heliograph (n.)

1848, "instrument for taking photographs of the sun," from helio- "sun" + -graph "something written." Earlier, "a description of the sun" (1706, implied in heliographic). From 1877 as the name of a movable mirror used in signaling. Related: Heliographical.

Heliography (1845) was the word for the product of a type of engraving process by chemical reaction from exposure to sunlight. It also was an early term for what came to be called photography (1840).

Others are reading

Definitions of heliograph from WordNet
heliograph (v.)
signal by means of a mirror and the using the sun's rays;
heliograph (n.)
an apparatus for sending telegraphic messages by using a mirror to turn the sun's rays off and on;