heliograph [Gr.,=sun-writer], signaling device using flashes of sunlight. It has two mirrors that are used to reflect sunlight on a distant point and a shutter through which the sunlight passes so that messages may be transmitted in telegraphic code by means of long and short flashes. It was used in ancient times and as recently as the 19th cent. by the U.S. army in the SW United States and by the British army in India.