日にっ光こう反はん射しゃ信しん号ごう

noun

heliogram

Historical term for a message sent by reflecting sunlight, typically using a mirror or heliograph. Rare in modern contexts.

• 日にっ光こう反はん射しゃ信しん号ごうは、19世せい紀きの軍ぐん事じ通つう信しんで使つかわれた。

  Heliograms were used in 19th-century military communication.