A direct-vision nephoscope constructed by L. Besson in 1897. It consists of a comb comprising equispaced vertical rods attached to a cross piece. The comb is affixed to one end of a column 2. 5–3 m (8–10 ft) long and is supported on a mounting that is free to rotate about its vertical axis. In use, the comb is turned so that the cloud appears to move parallel to the tips of the vertical rods.