between the synchronizing pulses of said pair of further reference signals and the synchronizing pulses of said second reference signal, g. means for supplying to each camera of said pair of further television cameras a timing signal, being a distinct one of said pair of further reference signals, in such manner that the timing of the synchronizing pulses of said timing signal controls the time at which said camera generates the electrical output signals corresponding to specific points in the scene viewed by said camera, h. means for controlling said controllable variable time delay between the synchronizing pulses of said second reference signal and the synchronizing pulses of said first reference signal by an amount dependent on the direction of viewing of said first television camera derived from the detection of the positions of said markers in the output of said first television camera, said amount being such that the variation in said time delay so produced results in a variation in the timing of the video output signals derived from said pair of further television cameras equal to the variation in the timing of the occurrence of said markers in the video output signal of said first television camera; whereby a displacement in the timing of said markers resulting from a motion of said first television camera will cause an equal displacement in the timing of the video output signals from said pair of further television cameras, i. means for setting said individually changeable time delays between the synchronizing pulses of said pair of further reference signals and the synchronizing pulses of said second reference signal such that the timings of each distinct one of said pair of further reference signals supplied to each one of said pair of further television cameras are such that, there being a first object viewed by the first one of said pair of further television cameras, being that camera placed to the left of the second one of said pair of further television cameras, said pair of further television cameras having horizontally overlapping fields of view, said first object being so positioned as to produce a video signal output from said first one of said pair of further television cameras at a time corresponding to the time of scanning of a part of the field of view close to the right side of the limit of the field of view of said first one of said pair of further television cameras, said first object being viewed by said second one of said pair of further television cameras then producing a video signal output from said second one of said pair of further television cameras at a time coincident with the time of production of the corresponding video signal output due to said first object from the first one of said pair of further television cameras; and conversely a second object being viewed close to the left limit of the field of view of said second one of said pair of further television cameras producing a video signal output coincident in time with the video signal output produced said second object when viewed by said first one of said pair of further television cameras; whereby said pair of further television cameras may have overlapping fields of view and the timings of said timing signals controlling the scannings of each of said pair of further television cameras may be so set that objects simultaneously appearing in the fields of view of both of said pair of further television cameras will produce video signal outputs coincident in time, j. means for generating a switch control signal so timed as to occur during the time of overlap of the fields of view of said pair of further television cameras such that said switch control signal will occur during the simultaneous presence of the video output signals from said first and second objects, both being viewed by both of said pair of further television cameras, in the video outputs of both of said pair of further television cameras, the timing of said switch control signal being directly related to the timings of said pair of further reference signals, said timings being set by way of said means for setting said individually changeable time delays such as to produce time coincidence of the video output signals from said pair of further television cameras deriving from objects within the overlapping fields of view of said pair of further television cameras, such as said first object and said second object, said switch control signal therefore being derivable from said pair of further reference signals by detection of the time of overlapping of the images of said pair of further television cameras; additionally means being provided to cause said switch control signal to be reset to its initial condition during the synchronizing pulses of said first reference signal; whereby the output of said first one of said pair of further television cameras will be selected following said synchronizing pulses of said first reference signal, being coincident with the start of scanning of a standard television display device on which the output of the system may be viewed, and k. means for switching between the video signal outputs of said pair of further television cameras controlled by said switch control signal such that only one of said video signal outputs is selected at any one time, the selection being performed at those times at which the video signal outputs from a single object appearing in the fields of view of both of said pair of further television cameras are coincident in time; whereby a single continuous video output signal corresponding to a field of view part of which is derived from one, and the remainder from the other, of said pair of further television cameras, may be derived.