Before the First World War the French introduced the collimateur. During that war the British introduced their first parallescope, which was a horizontal mirror placed a few feet from the gun; the layer aimed his sight at its reflection. In the 1950s the parallescope was replaced by the prism parallescope that was more robust and easily positioned. In the 1970s the US introduced a modern version of the French device and called it a collimator. In the same period infra-red beacons had some very limited use.