Abstract: A device for detection of self-movement has an optically convergent interface and a substantially linear marker. The convergent interface and linear marker are positioned so that the marker is inclined to, and extends progressively away from, the convergent interface on the same side as the focus or focal line thereof, at least to a position (P) at which the virtual image of the linear marker is significantly further displaced from the convergent interface than is the position (P) in real space. The interface may be a lens in plate or strip form which may be detachably mounted on a base which carries the marker, the base also forming a receptacle for storing the lens when detached. In an alternative embodiment, the device may comprise a solid transparent block with a front face shaped to provide the interface, and with the marker embedded within the block. The device can be used by sportsmen such as golfers to detect body sway during the execution of a stroke.