A sensing rod adapted to transmit light and receive reflected light and including circuitry adapted to detect variations in the intensity of the reflected light. A light transmitting and receiving tip is mounted on the sensing rod for axial movement relative thereto. Light conducting elements are provided for conducting the light detected from the axially movable tip to the aforementioned circuitry. The tip is resiliently urged into its outer projecting position from the front end of the sensing rod and is resiliently maintained in this position by a spring force which is adapted to yield when an external force is applied to the tip to permit the tip to move inside the sensing rod.