5921923 Indian signallers from the 25th Cavalry (Frontier Force), 1902 circa (b/w photo) by Unknown photographer, (20th century); National Army Museum, London; (add.info.: Indian signallers from the 25th Cavalry (Frontier Force), 1902 circa. Photograph, India, North West Frontier, 1902 circa. Sowars of the 25th Cavalry use a heliograph in the mountains of the North West Frontier. A heliograph used a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light could be used to send Morse code. It was an ideal tool in the mountainous frontier regions as it allowed long distance communication without a fixed infrastructure. The heliograph was first used by the Indian Army during the 1877 Jowaki Expedition against the Afridis. From an album of 87 photographs associated with Lieutenant F R Teesdale, 25th Cavalry (Frontier Force), Lahore Cantonment, 1902-1914.); by National Army Museum .