A

David

Darling

heliograph

The term 'heliograph' may apply to either of two devices:

 

A nineteenth-century instrument used for mainly military signaling, comprising essentially a mirror and a shutter to cut off the sunlight reflected from it. A further mirror permitted messages to be sent even when the Sun was behind the sender. The signals could be interpreted up to 50 kilometers away.

 

A device for recording the positions of sunspots.