Stand up and fight!

The helioscope was an invention of an 18th century battlefield. It was a mirror used to send light signals over the distance of several kilometres. On an 18th century battlefield armies would sometimes manoeuvre or simply wander around, not knowing where the enemy was and where the ally was. The helioscope was used as late as WW I, its main drawback being that it could only be used in daytime and in sunny weather.