Ottoman Army signallers use a heliograph (far left) to relay messages to troops in advanced positions at Huj in Palestine.
The heliograph was a wireless solar telegraph instrument that relayed Morse code using flashes of sunlight reflected by a mirror. The flashes were produced by pivoting the mirror or interrupting the beam of sunlight with a shutter. Heliographs were used by both British and Ottoman forces during the Sinai and Palestine campaigns, where they were effective over distances of more than 50 km.
Just to the right of the heliograph in the image is another signalling instument, probably a blinkgerat. These were powered by acetylene gas and therefore could be used when there was no sun. (See a higher resolution copy of this image here).