1. A device for detecting motion within an image comprising: a medium having an electrical property which varies as a function of an image incident on the medium and as a function of strain disturbances in the medium; means for causing the incidence of an image on the medium; means for causing strain disturbances in the medium; and means for measuring the electrical property of the medium while said image is incident thereon and while the strain disturbances are present in it to derive an electrical signal which is a Fourier transform representation of the image and means for deriving from said signal an indication of motion within the image.