Other forms of visual signalling existed, but had the same limitation of being visible to the enemy as well as to the intended recipient. The 1914 Field Service Pocket Book lists heliographs, discs, and lamps as alternatives to semaphore flags.(1) They all use Morse code. Heliographs use flashes of sunlight reflected from small mirrors. Discs were essentially a system of shutters that could be opened or closed to reveal a coloured signalling surface. Several models existed – one that could be hung up, and another that could be used from ground level. We are told that discs had the advantage of being operable “from cover and read using a periscope.”(2) I think I can picture how that would work, but I wouldn’t want to be sent onto the battlefield to put it into action without a good prior demonstration.
Lamps had the advantage of being usable at night as well as by day. A Begbie lamp burned paraffin, and its lens concentrated its light to project it considerable distances.(2) By 1916 they were giving way to electric versions.
Though their effectiveness depended on the terrain and on atmospheric conditions, visual signals had considerable range: the useful Pocket Book suggested three to seven miles (7 – 11 km) for flags, five to eight miles (8 – 13 km) for a Begbie lamp, and ten to fifteen miles (16 – 24 km) for limelight. (1) Limelight — the same used in the nineteenth century for theatre footlights — is produced by directing a flame of burning oxyhydrogen onto quicklime, or calcium oxide.